Network: automatically use CDN resources when local resources not available, and networking enabled

13 files changed, 355 insertions, 130 deletions

diff --git a/src/4D_api.cpp b/src/4D_api.cpp
index efb4a86a..cee8262e 100644
--- a/src/4D_api.cpp
+++ b/src/4D_api.cpp
@@ -262,7 +262,7 @@ similarly, but prefers the 2D resp. 3D interfaces if available.
             auto coordOperation = dynamic_cast<
             NS_PROJ::operation::CoordinateOperation*>(alt.pj->iso_obj.get());
             if( coordOperation ) {
-                if( coordOperation->gridsNeeded(dbContext).empty() ) {
+                if( coordOperation->gridsNeeded(dbContext, true).empty() ) {
                     if( P->iCurCoordOp != i ) {
                         std::string msg("Using coordinate operation ");
                         msg += alt.name;
@@ -1117,7 +1117,10 @@ PJ  *proj_create_crs_to_crs_from_pj (PJ_CONTEXT *ctx, const PJ *source_crs, cons
     proj_operation_factory_context_set_spatial_criterion(
         ctx, operation_ctx, PROJ_SPATIAL_CRITERION_PARTIAL_INTERSECTION);
     proj_operation_factory_context_set_grid_availability_use(
-        ctx, operation_ctx, PROJ_GRID_AVAILABILITY_DISCARD_OPERATION_IF_MISSING_GRID);
+        ctx, operation_ctx,
+        pj_context_is_network_enabled(ctx) ?
+            PROJ_GRID_AVAILABILITY_KNOWN_AVAILABLE:
+            PROJ_GRID_AVAILABILITY_DISCARD_OPERATION_IF_MISSING_GRID);
 
     auto op_list = proj_create_operations(ctx, source_crs, target_crs, operation_ctx);
 
diff --git a/src/apps/projinfo.cpp b/src/apps/projinfo.cpp
index fd9b2f46..f13e526b 100644
--- a/src/apps/projinfo.cpp
+++ b/src/apps/projinfo.cpp
@@ -87,7 +87,8 @@ static void usage() {
         << "                [--spatial-test contains|intersects]" << std::endl
         << "                [--crs-extent-use none|both|intersection|smallest]"
         << std::endl
-        << "                [--grid-check none|discard_missing|sort] "
+        << "                [--grid-check "
+           "none|discard_missing|sort|known_available] "
            "[--show-superseded]"
         << std::endl
         << "                [--pivot-crs always|if_no_direct_transformation|"
@@ -509,7 +510,7 @@ static void outputObject(
     auto op = dynamic_cast<CoordinateOperation *>(obj.get());
     if (op && dbContext && getenv("PROJINFO_NO_GRID_CHECK") == nullptr) {
         try {
-            auto setGrids = op->gridsNeeded(dbContext);
+            auto setGrids = op->gridsNeeded(dbContext, false);
             bool firstWarning = true;
             for (const auto &grid : setGrids) {
                 if (!grid.available) {
@@ -525,6 +526,7 @@ static void outputObject(
                         if (!grid.url.empty()) {
                             std::cout << " at " << grid.url;
                         }
+                        std::cout << ", or on CDN";
                     } else if (!grid.url.empty()) {
                         std::cout << " Can be obtained at " << grid.url;
                     }
@@ -539,8 +541,9 @@ static void outputObject(
 
 // ---------------------------------------------------------------------------
 
-static void outputOperationSummary(const CoordinateOperationNNPtr &op,
-                                   const DatabaseContextPtr &dbContext) {
+static void outputOperationSummary(
+    const CoordinateOperationNNPtr &op, const DatabaseContextPtr &dbContext,
+    CoordinateOperationContext::GridAvailabilityUse gridAvailabilityUse) {
     auto ids = op->identifiers();
     if (!ids.empty()) {
         std::cout << *(ids[0]->codeSpace()) << ":" << ids[0]->code();
@@ -586,10 +589,16 @@ static void outputOperationSummary(const CoordinateOperationNNPtr &op,
 
     if (dbContext && getenv("PROJINFO_NO_GRID_CHECK") == nullptr) {
         try {
-            auto setGrids = op->gridsNeeded(dbContext);
+            auto setGrids = op->gridsNeeded(dbContext, false);
             for (const auto &grid : setGrids) {
                 if (!grid.available) {
                     std::cout << ", at least one grid missing";
+                    if (gridAvailabilityUse ==
+                            CoordinateOperationContext::GridAvailabilityUse::
+                                KNOWN_AVAILABLE &&
+                        !grid.packageName.empty()) {
+                        std::cout << " on the system, but available on CDN";
+                    }
                     break;
                 }
             }
@@ -686,7 +695,7 @@ static void outputOperations(
     }
     if (summary) {
         for (const auto &op : list) {
-            outputOperationSummary(op, dbContext);
+            outputOperationSummary(op, dbContext, gridAvailabilityUse);
         }
     } else {
         bool first = true;
@@ -701,7 +710,7 @@ static void outputOperations(
                          "\xC2\xB0"
                       << (i + 1) << ":" << std::endl
                       << std::endl;
-            outputOperationSummary(op, dbContext);
+            outputOperationSummary(op, dbContext, gridAvailabilityUse);
             std::cout << std::endl;
             outputObject(dbContext, op, allowUseIntermediateCRS, outputOpt);
         }
@@ -734,7 +743,9 @@ int main(int argc, char **argv) {
         CoordinateOperationContext::SourceTargetCRSExtentUse::SMALLEST;
     bool buildBoundCRSToWGS84 = false;
     CoordinateOperationContext::GridAvailabilityUse gridAvailabilityUse =
-        CoordinateOperationContext::GridAvailabilityUse::USE_FOR_SORTING;
+        pj_context_is_network_enabled(nullptr)
+            ? CoordinateOperationContext::GridAvailabilityUse::KNOWN_AVAILABLE
+            : CoordinateOperationContext::GridAvailabilityUse::USE_FOR_SORTING;
     CoordinateOperationContext::IntermediateCRSUse allowUseIntermediateCRS =
         CoordinateOperationContext::IntermediateCRSUse::
             IF_NO_DIRECT_TRANSFORMATION;
@@ -947,6 +958,9 @@ int main(int argc, char **argv) {
             } else if (ci_equal(value, "sort")) {
                 gridAvailabilityUse = CoordinateOperationContext::
                     GridAvailabilityUse::USE_FOR_SORTING;
+            } else if (ci_equal(value, "known_available")) {
+                gridAvailabilityUse = CoordinateOperationContext::
+                    GridAvailabilityUse::KNOWN_AVAILABLE;
             } else {
                 std::cerr << "Unrecognized value for option --grid-check: "
                           << value << std::endl;
diff --git a/src/filemanager.cpp b/src/filemanager.cpp
index cd738d5e..d9a02632 100644
--- a/src/filemanager.cpp
+++ b/src/filemanager.cpp
@@ -69,7 +69,7 @@ NS_PROJ_START
 
 // ---------------------------------------------------------------------------
 
-File::File() = default;
+File::File(const std::string &name) : name_(name) {}
 
 // ---------------------------------------------------------------------------
 
@@ -85,7 +85,8 @@ class FileStdio : public File {
     FileStdio &operator=(const FileStdio &) = delete;
 
   protected:
-    FileStdio(PJ_CONTEXT *ctx, FILE *fp) : m_ctx(ctx), m_fp(fp) {}
+    FileStdio(const std::string &name, PJ_CONTEXT *ctx, FILE *fp)
+        : File(name), m_ctx(ctx), m_fp(fp) {}
 
   public:
     ~FileStdio() override;
@@ -130,7 +131,8 @@ unsigned long long FileStdio::tell() {
 
 std::unique_ptr<File> FileStdio::open(PJ_CONTEXT *ctx, const char *filename) {
     auto fp = fopen(filename, "rb");
-    return std::unique_ptr<File>(fp ? new FileStdio(ctx, fp) : nullptr);
+    return std::unique_ptr<File>(fp ? new FileStdio(filename, ctx, fp)
+                                    : nullptr);
 }
 
 // ---------------------------------------------------------------------------
@@ -145,7 +147,8 @@ class FileLegacyAdapter : public File {
     FileLegacyAdapter &operator=(const FileLegacyAdapter &) = delete;
 
   protected:
-    FileLegacyAdapter(PJ_CONTEXT *ctx, PAFile fp) : m_ctx(ctx), m_fp(fp) {}
+    FileLegacyAdapter(const std::string &name, PJ_CONTEXT *ctx, PAFile fp)
+        : File(name), m_ctx(ctx), m_fp(fp) {}
 
   public:
     ~FileLegacyAdapter() override;
@@ -189,7 +192,7 @@ unsigned long long FileLegacyAdapter::tell() {
 std::unique_ptr<File> FileLegacyAdapter::open(PJ_CONTEXT *ctx,
                                               const char *filename) {
     auto fid = pj_ctx_fopen(ctx, filename, "rb");
-    return std::unique_ptr<File>(fid ? new FileLegacyAdapter(ctx, fid)
+    return std::unique_ptr<File>(fid ? new FileLegacyAdapter(filename, ctx, fid)
                                      : nullptr);
 }
 
@@ -292,7 +295,7 @@ class NetworkFile : public File {
                 PROJ_NETWORK_HANDLE *handle,
                 unsigned long long lastDownloadOffset,
                 unsigned long long filesize)
-        : m_ctx(ctx), m_url(url), m_handle(handle),
+        : File(url), m_ctx(ctx), m_url(url), m_handle(handle),
           m_lastDownloadedOffset(lastDownloadOffset), m_filesize(filesize) {}
 
   public:
diff --git a/src/filemanager.hpp b/src/filemanager.hpp
index 949b223f..46391597 100644
--- a/src/filemanager.hpp
+++ b/src/filemanager.hpp
@@ -60,13 +60,16 @@ class FileManager {
 
 class File {
   protected:
-    File();
+    std::string name_;
+    explicit File(const std::string &name);
 
   public:
     virtual ~File();
     virtual size_t read(void *buffer, size_t sizeBytes) = 0;
     virtual bool seek(unsigned long long offset, int whence = SEEK_SET) = 0;
     virtual unsigned long long tell() = 0;
+
+    const std::string &name() const { return name_; }
 };
 
 //! @endcond Doxygen_Suppress
diff --git a/src/grids.cpp b/src/grids.cpp
index 91e51016..45e7e8b0 100644
--- a/src/grids.cpp
+++ b/src/grids.cpp
@@ -1299,14 +1299,15 @@ VerticalShiftGridSet::open(PJ_CONTEXT *ctx, const std::string &filename) {
         ctx->last_errno = 0; /* don't treat as a persistent error */
         return nullptr;
     }
-    if (ends_with(filename, "gtx") || ends_with(filename, "GTX")) {
-        auto grid = GTXVerticalShiftGrid::open(ctx, std::move(fp), filename);
+    const auto actualName(fp->name());
+    if (ends_with(actualName, "gtx") || ends_with(actualName, "GTX")) {
+        auto grid = GTXVerticalShiftGrid::open(ctx, std::move(fp), actualName);
         if (!grid) {
             return nullptr;
         }
         auto set =
             std::unique_ptr<VerticalShiftGridSet>(new VerticalShiftGridSet());
-        set->m_name = filename;
+        set->m_name = actualName;
         set->m_format = "gtx";
         set->m_grids.push_back(std::unique_ptr<VerticalShiftGrid>(grid));
         return set;
@@ -1324,7 +1325,7 @@ VerticalShiftGridSet::open(PJ_CONTEXT *ctx, const std::string &filename) {
 
     if (IsTIFF(header_size, header)) {
 #ifdef TIFF_ENABLED
-        auto set = GTiffVGridShiftSet::open(ctx, std::move(fp), filename);
+        auto set = GTiffVGridShiftSet::open(ctx, std::move(fp), actualName);
         if (!set)
             pj_ctx_set_errno(ctx, PJD_ERR_FAILED_TO_LOAD_GRID);
         return set;
@@ -2130,6 +2131,7 @@ HorizontalShiftGridSet::open(PJ_CONTEXT *ctx, const std::string &filename) {
         ctx->last_errno = 0; /* don't treat as a persistent error */
         return nullptr;
     }
+    const auto actualName(fp->name());
 
     char header[160];
     /* -------------------------------------------------------------------- */
@@ -2151,35 +2153,35 @@ HorizontalShiftGridSet::open(PJ_CONTEXT *ctx, const std::string &filename) {
     if (header_size >= 144 + 16 && strncmp(header + 0, "HEADER", 6) == 0 &&
         strncmp(header + 96, "W GRID", 6) == 0 &&
         strncmp(header + 144, "TO      NAD83   ", 16) == 0) {
-        auto grid = NTv1Grid::open(ctx, std::move(fp), filename);
+        auto grid = NTv1Grid::open(ctx, std::move(fp), actualName);
         if (!grid) {
             return nullptr;
         }
         auto set = std::unique_ptr<HorizontalShiftGridSet>(
             new HorizontalShiftGridSet());
-        set->m_name = filename;
+        set->m_name = actualName;
         set->m_format = "ntv1";
         set->m_grids.push_back(std::unique_ptr<HorizontalShiftGrid>(grid));
         return set;
     } else if (header_size >= 9 && strncmp(header + 0, "CTABLE V2", 9) == 0) {
-        auto grid = CTable2Grid::open(ctx, std::move(fp), filename);
+        auto grid = CTable2Grid::open(ctx, std::move(fp), actualName);
         if (!grid) {
             return nullptr;
         }
         auto set = std::unique_ptr<HorizontalShiftGridSet>(
             new HorizontalShiftGridSet());
-        set->m_name = filename;
+        set->m_name = actualName;
         set->m_format = "ctable2";
         set->m_grids.push_back(std::unique_ptr<HorizontalShiftGrid>(grid));
         return set;
     } else if (header_size >= 48 + 7 &&
                strncmp(header + 0, "NUM_OREC", 8) == 0 &&
                strncmp(header + 48, "GS_TYPE", 7) == 0) {
-        return NTv2GridSet::open(ctx, std::move(fp), filename);
+        return NTv2GridSet::open(ctx, std::move(fp), actualName);
     } else if (IsTIFF(header_size,
                       reinterpret_cast<const unsigned char *>(header))) {
 #ifdef TIFF_ENABLED
-        auto set = GTiffHGridShiftSet::open(ctx, std::move(fp), filename);
+        auto set = GTiffHGridShiftSet::open(ctx, std::move(fp), actualName);
         if (!set)
             pj_ctx_set_errno(ctx, PJD_ERR_FAILED_TO_LOAD_GRID);
         return set;
@@ -2450,6 +2452,7 @@ GenericShiftGridSet::open(PJ_CONTEXT *ctx, const std::string &filename) {
         ctx->last_errno = 0; /* don't treat as a persistent error */
         return nullptr;
     }
+    const auto actualName(fp->name());
 
     /* -------------------------------------------------------------------- */
     /*      Load a header, to determine the file type.                      */
@@ -2463,7 +2466,8 @@ GenericShiftGridSet::open(PJ_CONTEXT *ctx, const std::string &filename) {
 
     if (IsTIFF(header_size, header)) {
 #ifdef TIFF_ENABLED
-        auto set = GTiffGenericGridShiftSet::open(ctx, std::move(fp), filename);
+        auto set =
+            GTiffGenericGridShiftSet::open(ctx, std::move(fp), actualName);
         if (!set)
             pj_ctx_set_errno(ctx, PJD_ERR_FAILED_TO_LOAD_GRID);
         return set;
diff --git a/src/iso19111/c_api.cpp b/src/iso19111/c_api.cpp
index 9db9e5b3..5df4c513 100644
--- a/src/iso19111/c_api.cpp
+++ b/src/iso19111/c_api.cpp
@@ -177,7 +177,11 @@ static PJ *pj_obj_create(PJ_CONTEXT *ctx, const IdentifiedObjectNNPtr &objIn) {
             auto formatter = PROJStringFormatter::create(
                 PROJStringFormatter::Convention::PROJ_5, dbContext);
             auto projString = coordop->exportToPROJString(formatter.get());
+            if (pj_context_is_network_enabled(ctx)) {
+                ctx->defer_grid_opening = true;
+            }
             auto pj = pj_create_internal(ctx, projString.c_str());
+            ctx->defer_grid_opening = false;
             if (pj) {
                 pj->iso_obj = objIn;
                 if (ctx->cpp_context) {
@@ -766,7 +770,7 @@ int PROJ_DLL proj_grid_get_info_from_database(
         bool open_license;
         bool available;
         if (!db_context->lookForGridInfo(
-                grid_name, ctx->cpp_context->lastGridFullName_,
+                grid_name, false, ctx->cpp_context->lastGridFullName_,
                 ctx->cpp_context->lastGridPackageName_,
                 ctx->cpp_context->lastGridUrl_, direct_download, open_license,
                 available)) {
@@ -6571,7 +6575,10 @@ int proj_coordoperation_is_instantiable(PJ_CONTEXT *ctx,
     }
     auto dbContext = getDBcontextNoException(ctx, __FUNCTION__);
     try {
-        auto ret = op->isPROJInstantiable(dbContext) ? 1 : 0;
+        auto ret = op->isPROJInstantiable(dbContext,
+                                          pj_context_is_network_enabled(ctx))
+                       ? 1
+                       : 0;
         if (ctx->cpp_context) {
             ctx->cpp_context->autoCloseDbIfNeeded();
         }
@@ -6883,7 +6890,8 @@ int proj_coordoperation_get_grid_used_count(PJ_CONTEXT *ctx,
     try {
         if (!coordoperation->gridsNeededAsked) {
             coordoperation->gridsNeededAsked = true;
-            const auto gridsNeeded = co->gridsNeeded(dbContext);
+            const auto gridsNeeded =
+                co->gridsNeeded(dbContext, pj_context_is_network_enabled(ctx));
             for (const auto &gridDesc : gridsNeeded) {
                 coordoperation->gridsNeeded.emplace_back(gridDesc);
             }
@@ -7220,6 +7228,12 @@ void PROJ_DLL proj_operation_factory_context_set_grid_availability_use(
                 CoordinateOperationContext::GridAvailabilityUse::
                     IGNORE_GRID_AVAILABILITY);
             break;
+
+        case PROJ_GRID_AVAILABILITY_KNOWN_AVAILABLE:
+            factory_ctx->operationContext->setGridAvailabilityUse(
+                CoordinateOperationContext::GridAvailabilityUse::
+                    KNOWN_AVAILABLE);
+            break;
         }
     } catch (const std::exception &e) {
         proj_log_error(ctx, __FUNCTION__, e.what());
diff --git a/src/iso19111/coordinateoperation.cpp b/src/iso19111/coordinateoperation.cpp
index 6120c768..7c0515c7 100644
--- a/src/iso19111/coordinateoperation.cpp
+++ b/src/iso19111/coordinateoperation.cpp
@@ -788,13 +788,15 @@ void CoordinateOperation::setAccuracies(
  * available.
  */
 bool CoordinateOperation::isPROJInstantiable(
-    const io::DatabaseContextPtr &databaseContext) const {
+    const io::DatabaseContextPtr &databaseContext,
+    bool considerKnownGridsAsAvailable) const {
     try {
         exportToPROJString(io::PROJStringFormatter::create().get());
     } catch (const std::exception &) {
         return false;
     }
-    for (const auto &gridDesc : gridsNeeded(databaseContext)) {
+    for (const auto &gridDesc :
+         gridsNeeded(databaseContext, considerKnownGridsAsAvailable)) {
         if (!gridDesc.available) {
             return false;
         }
@@ -2013,8 +2015,9 @@ bool SingleOperation::_isEquivalentTo(const util::IComparable *other,
 
 // ---------------------------------------------------------------------------
 
-std::set<GridDescription> SingleOperation::gridsNeeded(
-    const io::DatabaseContextPtr &databaseContext) const {
+std::set<GridDescription>
+SingleOperation::gridsNeeded(const io::DatabaseContextPtr &databaseContext,
+                             bool considerKnownGridsAsAvailable) const {
     std::set<GridDescription> res;
     for (const auto &genOpParamvalue : parameterValues()) {
         auto opParamvalue = dynamic_cast<const OperationParameterValue *>(
@@ -2026,9 +2029,9 @@ std::set<GridDescription> SingleOperation::gridsNeeded(
                 desc.shortName = value->valueFile();
                 if (databaseContext) {
                     databaseContext->lookForGridInfo(
-                        desc.shortName, desc.fullName, desc.packageName,
-                        desc.url, desc.directDownload, desc.openLicense,
-                        desc.available);
+                        desc.shortName, considerKnownGridsAsAvailable,
+                        desc.fullName, desc.packageName, desc.url,
+                        desc.directDownload, desc.openLicense, desc.available);
                 }
                 res.insert(desc);
             }
@@ -10209,10 +10212,12 @@ bool ConcatenatedOperation::_isEquivalentTo(
 // ---------------------------------------------------------------------------
 
 std::set<GridDescription> ConcatenatedOperation::gridsNeeded(
-    const io::DatabaseContextPtr &databaseContext) const {
+    const io::DatabaseContextPtr &databaseContext,
+    bool considerKnownGridsAsAvailable) const {
     std::set<GridDescription> res;
     for (const auto &operation : operations()) {
-        const auto l_gridsNeeded = operation->gridsNeeded(databaseContext);
+        const auto l_gridsNeeded = operation->gridsNeeded(
+            databaseContext, considerKnownGridsAsAvailable);
         for (const auto &gridDesc : l_gridsNeeded) {
             res.insert(gridDesc);
         }
@@ -11132,7 +11137,10 @@ struct FilterResults {
             bool gridsKnown = true;
             if (context->getAuthorityFactory()) {
                 const auto gridsNeeded = op->gridsNeeded(
-                    context->getAuthorityFactory()->databaseContext());
+                    context->getAuthorityFactory()->databaseContext(),
+                    gridAvailabilityUse ==
+                        CoordinateOperationContext::GridAvailabilityUse::
+                            KNOWN_AVAILABLE);
                 for (const auto &gridDesc : gridsNeeded) {
                     hasGrids = true;
                     if (gridAvailabilityUse ==
@@ -11254,6 +11262,7 @@ struct FilterResults {
         CoordinateOperationPtr lastOp;
 
         bool first = true;
+        const auto gridAvailabilityUse = context->getGridAvailabilityUse();
         for (const auto &op : res) {
             const auto curAccuracy = getAccuracy(op);
             bool dummy = false;
@@ -11266,7 +11275,10 @@ struct FilterResults {
 
             if (context->getAuthorityFactory()) {
                 const auto gridsNeeded = op->gridsNeeded(
-                    context->getAuthorityFactory()->databaseContext());
+                    context->getAuthorityFactory()->databaseContext(),
+                    gridAvailabilityUse ==
+                        CoordinateOperationContext::GridAvailabilityUse::
+                            KNOWN_AVAILABLE);
                 for (const auto &gridDesc : gridsNeeded) {
                     curHasGrids = true;
                     curSetOfGrids.insert(gridDesc.shortName);
@@ -11571,6 +11583,7 @@ CoordinateOperationFactory::Private::findOpsInRegistryDirect(
     buildCRSIds(sourceCRS, context, sourceIds);
     buildCRSIds(targetCRS, context, targetIds);
 
+    const auto gridAvailabilityUse = context.context->getGridAvailabilityUse();
     for (const auto &idSrc : sourceIds) {
         const auto &srcAuthName = idSrc.first;
         const auto &srcCode = idSrc.second;
@@ -11590,9 +11603,16 @@ CoordinateOperationFactory::Private::findOpsInRegistryDirect(
                     tmpAuthFactory->createFromCoordinateReferenceSystemCodes(
                         srcAuthName, srcCode, targetAuthName, targetCode,
                         context.context->getUsePROJAlternativeGridNames(),
-                        context.context->getGridAvailabilityUse() ==
+                        gridAvailabilityUse ==
+                                CoordinateOperationContext::
+                                    GridAvailabilityUse::
+                                        DISCARD_OPERATION_IF_MISSING_GRID ||
+                            gridAvailabilityUse ==
+                                CoordinateOperationContext::
+                                    GridAvailabilityUse::KNOWN_AVAILABLE,
+                        gridAvailabilityUse ==
                             CoordinateOperationContext::GridAvailabilityUse::
-                                DISCARD_OPERATION_IF_MISSING_GRID,
+                                KNOWN_AVAILABLE,
                         context.context->getDiscardSuperseded(), true, false,
                         context.extent1, context.extent2);
                 res.insert(res.end(), resTmp.begin(), resTmp.end());
@@ -11635,6 +11655,7 @@ CoordinateOperationFactory::Private::findOpsInRegistryDirectTo(
     std::list<std::pair<std::string, std::string>> ids;
     buildCRSIds(targetCRS, context, ids);
 
+    const auto gridAvailabilityUse = context.context->getGridAvailabilityUse();
     for (const auto &id : ids) {
         const auto &targetAuthName = id.first;
         const auto &targetCode = id.second;
@@ -11648,9 +11669,15 @@ CoordinateOperationFactory::Private::findOpsInRegistryDirectTo(
             auto res = tmpAuthFactory->createFromCoordinateReferenceSystemCodes(
                 std::string(), std::string(), targetAuthName, targetCode,
                 context.context->getUsePROJAlternativeGridNames(),
-                context.context->getGridAvailabilityUse() ==
-                    CoordinateOperationContext::GridAvailabilityUse::
-                        DISCARD_OPERATION_IF_MISSING_GRID,
+
+                gridAvailabilityUse ==
+                        CoordinateOperationContext::GridAvailabilityUse::
+                            DISCARD_OPERATION_IF_MISSING_GRID ||
+                    gridAvailabilityUse ==
+                        CoordinateOperationContext::GridAvailabilityUse::
+                            KNOWN_AVAILABLE,
+                gridAvailabilityUse == CoordinateOperationContext::
+                                           GridAvailabilityUse::KNOWN_AVAILABLE,
                 context.context->getDiscardSuperseded(), true, true,
                 context.extent1, context.extent2);
             if (!res.empty()) {
@@ -11698,6 +11725,7 @@ CoordinateOperationFactory::Private::findsOpsInRegistryWithIntermediate(
     buildCRSIds(sourceCRS, context, sourceIds);
     buildCRSIds(targetCRS, context, targetIds);
 
+    const auto gridAvailabilityUse = context.context->getGridAvailabilityUse();
     for (const auto &idSrc : sourceIds) {
         const auto &srcAuthName = idSrc.first;
         const auto &srcCode = idSrc.second;
@@ -11717,21 +11745,28 @@ CoordinateOperationFactory::Private::findsOpsInRegistryWithIntermediate(
 
             std::vector<CoordinateOperationNNPtr> res;
             if (useCreateBetweenGeodeticCRSWithDatumBasedIntermediates) {
-                res = tmpAuthFactory
-                          ->createBetweenGeodeticCRSWithDatumBasedIntermediates(
-                              sourceCRS, srcAuthName, srcCode, targetCRS,
-                              targetAuthName, targetCode,
-                              context.context->getUsePROJAlternativeGridNames(),
-                              context.context->getGridAvailabilityUse() ==
-                                  CoordinateOperationContext::
-                                      GridAvailabilityUse::
-                                          DISCARD_OPERATION_IF_MISSING_GRID,
-                              context.context->getDiscardSuperseded(),
-                              authFactory->getAuthority() != "any" &&
-                                      authorities.size() > 1
-                                  ? authorities
-                                  : std::vector<std::string>(),
-                              context.extent1, context.extent2);
+                res =
+                    tmpAuthFactory
+                        ->createBetweenGeodeticCRSWithDatumBasedIntermediates(
+                            sourceCRS, srcAuthName, srcCode, targetCRS,
+                            targetAuthName, targetCode,
+                            context.context->getUsePROJAlternativeGridNames(),
+                            gridAvailabilityUse ==
+                                    CoordinateOperationContext::
+                                        GridAvailabilityUse::
+                                            DISCARD_OPERATION_IF_MISSING_GRID ||
+                                gridAvailabilityUse ==
+                                    CoordinateOperationContext::
+                                        GridAvailabilityUse::KNOWN_AVAILABLE,
+                            gridAvailabilityUse ==
+                                CoordinateOperationContext::
+                                    GridAvailabilityUse::KNOWN_AVAILABLE,
+                            context.context->getDiscardSuperseded(),
+                            authFactory->getAuthority() != "any" &&
+                                    authorities.size() > 1
+                                ? authorities
+                                : std::vector<std::string>(),
+                            context.extent1, context.extent2);
             } else {
                 io::AuthorityFactory::ObjectType intermediateObjectType =
                     io::AuthorityFactory::ObjectType::CRS;
@@ -11749,9 +11784,15 @@ CoordinateOperationFactory::Private::findsOpsInRegistryWithIntermediate(
                 res = tmpAuthFactory->createFromCRSCodesWithIntermediates(
                     srcAuthName, srcCode, targetAuthName, targetCode,
                     context.context->getUsePROJAlternativeGridNames(),
-                    context.context->getGridAvailabilityUse() ==
+                    gridAvailabilityUse ==
+                            CoordinateOperationContext::GridAvailabilityUse::
+                                DISCARD_OPERATION_IF_MISSING_GRID ||
+                        gridAvailabilityUse ==
+                            CoordinateOperationContext::GridAvailabilityUse::
+                                KNOWN_AVAILABLE,
+                    gridAvailabilityUse ==
                         CoordinateOperationContext::GridAvailabilityUse::
-                            DISCARD_OPERATION_IF_MISSING_GRID,
+                            KNOWN_AVAILABLE,
                     context.context->getDiscardSuperseded(),
                     context.context->getIntermediateCRS(),
                     intermediateObjectType,
@@ -14167,15 +14208,21 @@ void CoordinateOperationFactory::Private::createOperationsCompoundToGeog(
                 componentsSrc[1],
                 targetCRS->promoteTo3D(std::string(), dbContext), context);
             bool foundRegisteredTransformWithAllGridsAvailable = false;
+            const auto gridAvailabilityUse =
+                context.context->getGridAvailabilityUse();
             const bool ignoreMissingGrids =
-                context.context->getGridAvailabilityUse() ==
+                gridAvailabilityUse ==
                 CoordinateOperationContext::GridAvailabilityUse::
                     IGNORE_GRID_AVAILABILITY;
             for (const auto &op : verticalTransforms) {
                 if (hasIdentifiers(op) && dbContext) {
                     bool missingGrid = false;
                     if (!ignoreMissingGrids) {
-                        const auto gridsNeeded = op->gridsNeeded(dbContext);
+                        const auto gridsNeeded = op->gridsNeeded(
+                            dbContext,
+                            gridAvailabilityUse ==
+                                CoordinateOperationContext::
+                                    GridAvailabilityUse::KNOWN_AVAILABLE);
                         for (const auto &gridDesc : gridsNeeded) {
                             if (!gridDesc.available) {
                                 missingGrid = true;
@@ -14204,7 +14251,11 @@ void CoordinateOperationFactory::Private::createOperationsCompoundToGeog(
                     if (hasIdentifiers(op) && dbContext) {
                         bool missingGrid = false;
                         if (!ignoreMissingGrids) {
-                            const auto gridsNeeded = op->gridsNeeded(dbContext);
+                            const auto gridsNeeded = op->gridsNeeded(
+                                dbContext,
+                                gridAvailabilityUse ==
+                                    CoordinateOperationContext::
+                                        GridAvailabilityUse::KNOWN_AVAILABLE);
                             for (const auto &gridDesc : gridsNeeded) {
                                 if (!gridDesc.available) {
                                     missingGrid = true;
@@ -15034,8 +15085,9 @@ CoordinateOperationNNPtr PROJBasedOperation::_shallowClone() const {
 
 // ---------------------------------------------------------------------------
 
-std::set<GridDescription> PROJBasedOperation::gridsNeeded(
-    const io::DatabaseContextPtr &databaseContext) const {
+std::set<GridDescription>
+PROJBasedOperation::gridsNeeded(const io::DatabaseContextPtr &databaseContext,
+                                bool considerKnownGridsAsAvailable) const {
     std::set<GridDescription> res;
 
     try {
@@ -15048,7 +15100,8 @@ std::set<GridDescription> PROJBasedOperation::gridsNeeded(
             desc.shortName = shortName;
             if (databaseContext) {
                 databaseContext->lookForGridInfo(
-                    desc.shortName, desc.fullName, desc.packageName, desc.url,
+                    desc.shortName, considerKnownGridsAsAvailable,
+                    desc.fullName, desc.packageName, desc.url,
                     desc.directDownload, desc.openLicense, desc.available);
             }
             res.insert(desc);
diff --git a/src/iso19111/factory.cpp b/src/iso19111/factory.cpp
index 57850303..dae8680c 100644
--- a/src/iso19111/factory.cpp
+++ b/src/iso19111/factory.cpp
@@ -1018,14 +1018,14 @@ bool DatabaseContext::lookForGridAlternative(const std::string &officialName,
 
 // ---------------------------------------------------------------------------
 
-bool DatabaseContext::lookForGridInfo(const std::string &projFilename,
-                                      std::string &fullFilename,
-                                      std::string &packageName,
-                                      std::string &url, bool &directDownload,
-                                      bool &openLicense,
-                                      bool &gridAvailable) const {
+bool DatabaseContext::lookForGridInfo(
+    const std::string &projFilename, bool considerKnownGridsAsAvailable,
+    std::string &fullFilename, std::string &packageName, std::string &url,
+    bool &directDownload, bool &openLicense, bool &gridAvailable) const {
     Private::GridInfoCache info;
-    if (d->getGridInfoFromCache(projFilename, info)) {
+    const std::string key(projFilename +
+                          (considerKnownGridsAsAvailable ? "true" : "false"));
+    if (d->getGridInfoFromCache(key, info)) {
         fullFilename = info.fullFilename;
         packageName = info.packageName;
         url = info.url;
@@ -1041,16 +1041,20 @@ bool DatabaseContext::lookForGridInfo(const std::string &projFilename,
     openLicense = false;
     directDownload = false;
 
-    fullFilename.resize(2048);
-    if (d->pjCtxt() == nullptr) {
-        d->setPjCtxt(pj_get_default_ctx());
+    if (considerKnownGridsAsAvailable) {
+        fullFilename = projFilename;
+    } else {
+        fullFilename.resize(2048);
+        if (d->pjCtxt() == nullptr) {
+            d->setPjCtxt(pj_get_default_ctx());
+        }
+        int errno_before = proj_context_errno(d->pjCtxt());
+        gridAvailable =
+            pj_find_file(d->pjCtxt(), projFilename.c_str(), &fullFilename[0],
+                         fullFilename.size() - 1) != 0;
+        proj_context_errno_set(d->pjCtxt(), errno_before);
+        fullFilename.resize(strlen(fullFilename.c_str()));
     }
-    int errno_before = proj_context_errno(d->pjCtxt());
-    gridAvailable =
-        pj_find_file(d->pjCtxt(), projFilename.c_str(), &fullFilename[0],
-                     fullFilename.size() - 1) != 0;
-    proj_context_errno_set(d->pjCtxt(), errno_before);
-    fullFilename.resize(strlen(fullFilename.c_str()));
 
     auto res =
         d->run("SELECT "
@@ -1074,6 +1078,10 @@ bool DatabaseContext::lookForGridInfo(const std::string &projFilename,
         openLicense = (row[3].empty() ? row[4] : row[3]) == "1";
         directDownload = (row[5].empty() ? row[6] : row[5]) == "1";
 
+        if (considerKnownGridsAsAvailable && !packageName.empty()) {
+            gridAvailable = true;
+        }
+
         info.fullFilename = fullFilename;
         info.packageName = packageName;
         info.url = url;
@@ -1082,7 +1090,7 @@ bool DatabaseContext::lookForGridInfo(const std::string &projFilename,
     }
     info.gridAvailable = gridAvailable;
     info.found = ret;
-    d->cache(projFilename, info);
+    d->cache(key, info);
     return ret;
 }
 
@@ -1264,8 +1272,8 @@ struct AuthorityFactory::Private {
         return AuthorityFactory::create(context_, auth_name);
     }
 
-    bool
-    rejectOpDueToMissingGrid(const operation::CoordinateOperationNNPtr &op);
+    bool rejectOpDueToMissingGrid(const operation::CoordinateOperationNNPtr &op,
+                                  bool considerKnownGridsAsAvailable);
 
     UnitOfMeasure createUnitOfMeasure(const std::string &auth_name,
                                       const std::string &code);
@@ -1392,8 +1400,10 @@ util::PropertyMap AuthorityFactory::Private::createProperties(
 // ---------------------------------------------------------------------------
 
 bool AuthorityFactory::Private::rejectOpDueToMissingGrid(
-    const operation::CoordinateOperationNNPtr &op) {
-    for (const auto &gridDesc : op->gridsNeeded(context())) {
+    const operation::CoordinateOperationNNPtr &op,
+    bool considerKnownGridsAsAvailable) {
+    for (const auto &gridDesc :
+         op->gridsNeeded(context(), considerKnownGridsAsAvailable)) {
         if (!gridDesc.available) {
             return true;
         }
@@ -3381,7 +3391,7 @@ AuthorityFactory::createFromCoordinateReferenceSystemCodes(
     const std::string &sourceCRSCode, const std::string &targetCRSCode) const {
     return createFromCoordinateReferenceSystemCodes(
         d->authority(), sourceCRSCode, d->authority(), targetCRSCode, false,
-        false, false);
+        false, false, false);
 }
 
 // ---------------------------------------------------------------------------
@@ -3410,6 +3420,8 @@ AuthorityFactory::createFromCoordinateReferenceSystemCodes(
  * should be substituted to the official grid names.
  * @param discardIfMissingGrid Whether coordinate operations that reference
  * missing grids should be removed from the result set.
+ * @param considerKnownGridsAsAvailable Whether known grids should be considered
+ * as available (typically when network is enabled).
  * @param discardSuperseded Whether cordinate operations that are superseded
  * (but not deprecated) should be removed from the result set.
  * @param tryReverseOrder whether to search in the reverse order too (and thus
@@ -3430,8 +3442,8 @@ AuthorityFactory::createFromCoordinateReferenceSystemCodes(
     const std::string &sourceCRSAuthName, const std::string &sourceCRSCode,
     const std::string &targetCRSAuthName, const std::string &targetCRSCode,
     bool usePROJAlternativeGridNames, bool discardIfMissingGrid,
-    bool discardSuperseded, bool tryReverseOrder,
-    bool reportOnlyIntersectingTransformations,
+    bool considerKnownGridsAsAvailable, bool discardSuperseded,
+    bool tryReverseOrder, bool reportOnlyIntersectingTransformations,
     const metadata::ExtentPtr &intersectingExtent1,
     const metadata::ExtentPtr &intersectingExtent2) const {
 
@@ -3442,6 +3454,7 @@ AuthorityFactory::createFromCoordinateReferenceSystemCodes(
     cacheKey += targetCRSCode;
     cacheKey += (usePROJAlternativeGridNames ? '1' : '0');
     cacheKey += (discardIfMissingGrid ? '1' : '0');
+    cacheKey += (considerKnownGridsAsAvailable ? '1' : '0');
     cacheKey += (discardSuperseded ? '1' : '0');
     cacheKey += (tryReverseOrder ? '1' : '0');
     cacheKey += (reportOnlyIntersectingTransformations ? '1' : '0');
@@ -3680,7 +3693,8 @@ AuthorityFactory::createFromCoordinateReferenceSystemCodes(
                     target_crs_code != targetCRSCode))) {
             op = op->inverse();
         }
-        if (!discardIfMissingGrid || !d->rejectOpDueToMissingGrid(op)) {
+        if (!discardIfMissingGrid ||
+            !d->rejectOpDueToMissingGrid(op, considerKnownGridsAsAvailable)) {
             list.emplace_back(op);
         }
     }
@@ -3745,6 +3759,8 @@ static bool useIrrelevantPivot(const operation::CoordinateOperationNNPtr &op,
  * should be substituted to the official grid names.
  * @param discardIfMissingGrid Whether coordinate operations that reference
  * missing grids should be removed from the result set.
+ * @param considerKnownGridsAsAvailable Whether known grids should be considered
+ * as available (typically when network is enabled).
  * @param discardSuperseded Whether cordinate operations that are superseded
  * (but not deprecated) should be removed from the result set.
  * @param intermediateCRSAuthCodes List of (auth_name, code) of CRS that can be
@@ -3773,7 +3789,7 @@ AuthorityFactory::createFromCRSCodesWithIntermediates(
     const std::string &sourceCRSAuthName, const std::string &sourceCRSCode,
     const std::string &targetCRSAuthName, const std::string &targetCRSCode,
     bool usePROJAlternativeGridNames, bool discardIfMissingGrid,
-    bool discardSuperseded,
+    bool considerKnownGridsAsAvailable, bool discardSuperseded,
     const std::vector<std::pair<std::string, std::string>>
         &intermediateCRSAuthCodes,
     ObjectType allowedIntermediateObjectType,
@@ -4221,7 +4237,8 @@ AuthorityFactory::createFromCRSCodesWithIntermediates(
 
     std::vector<operation::CoordinateOperationNNPtr> list;
     for (const auto &op : listTmp) {
-        if (!discardIfMissingGrid || !d->rejectOpDueToMissingGrid(op)) {
+        if (!discardIfMissingGrid ||
+            !d->rejectOpDueToMissingGrid(op, considerKnownGridsAsAvailable)) {
             list.emplace_back(op);
         }
     }
@@ -4239,7 +4256,8 @@ AuthorityFactory::createBetweenGeodeticCRSWithDatumBasedIntermediates(
     const std::string &sourceCRSCode, const crs::CRSNNPtr &targetCRS,
     const std::string &targetCRSAuthName, const std::string &targetCRSCode,
     bool usePROJAlternativeGridNames, bool discardIfMissingGrid,
-    bool discardSuperseded, const std::vector<std::string> &allowedAuthorities,
+    bool considerKnownGridsAsAvailable, bool discardSuperseded,
+    const std::vector<std::string> &allowedAuthorities,
     const metadata::ExtentPtr &intersectingExtent1,
     const metadata::ExtentPtr &intersectingExtent2) const {
 
@@ -4822,7 +4840,8 @@ AuthorityFactory::createBetweenGeodeticCRSWithDatumBasedIntermediates(
 
     std::vector<operation::CoordinateOperationNNPtr> list;
     for (const auto &op : listTmp) {
-        if (!discardIfMissingGrid || !d->rejectOpDueToMissingGrid(op)) {
+        if (!discardIfMissingGrid ||
+            !d->rejectOpDueToMissingGrid(op, considerKnownGridsAsAvailable)) {
             list.emplace_back(op);
         }
     }
diff --git a/src/open_lib.cpp b/src/open_lib.cpp
index 6bdd5510..926505ba 100644
--- a/src/open_lib.cpp
+++ b/src/open_lib.cpp
@@ -203,6 +203,27 @@ static const char *get_path_from_win32_projlib(const char *name, std::string& ou
 /*                      pj_open_lib_internal()                          */
 /************************************************************************/
 
+#ifdef WIN32
+static const char dir_chars[] = "/\\";
+static const char dirSeparator = ';';
+#else
+static const char dir_chars[] = "/";
+static const char dirSeparator = ':';
+#endif
+
+static bool is_tilde_slash(const char* name)
+{
+    return *name == '~' && strchr(dir_chars,name[1]);
+}
+
+static bool is_rel_or_absolute_filename(const char *name)
+{
+    return strchr(dir_chars,*name)
+            || (*name == '.' && strchr(dir_chars,name[1]))
+            || (!strncmp(name, "..", 2) && strchr(dir_chars,name[2]))
+            || (name[0] != '\0' && name[1] == ':' && strchr(dir_chars,name[2]));
+}
+
 static void*
 pj_open_lib_internal(projCtx ctx, const char *name, const char *mode,
                      void* (*open_file)(projCtx, const char*, const char*),
@@ -211,13 +232,6 @@ pj_open_lib_internal(projCtx ctx, const char *name, const char *mode,
         std::string fname;
         const char *sysname = nullptr;
         void* fid = nullptr;
-#ifdef WIN32
-        static const char dir_chars[] = "/\\";
-        const char dirSeparator = ';';
-#else
-        static const char dir_chars[] = "/";
-        const char dirSeparator = ':';
-#endif
 
         if( ctx == nullptr ) {
             ctx = pj_get_default_ctx();
@@ -227,7 +241,7 @@ pj_open_lib_internal(projCtx ctx, const char *name, const char *mode,
             out_full_filename[0] = '\0';
 
         /* check if ~/name */
-        if (*name == '~' && strchr(dir_chars,name[1]) )
+        if (is_tilde_slash(name))
             if ((sysname = getenv("HOME")) != nullptr) {
                 fname = sysname;
                 fname += DIR_CHAR;
@@ -236,11 +250,8 @@ pj_open_lib_internal(projCtx ctx, const char *name, const char *mode,
             } else
                 return nullptr;
 
-        /* or fixed path: /name, ./name or ../name */
-        else if (strchr(dir_chars,*name)
-                || (*name == '.' && strchr(dir_chars,name[1])) 
-                || (!strncmp(name, "..", 2) && strchr(dir_chars,name[2]))
-                || (name[0] != '\0' && name[1] == ':' && strchr(dir_chars,name[2]))
+        /* or fixed path: /name, ./name or ../name or http[s]:// */
+        else if (is_rel_or_absolute_filename(name)
                 || starts_with(name, "http://")
                 || starts_with(name, "https://"))
             sysname = name;
@@ -344,11 +355,31 @@ static void* pj_open_file_with_manager(projCtx ctx, const char *name,
 std::unique_ptr<NS_PROJ::File> NS_PROJ::FileManager::open_resource_file(
                                                 projCtx ctx, const char *name)
 {
-    return std::unique_ptr<NS_PROJ::File>(
+    auto file = std::unique_ptr<NS_PROJ::File>(
         reinterpret_cast<NS_PROJ::File*>(
                pj_open_lib_internal(ctx, name, "rb",
                                     pj_open_file_with_manager,
                                     nullptr, 0)));
+    if( file == nullptr &&
+        !is_tilde_slash(name) &&
+        !is_rel_or_absolute_filename(name) &&
+        !starts_with(name, "http://") &&
+        !starts_with(name, "https://") &&
+        pj_context_is_network_enabled(ctx) ) {
+        std::string remote_file("https://cdn.proj.org/");
+        remote_file += name;
+        auto pos = remote_file.rfind('.');
+        if( pos + 4 == remote_file.size() ) {
+            remote_file = remote_file.substr(0, pos) + ".tif";
+            file = open(ctx, remote_file.c_str());
+            if( file ) {
+                pj_log( ctx, PJ_LOG_DEBUG_MAJOR,
+                        "Using %s", remote_file.c_str() );
+                pj_ctx_set_errno( ctx, 0 );
+            }
+        }
+    }
+    return file;
 }
 
 /************************************************************************/
diff --git a/src/proj.h b/src/proj.h
index 4f7878a4..7210318c 100644
--- a/src/proj.h
+++ b/src/proj.h
@@ -697,6 +697,12 @@ typedef enum {
     /** Ignore grid availability at all. Results will be presented as if
         * all grids were available. */
     PROJ_GRID_AVAILABILITY_IGNORED,
+
+    /** Results will be presented as if grids known to PROJ (that is
+    * registered in the grid_alternatives table of its database) were
+    * available. Used typically when networking is enabled.
+    */
+    PROJ_GRID_AVAILABILITY_KNOWN_AVAILABLE,
 } PROJ_GRID_AVAILABILITY_USE;
 
 /** \brief PROJ string version. */
diff --git a/src/proj_internal.h b/src/proj_internal.h
index d54d8fb9..983f1c07 100644
--- a/src/proj_internal.h
+++ b/src/proj_internal.h
@@ -697,6 +697,7 @@ struct projCtx_t {
     void* file_finder_user_data = nullptr;
 
     projNetworkCallbacksAndData networking{};
+    bool defer_grid_opening = false; // set by pj_obj_create()
 
     int projStringParserCreateFromPROJStringRecursionCounter = 0; // to avoid potential infinite recursion in PROJStringParser::createFromPROJString()
 
@@ -826,7 +827,8 @@ PJ *pj_create_argv_internal (PJ_CONTEXT *ctx, int argc, char **argv);
 
 void pj_pipeline_assign_context_to_steps( PJ* P, PJ_CONTEXT* ctx );
 
-bool pj_context_is_network_enabled(PJ_CONTEXT* ctx);
+// For use by projinfo
+bool PROJ_DLL pj_context_is_network_enabled(PJ_CONTEXT* ctx);
 
 /* classic public API */
 #include "proj_api.h"
diff --git a/src/transformations/hgridshift.cpp b/src/transformations/hgridshift.cpp
index e9983df6..3b6e366f 100644
--- a/src/transformations/hgridshift.cpp
+++ b/src/transformations/hgridshift.cpp
@@ -17,6 +17,7 @@ struct hgridshiftData {
     double t_final = 0;
     double t_epoch = 0;
     ListOfHGrids grids{};
+    bool defer_grid_opening = false;
 };
 } // anonymous namespace
 
@@ -25,6 +26,14 @@ static PJ_XYZ forward_3d(PJ_LPZ lpz, PJ *P) {
     PJ_COORD point = {{0,0,0,0}};
     point.lpz = lpz;
 
+    if ( Q->defer_grid_opening ) {
+        Q->defer_grid_opening = false;
+        Q->grids = proj_hgrid_init(P, "grids");
+        if ( proj_errno(P) ) {
+            return proj_coord_error().xyz;
+        }
+    }
+
     if (!Q->grids.empty()) {
         /* Only try the gridshift if at least one grid is loaded,
          * otherwise just pass the coordinate through unchanged. */
@@ -40,6 +49,14 @@ static PJ_LPZ reverse_3d(PJ_XYZ xyz, PJ *P) {
     PJ_COORD point = {{0,0,0,0}};
     point.xyz = xyz;
 
+    if ( Q->defer_grid_opening ) {
+        Q->defer_grid_opening = false;
+        Q->grids = proj_hgrid_init(P, "grids");
+        if ( proj_errno(P) ) {
+            return proj_coord_error().lpz;
+        }
+    }
+
     if (!Q->grids.empty()) {
         /* Only try the gridshift if at least one grid is loaded,
          * otherwise just pass the coordinate through unchanged. */
@@ -114,9 +131,9 @@ PJ *TRANSFORMATION(hgridshift,0) {
         return destructor (P, PJD_ERR_NO_ARGS);
     }
 
-   /* TODO: Refactor into shared function that can be used  */
-   /*       by both vgridshift and hgridshift               */
-   if (pj_param(P->ctx, P->params, "tt_final").i) {
+    /* TODO: Refactor into shared function that can be used  */
+    /*       by both vgridshift and hgridshift               */
+    if (pj_param(P->ctx, P->params, "tt_final").i) {
         Q->t_final = pj_param (P->ctx, P->params, "dt_final").f;
         if (Q->t_final == 0) {
             /* a number wasn't passed to +t_final, let's see if it was "now" */
@@ -131,16 +148,21 @@ PJ *TRANSFORMATION(hgridshift,0) {
         }
     }
 
-   if (pj_param(P->ctx, P->params, "tt_epoch").i)
+    if (pj_param(P->ctx, P->params, "tt_epoch").i)
         Q->t_epoch = pj_param (P->ctx, P->params, "dt_epoch").f;
 
 
-    Q->grids = proj_hgrid_init(P, "grids");
-    /* Was gridlist compiled properly? */
-    if ( proj_errno(P) ) {
-        proj_log_error(P, "hgridshift: could not find required grid(s).");
-        return destructor(P, PJD_ERR_FAILED_TO_LOAD_GRID);
+    if( P->ctx->defer_grid_opening ) {
+        Q->defer_grid_opening = true;
     }
+    else {
+        Q->grids = proj_hgrid_init(P, "grids");
+        /* Was gridlist compiled properly? */
+        if ( proj_errno(P) ) {
+            proj_log_error(P, "hgridshift: could not find required grid(s).");
+            return destructor(P, PJD_ERR_FAILED_TO_LOAD_GRID);
+        }
+     }
 
     return P;
 }
diff --git a/src/transformations/vgridshift.cpp b/src/transformations/vgridshift.cpp
index b964f45b..f35832e1 100644
--- a/src/transformations/vgridshift.cpp
+++ b/src/transformations/vgridshift.cpp
@@ -18,14 +18,51 @@ struct vgridshiftData {
     double t_epoch = 0;
     double forward_multiplier = 0;
     ListOfVGrids grids{};
+    bool defer_grid_opening = false;
 };
 } // anonymous namespace
 
+static void deal_with_vertcon_gtx_hack(PJ *P)
+{
+    struct vgridshiftData *Q = (struct vgridshiftData *) P->opaque;
+    // The .gtx VERTCON files stored millimetres, but the .tif files
+    // are in metres.
+    if( Q->forward_multiplier != 0.001 ) {
+        return;
+    }
+    const char* gridname = pj_param(P->ctx, P->params, "sgrids").s;
+    if( !gridname ) {
+        return;
+    }
+    if( strcmp(gridname, "vertconw.gtx") != 0 &&
+        strcmp(gridname, "vertconc.gtx") != 0 &&
+        strcmp(gridname, "vertcone.gtx") != 0 ) {
+        return;
+    }
+    if( Q->grids.empty() ) {
+        return;
+    }
+    const auto& grids = Q->grids[0]->grids();
+    if( !grids.empty() &&
+        grids[0]->name().find(".tif") != std::string::npos ) {
+        Q->forward_multiplier = 1.0;
+    }
+}
+
 static PJ_XYZ forward_3d(PJ_LPZ lpz, PJ *P) {
     struct vgridshiftData *Q = (struct vgridshiftData *) P->opaque;
     PJ_COORD point = {{0,0,0,0}};
     point.lpz = lpz;
 
+    if ( Q->defer_grid_opening ) {
+        Q->defer_grid_opening = false;
+        Q->grids = proj_vgrid_init(P, "grids");
+        deal_with_vertcon_gtx_hack(P);
+        if ( proj_errno(P) ) {
+            return proj_coord_error().xyz;
+        }
+    }
+
     if (!Q->grids.empty()) {
         /* Only try the gridshift if at least one grid is loaded,
          * otherwise just pass the coordinate through unchanged. */
@@ -41,6 +78,15 @@ static PJ_LPZ reverse_3d(PJ_XYZ xyz, PJ *P) {
     PJ_COORD point = {{0,0,0,0}};
     point.xyz = xyz;
 
+    if ( Q->defer_grid_opening ) {
+        Q->defer_grid_opening = false;
+        Q->grids = proj_vgrid_init(P, "grids");
+        deal_with_vertcon_gtx_hack(P);
+        if ( proj_errno(P) ) {
+            return proj_coord_error().lpz;
+        }
+    }
+
     if (!Q->grids.empty()) {
         /* Only try the gridshift if at least one grid is loaded,
          * otherwise just pass the coordinate through unchanged. */
@@ -132,13 +178,18 @@ PJ *TRANSFORMATION(vgridshift,0) {
         Q->forward_multiplier = pj_param(P->ctx, P->params, "dmultiplier").f;
     }
 
-    /* Build gridlist. P->vgridlist_geoid can be empty if +grids only ask for optional grids. */
-    Q->grids = proj_vgrid_init(P, "grids");
-
-    /* Was gridlist compiled properly? */
-    if ( proj_errno(P) ) {
-        proj_log_error(P, "vgridshift: could not find required grid(s).");
-        return destructor(P, PJD_ERR_FAILED_TO_LOAD_GRID);
+    if( P->ctx->defer_grid_opening ) {
+        Q->defer_grid_opening = true;
+    }
+    else {
+        /* Build gridlist. P->vgridlist_geoid can be empty if +grids only ask for optional grids. */
+        Q->grids = proj_vgrid_init(P, "grids");
+
+        /* Was gridlist compiled properly? */
+        if ( proj_errno(P) ) {
+            proj_log_error(P, "vgridshift: could not find required grid(s).");
+            return destructor(P, PJD_ERR_FAILED_TO_LOAD_GRID);
+        }
     }
 
     P->fwd4d = forward_4d;