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diff --git a/html/faq.html b/html/faq.html index da4ff0e4..655080da 100644 --- a/html/faq.html +++ b/html/faq.html @@ -104,6 +104,37 @@ include expanding the definition of +datum clause. <p> </ol> +<!--------------------------------------------------------------------------> + +<h2>How do I use EPSG coordinate system codes with PROJ.4?</h2> + +There is somewhat imperfect translation between 2d geographic and +projected coordinate system codes and PROJ.4 descriptions of the +coordinate system available in the <tt>epsg</tt> definition file that +normally lives in the <tt>proj/nad</tt> directory. If installed (it is +installed by default on Unix), it is possible to use EPSG numbers like this: + +<code> +% 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 +</code> + +The <tt>proj/nad/epsg</tt> 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!<p> + <!--------------------------------------------------------------------------> |