The proj command
itself can report the list of projections using the -lp, option,
the list of ellipsoids with the -le option, the list of units with
the -lu option, and the list of built-in datums with the -ld
option.
The GeoTIFF
Projections Pages include most of the common PROJ.4 projections, and
a definition of the projection specific options for each.
How do I do datum shifts between NAD27 and NAD83?
While the nad2nad program can be used in some cases, the cs2cs
is now the preferred mechanism. The following example demonstrates using
the default shift parameters for NAD27 to NAD83:
% cs2cs +proj=latlong +datum=NAD27 +to +proj=latlong +datum=NAD83
-117 30
producing:
117d0'2.901"W 30d0'0.407"N 0.000
In order for datum shifting to work properly the various grid shift files
must be available. See below.
How do I build/configure PROJ.4 to support datum shifting.
After downloading and unpacking the PROJ.4 source, also download and unpack
the set of datum shift files. This would be a file like
ftp://ftp.remotesensing.org/pub/proj/proj-nad27-1.1.tar.gz. This
file should be unpacked within the proj/nad directory.
Then proceed with the configuration, build and install. This will ensure
that the build system knows about the grid shift files, and applies the
ascii to binary preprocessing step.
On Windows the extra nadshift target must be used. For instance
nmake /f makefile.vc nadshift in the proj/src directory.
A default build and install on Unix will normally build knowledge of the
directory where the grid shift files are installed into the PROJ.4 library
(usually /usr/local/share/proj). On Windows the library is normally built
thinking that C:\PROJ\NAD is the installed directory for the grid shift files.
If the built in concept of the PROJ.4 data directory is incorrect, the PROJ_LIB
environment can be defined with the correct directory.
How do I debug problems with NAD27/NAD83 datum shifting?
- Try a datum shifting operation in relative isolation, such as with the
cs2cs command listed above. Do you get reasonable results? If not it is
likely the grid shift files aren't being found. Perhaps you need to define
PROJ_LIB?
- The cs2cs command and the underlying pj_transform() API know how to
do a grid shift as part of a more complex coordinate transformation; however,
it is imperative that both the source and destination coordinate system be
defined with appropriate datum information. That means that implicitly or
explicitly there must be a +datum= clause, a +nadgrids= clause or
a +towgs84= clause. For instance "cs2cs +proj=latlong +datum=NAD27 +to
+proj=latlong +ellps=WGS84" won't work because defining the output coordinate
system as using the ellipse WGS84 isn't the same as defining it to use the
datum WGS84 (use +datum=WGS84). If either the input or output are not
identified as having a datum, the datum shifting (and ellipsoid change) step
is just quietly skipped!
- The PROJ_DEBUG environment can be defined (any value) to force extra
output from
the PROJ.4 library to stderr (the text console normally) with information on
what data files are being opened and in some cases why a transformation fails.
Note that PROJ_DEBUG support is not yet very mature in the PROJ.4 library.
- The "-v" flag to cs2cs can be useful in establishing more detail on
what parameters being used internally for a coordinate system. This will
include expanding the definition of +datum clause.
How do I use EPSG coordinate system codes with PROJ.4?
There is somewhat imperfect translation between 2d geographic and
projected coordinate system codes and PROJ.4 descriptions of the
coordinate system available in the epsg definition file that
normally lives in the proj/nad directory. If installed (it is
installed by default on Unix), it is possible to use EPSG numbers like this:
% cs2cs -v +init=epsg:26711
# ---- From Coordinate System ----
#Universal Transverse Mercator (UTM)
# Cyl, Sph
# zone= south
# +init=epsg:26711 +proj=utm +zone=11 +ellps=clrk66 +datum=NAD27 +units=m
# +no_defs +nadgrids=conus,ntv1_can.dat
#--- following specified but NOT used
# +ellps=clrk66
# ---- To Coordinate System ----
#Lat/long (Geodetic)
#
# +proj=latlong +datum=NAD27 +ellps=clrk66 +nadgrids=conus,ntv1_can.dat
The proj/nad/epsg file can be browsed and searched in a text editor
for coordinate systems. There are known to be problems with some coordinate
systems, and any coordinate systems with odd axes, a non-greenwich prime
meridian or other quirkyness are unlikely to work properly. Caveat Emptor!
How do I use 3 parameter and 7 parameter datum shifting
Datum shifts can be approximated with 3 and 7 parameter transformations.
Their use is more fully described in the
towgs84 discussions.
Requests to add items to the frequently asked questions list
can be entered in bugzilla.