From: Glynn Clements <glynn*gclements.plus.com>
Date: Mon, 18 Dec 2006 17:28:43 +0000
List-Archive: <http://grass.itc.it/pipermail/grass-dev>


The relevant files (all installed into $GISBASE/etc) are:

1. parms.table

One line for each known projection. A sample entry:

	AEA:Albers Equal Area:LAT0=ask,23.0;LAT1=ask,29.5;LAT2=ask,45.5;LON0=ask,-96.0;X0=ask,0.0;Y0=ask,0.0

The first field is the projection name, as used by the old geo.h file.
AFAICT, the corresponding +proj= parameter is identical except in
lower case.

The remainder of the line (after the colon) is a semicolon-separated
list of parameter definitions of the form parm=spec. The LHS is the
parameter name as used by geo.h; these don't necessarily match the
corresponding the proj parameter, but you can translate them using the
desc.table file (see below).

The RHS has two parts, separated by a comma. The first part is either
"ask" or "noask", corresponding to whether or not g.setproj should
prompt the user for a value. The second part is either "nodfl" or a
default value. Note that the combination "noask,nodfl" never occurs;
if an option is marked as "noask", there will always be a default
value.

2. desc.table

One line for each known parameter (other than +proj=). A sample entry:

	LON0:lon:lon_0:Central Meridian

The first field is the parameter name as used by the old geo.h file,
and as used in parms.table.

The second field is the type, and can be one of:

	bool int float lat lon zone

This determines the set of valid options for the parameter (the "zone"
type corresponds to the UTM zone).

The third field is the name of the corresponding proj parameter
(without the leading "+"), and is the key stored in the PROJ_INFO
file.

The fourth field is a textual description of the parameter.


3. units.table

One line for each known distance unit. A sample entry:

	feet:foot:0.3048

The three fields are (in case it isn't obvious) the plural form of the
name, the singular form, and the size of the unit in metres.

However, note that g.setproj still has a fair number of special-case
hacks. Some of those are useful-but-not-essential (e.g. automatically
selecting the correct ellipsoid for certain projections), while others
are necessary (e.g. the zone= and south parameters for UTM need to go
into the WIND/DEFAULT_WIND files as well as the PROJ_INFO file).