[GRASS-user] help needed with polar projections

[Maciej Sieczka]

Henning Lorenz wrote:

> You have specified your region in longitude/latitude while you work in
> Polar Stereographic Projection whose coordinate system has "meters" as
> unit. You have to convert your lat/long values first, and don't
> forget to set the resolution to a proper value!

Henning,

Please note Daniel is reprojecting a vector, so region settings don't
matter.

> On Thu, 11 Jan 2007 13:24:24 -0500, Daniel.Gillespie wrote:

>> I am new Grass user, and am having some problems trying to set up a
>> polar projection.  I am using Grass 6.0.2 that I compiled,

Daniel,

Please use a newer version. The latest stable is 6.2.1 and it is highly
recommended over 6.02.

>> I have tried to set up a Universal Polar Stereographic (UPS) projection,
>> as well as an Antarctic Polar Stereographic projection.  The UPS
>> projection did not appear to work at all.  The Antartic Polar
>> Stereographic projection didn't fail, but when I reproject a vector file
>> with Antarctica, the results are not what I expected.  I think that I
>> may have set the default region incorrectly,

The region (both input and output) is not taken into account when
reprojecting vectors. It matters only when reprojecting rasters (r.proj).

>> because only a small
>> portion of Antarctica appears in the monitor.  The details of the
>> projections and region follow below.

>> UPS 
>> 	GRASS 6.0.2 (SouthPole2):~ >  g.proj -w
>> 	WARNING: OGR can't parse PROJ.4-style parameter string:
>>         	 +proj=ups +south +a=6378137 +rf=298.257223563 +no_defs
>>          	 +towgs84=0.000,0.000,0.000 +to_meter=1.0000000000
>>          	 (OGR Error code was 5)
>> 	WARNING: g.proj: Unable to convert to WKT

Based on EPSG code 32761 [1], that should rather be:

+proj=stere +lat_0=-90 +lat_ts=-90 +lon_0=0 +k=0.994 +x_0=2000000
+y_0=2000000 +ellps=WGS84 +datum=WGS84 +units=m

To update your projection settings use g.proj -c in the PERMANENT
mapset of your location, or edit the PROJ_INFO manually.

>> Antarctic Polar Stereographic
>> 	GRASS 6.0.2 (SouthPole4):~ > g.proj -w
>> 	PROJCS["Stereographic",
>>     	   GEOGCS["wgs84",
>>               DATUM["unknown",
>>                   SPHEROID["wgs84",6378137,298.257223563]],
>>               PRIMEM["Greenwich",0],
>>               UNIT["degree",0.0174532925199433]],
>>     	PROJECTION["Polar_Stereographic"],
>>     	PARAMETER["latitude_of_origin",-71],
>>     	PARAMETER["central_meridian",70],
>>     	PARAMETER["scale_factor",1],
>>     	PARAMETER["false_easting",0],
>>     	PARAMETER["false_northing",0],
>>     	UNIT["meters",1]]

This seems OK, but please note the last line should be:

>>	UNIT["metre",1]]

instead. I recall it is a bug fixed after 6.02. At least it is in my
6.3 CVS.

Maciek



[1] I'm guessing by the output of this command:

$ grep UPS -A1 /usr/local/share/proj/epsg

# WGS 84 / UPS North
<32661> +proj=stere +lat_0=90 +lat_ts=90 +lon_0=0 +k=0.994 +x_0=2000000
+y_0=2000000 +ellps=WGS84 +datum=WGS84 +units=m +no_defs  <>
--
# WGS 84 / UPS South
<32761> +proj=stere +lat_0=-90 +lat_ts=-90 +lon_0=0 +k=0.994
+x_0=2000000 +y_0=2000000 +ellps=WGS84 +datum=WGS84 +units=m +no_defs  <>

Your epsg file might be located elsewhere, depending on where you have
PROJ.4 installed.