From b059707c85bd788795a2ec660b6d14dc1abc5bc0 Mon Sep 17 00:00:00 2001 From: Frank Warmerdam Date: Tue, 30 Apr 2002 16:55:47 +0000 Subject: New git-svn-id: http://svn.osgeo.org/metacrs/proj/trunk@1010 4e78687f-474d-0410-85f9-8d5e500ac6b2 --- html/faq.html | 116 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 116 insertions(+) create mode 100644 html/faq.html (limited to 'html') diff --git a/html/faq.html b/html/faq.html new file mode 100644 index 00000000..da4ff0e4 --- /dev/null +++ b/html/faq.html @@ -0,0 +1,116 @@ + + +PROJ.4 - Frequently Asked Questions + + +

PROJ.4 - Frequently Asked Questions

+ + + +

Where can I find the list of projections and their arguments?

+ +There is no simple single location to find all the required information. The +PostScript/PDF documents listed on the main PROJ.4 +page under documentation are the authoritative source but projections and +options are spread over several documents in a form more related to their +order of implementation than anything else. + +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 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.
+ +

+ + + + +

+ +Requests to add items to the frequently asked questions list + +can be entered in bugzilla.

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