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| author | Frank Warmerdam <warmerdam@pobox.com> | 2002-04-30 16:55:47 +0000 |
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| committer | Frank Warmerdam <warmerdam@pobox.com> | 2002-04-30 16:55:47 +0000 |
| commit | b059707c85bd788795a2ec660b6d14dc1abc5bc0 (patch) | |
| tree | 984a11a973cc2d70f66c2b549f8d85739700a4b8 /html | |
| parent | 163bcc6f01dca1a51b67f95161544ea0f54fcb82 (diff) | |
| download | PROJ-b059707c85bd788795a2ec660b6d14dc1abc5bc0.tar.gz PROJ-b059707c85bd788795a2ec660b6d14dc1abc5bc0.zip | |
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git-svn-id: http://svn.osgeo.org/metacrs/proj/trunk@1010 4e78687f-474d-0410-85f9-8d5e500ac6b2
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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 @@ +<html> +<head> +<title>PROJ.4 - Frequently Asked Questions</title> +</head> +<body BGCOLOR="#FFFFFF"> +<h1>PROJ.4 - Frequently Asked Questions</h1> + +<!--------------------------------------------------------------------------> + +<h2>Where can I find the list of projections and their arguments?</h2> + +There is no simple single location to find all the required information. The +PostScript/PDF documents listed on the <a href="index.html">main</a> 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.<pp> + +The <b>proj</b> command +itself can report the list of projections using the <tt>-lp,</tt> option, +the list of ellipsoids with the <tt>-le</tt> option, the list of units with +the <tt>-lu</tt> option, and the list of built-in datums with the <tt>-ld</tt> +option.<p> + +The <a href="http://www.remotesensing.org/geotiff/proj_list/">GeoTIFF +Projections Pages</a> include most of the common PROJ.4 projections, and +a definition of the projection specific options for each.<p> + +<!--------------------------------------------------------------------------> + +<h2>How do I do datum shifts between NAD27 and NAD83?</h2> + +While the <b>nad2nad</b> program can be used in some cases, the <b>cs2cs</b> +is now the preferred mechanism. The following example demonstrates using +the default shift parameters for NAD27 to NAD83:<p> + +<code> +% cs2cs +proj=latlong +datum=NAD27 +to +proj=latlong +datum=NAD83 +-117 30 +</code> + +producing: + +<code> +117d0'2.901"W 30d0'0.407"N 0.000 +</code> + +In order for datum shifting to work properly the various grid shift files +must be available. See below.<p> + +<!--------------------------------------------------------------------------> + +<h2>How do I build/configure PROJ.4 to support datum shifting.</h2> + +After downloading and unpacking the PROJ.4 source, also download and unpack +the set of datum shift files. This would be a file like +<a href="ftp://ftp.remotesensing.org/pub/proj/proj-nad27-1.1.tar.gz"> +ftp://ftp.remotesensing.org/pub/proj/proj-nad27-1.1.tar.gz</a>. This +file should be unpacked <i>within</i> the <tt>proj/nad</tt> 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.<p> + +On Windows the extra <tt>nadshift</tt> target must be used. For instance +<tt>nmake /f makefile.vc nadshift</tt> in the <tt>proj/src</tt> directory. +<p> + +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.<p> + +<!--------------------------------------------------------------------------> + +<h2>How do I debug problems with NAD27/NAD83 datum shifting?</h2> + +<ol> +<li> 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? + +<li> 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!<p> + +<li> 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.<p> + +<li> 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. <p> + +</ol> + + +<!--------------------------------------------------------------------------> + +<hr> + +Requests to add items to the frequently asked questions list +<a href="http://bugzilla.remotesensing.org/enter_bug.cgi?product=PROJ.4"> +can be entered</a> in bugzilla.<p> +</body> +</html> |