[GRASS-user] help needed with polar projections

GRASS GRASS user
1

Hello:

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, and am
working on a RedHat Linux system.

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

Any suggestions for how to set up a UPS projection or other polar
projections, or how to define the region setting properly for the
Antarctic polar projection would be much appreciated!

Many thanks,

Dan Gillespie

..................................

Here are the projection parameters for the 2 locations:

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

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]]
  
  Here is the default region for this location:
  N: 0
  S: -90
  W: -180
  E: 180
  grid resolution NS: 1
      EW: 1
  rows: 90
  columns: 360

2

Hi Daniel!

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

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

Hello:

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, and am
working on a RedHat Linux system.

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

Any suggestions for how to set up a UPS projection or other polar
projections, or how to define the region setting properly for the
Antarctic polar projection would be much appreciated!

Many thanks,

Dan Gillespie

..................................

Here are the projection parameters for the 2 locations:

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

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]]
  
  Here is the default region for this location:
  N: 0
  S: -90
  W: -180
  E: 180
  grid resolution NS: 1
      EW: 1
  rows: 90
  columns: 360

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3

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.