nz_linz: add geoid models and vertical offset grids

16 files changed, 136 insertions, 8 deletions

diff --git a/nz_linz/README.nz_linz b/nz_linz/README.nz_linz
index f35df84..b3d7c4e 100644
--- a/nz_linz/README.nz_linz
+++ b/nz_linz/README.nz_linz
@@ -1,17 +1,145 @@
 # README.nz_linz
 
 The files in this section result from the conversion of datasets originating
-from [LINZ](https://www.linz.govt.nz)
+from [Land Information New Zealand]
+
+The New Zealand grids are
+required for coordinate transformations between New Zealand geographic  
+and vertical coordinate systems.
+
+The New Zealand grids are sourced from [Land Information New Zealand]
+and are also available from LINZ as a separate resource [proj-datumgrid-nz].
+
+The New Zealand grids are published under the  
+[Creative Commons Attribution 4.0 International licence].
 
 ## Included grids
 
-### New Zealand: NZGD49 -> NZGD2000
+### NZGD1949 to NZGD2000 transformation  
+
+*Filename*: nzgd2kgrid0005.tif  
+*Reference*: https://www.linz.govt.nz/data/geodetic-system/coordinate-conversion/geodetic-datum-conversions/nzgd1949-nzgd2000
+
+Distortion grid to convert New Zealand Geodetic Datum 1949 longitude and latitude  
+to New Zealand Geodetic Datum 2000 longitude and latitude.
+
+#### Usage
+
+Conversion from NZGD1949 to NZGD2000 using proj strings
+  
+  echo '173 -41 0' | cs2cs -v -f %.8f +proj=longlat +ellps=intl +datum=nzgd49 +nadgrids=nzgd2kgrid0005.tif +to +proj=longlat +ellps=GRS80 +towgs84=0,0,0  
+
+Conversion from NZGD1949 to NZGD2000 using EPSG codes.  Note that this uses  
+EPSG specified coordinate order for coordinate systems (latitude/longitude in this case).
+
+  echo '-41 173 0' | cs2cs -f %.8f EPSG:4272 EPSG:4167
+
+### Quasigeoid grids
+
+Two geoid grid files are included:
+
+*Geoid*: New Zealand Quasigeoid 2009  
+*Filename*: nzgeoid2009.tif  
+*Reference*: https://www.linz.govt.nz/data/geodetic-system/datums-projections-and-heights/vertical-datums/new-zealand-quasigeoid-2009-nzgeoid2009
+
+*Geoid*: New Zealand Quasigeoid 2016  
+*Filename*: nzgeoid2016.tif  
+*Reference*: https://www.linz.govt.nz/data/geodetic-system/datums-projections-and-heights/vertical-datums/new-zealand-quasigeoid-2016-nzgeoid2016
+
+These grids define the height of the New Zealand Quasigeoid relative to the ellipsoidal  
+height surface on which NZGD2000 is based (nominally a GRS80 ellipsoid aligned ITRF96  
+at epoch 2000.0). See https://www.linz.govt.nz/nzgd2000 for more information on the  
+NZGD2000 datum.  
+The current NZGD2000 deformation model has this as the zero elevation of NZGD2000  
+coordinates at the current epoch.  
+
+These quasigeoids are the reference surfaces the New Zealand Vertical Datum 2009  
+(NZVD2009) and New Zealand Vertical Datum 2016 (NZVD2016).  
+
+#### Usage
+
+To convert a NZGD2000 ellipsoidal height 100.0 to a NZVD2016 orthometric height at 173W 41S
+
+   echo '173 -41 100.0' | cs2cs -v -f %.8f +proj=longlat +ellps=GRS80 +to +proj=longlat +ellps=GRS80 +geoidgrids=nzgeoid2016.tif
+
+To convert a NZVD2016 orthometric height to a NZGD2000 ellipsoidal height
+
+   echo '173 -41 100.0' | cs2cs -v -f %.8f +proj=longlat +ellps=GRS80 +geoidgrids=nzgeoid2016.tif +to +proj=longlat +ellps=GRS80
+
+To convert a NZGD2000 ellipsoidal height 100.0 to a NZVD2016 height at 173W 41S using EPSG codes (requires proj > 6.2)
+
+   echo '-41 173 100.0' | cs2cs -f %.8f EPSG:4167 EPSG:4167+7839
+
+
+### Local vertical datum transformation grids
+
+Thirteen grids are provided to transform heights between NZVD2016 and  
+the [New Zealand local vertical datums].  
+
+These grids represent the systematic errors in the local vertical datums.  
+The local vertical datums are each based on levelling from a tide gauge.  
+They include systematic errors from the levelling as well as a potential of the local  
+tidal signal at the port from the global mean gravitational equipotential surface.  The  
+NZVD2016 datum is based on gravity measurements from a national airborne gravity  
+program and is much less prone to regional systematic errors.  The transformation grids  
+are derived from comparing the benchmark heights at which both levelling data and GNSS  
+(Global Navigation Satellite system) data are available.  
+
+The height determined from the grid model is added to an NZVD2016 height to obtain
+the corresponding local vertical datum shift.
+
+The following grids are available:
+
+*Local vertical datum*: Auckland 1946  
+*Filename*: auckht1946-nzvd2016.tif
+
+*Local vertical datum*: Bluff 1955  
+*Filename*: blufht1955-nzvd2016.tif
+
+*Local vertical datum*: Dunedin 1958  
+*Filename*: duneht1958-nzvd2016.tif
+
+*Local vertical datum*: Dunedin-Bluff 1960  
+*Filename*: dublht1960-nzvd2016.tif
+
+*Local vertical datum*: Gisborne 1926  
+*Filename*: gisbht1926-nzvd2016.tif
+
+*Local vertical datum*: Lyttelton 1937  
+*Filename*: lyttht1937-nzvd2016.tif
+
+*Local vertical datum*: Moturiki 1953  
+*Filename*: motuht1953-nzvd2016.tif
+
+*Local vertical datum*: Napier 1962  
+*Filename*: napiht1962-nzvd2016.tif
+
+*Local vertical datum*: Nelson 1955  
+*Filename*: nelsht1955-nzvd2016.tif
+
+*Local vertical datum*: One Tree Point 1964  
+*Filename*: ontpht1964-nzvd2016.tif
+
+*Local vertical datum*: Stewart Island 1977  
+*Filename*: stisht1977-nzvd2016.tif
+
+*Local vertical datum*: Taranaki 1970  
+*Filename*: taraht1970-nzvd2016.tif
+
+*Local vertical datum*: Wellington 1953  
+*Filename*: wellht1953-nzvd2016.tif
+
+#### Usage
+
+To convert from a local vertical datum height to a NZVD2016 height.
+
+   echo '175 -37 0' | cct +proj=vgridshift +inv +grids=auckht1946-nzvd2016.tif +multiplier=1
 
-*Source*: [LINZ](https://www.linz.govt.nz/data/geodetic-system/download-geodetic-software/gd2000it-download)  
-*Format*: GeoTIFF converted from NTv2  
-*License*: [Creative Commons Attribution 3.0 New Zealand](https://creativecommons.org/licenses/by/3.0/nz/)
+To convert from a NZVD2016 height to a local vertical datum height.
 
-This grid file was computed by Land Information New Zealand at approximately
-a 20 km interval for the conversion between NZGD49 and NZGD2000.
+   echo '175 -37 0' | cct +proj=vgridshift +grids=auckht1946-nzvd2016.tif +multiplier=1
 
-* nzgd2kgrid0005.tif
+[Land Information New Zealand]: https://www.linz.govt.nz
+[proj-datumgrid-nz]: https://www.geodesy.linz.govt.nz/download/proj-datumgrid-nz
+[New Zealand local vertical datums]: https://www.linz.govt.nz/data/geodetic-system/datums-projections-and-heights/vertical-datums/vertical-datum-relationship-grids
+[Creative Commons Attribution 4.0 International licence]: https://data.linz.govt.nz/license/attribution-4-0-international/
diff --git a/nz_linz/auckht1946-nzvd2016.tif b/nz_linz/auckht1946-nzvd2016.tif
new file mode 100644
index 0000000..e56c928
--- /dev/null
+++ b/nz_linz/auckht1946-nzvd2016.tif
diff --git a/nz_linz/blufht1955-nzvd2016.tif b/nz_linz/blufht1955-nzvd2016.tif
new file mode 100644
index 0000000..bdaa013
--- /dev/null
+++ b/nz_linz/blufht1955-nzvd2016.tif
diff --git a/nz_linz/dublht1960-nzvd2016.tif b/nz_linz/dublht1960-nzvd2016.tif
new file mode 100644
index 0000000..35f89e2
--- /dev/null
+++ b/nz_linz/dublht1960-nzvd2016.tif
diff --git a/nz_linz/duneht1958-nzvd2016.tif b/nz_linz/duneht1958-nzvd2016.tif
new file mode 100644
index 0000000..b7a1b65
--- /dev/null
+++ b/nz_linz/duneht1958-nzvd2016.tif
diff --git a/nz_linz/gisbht1926-nzvd2016.tif b/nz_linz/gisbht1926-nzvd2016.tif
new file mode 100644
index 0000000..5b9fd9d
--- /dev/null
+++ b/nz_linz/gisbht1926-nzvd2016.tif
diff --git a/nz_linz/lyttht1937-nzvd2016.tif b/nz_linz/lyttht1937-nzvd2016.tif
new file mode 100644
index 0000000..20ad294
--- /dev/null
+++ b/nz_linz/lyttht1937-nzvd2016.tif
diff --git a/nz_linz/motuht1953-nzvd2016.tif b/nz_linz/motuht1953-nzvd2016.tif
new file mode 100644
index 0000000..f90ec77
--- /dev/null
+++ b/nz_linz/motuht1953-nzvd2016.tif
diff --git a/nz_linz/napiht1962-nzvd2016.tif b/nz_linz/napiht1962-nzvd2016.tif
new file mode 100644
index 0000000..a6f898c
--- /dev/null
+++ b/nz_linz/napiht1962-nzvd2016.tif
diff --git a/nz_linz/nelsht1955-nzvd2016.tif b/nz_linz/nelsht1955-nzvd2016.tif
new file mode 100644
index 0000000..bad2587
--- /dev/null
+++ b/nz_linz/nelsht1955-nzvd2016.tif
diff --git a/nz_linz/nzgeoid2009.tif b/nz_linz/nzgeoid2009.tif
new file mode 100644
index 0000000..8e2daff
--- /dev/null
+++ b/nz_linz/nzgeoid2009.tif
diff --git a/nz_linz/nzgeoid2016.tif b/nz_linz/nzgeoid2016.tif
new file mode 100644
index 0000000..8a6c215
--- /dev/null
+++ b/nz_linz/nzgeoid2016.tif
diff --git a/nz_linz/ontpht1964-nzvd2016.tif b/nz_linz/ontpht1964-nzvd2016.tif
new file mode 100644
index 0000000..607f3ee
--- /dev/null
+++ b/nz_linz/ontpht1964-nzvd2016.tif
diff --git a/nz_linz/stisht1977-nzvd2016.tif b/nz_linz/stisht1977-nzvd2016.tif
new file mode 100644
index 0000000..a9e7241
--- /dev/null
+++ b/nz_linz/stisht1977-nzvd2016.tif
diff --git a/nz_linz/taraht1970-nzvd2016.tif b/nz_linz/taraht1970-nzvd2016.tif
new file mode 100644
index 0000000..1d6bb18
--- /dev/null
+++ b/nz_linz/taraht1970-nzvd2016.tif
diff --git a/nz_linz/wellht1953-nzvd2016.tif b/nz_linz/wellht1953-nzvd2016.tif
new file mode 100644
index 0000000..3cfce3b
--- /dev/null
+++ b/nz_linz/wellht1953-nzvd2016.tif