[GRASS-dev] custom CRS

[Moritz Lennert]

Le 18 mars 2017 10:06:09 GMT+01:00, Paulo van Breugel <p.vanbreugel at gmail.com> a écrit :
>Dear all,
>
>I have a shapefile which is in CRS ESRI:37203 [1], which is defined in 
>the prj file as
>
>GEOGCS["GCS_Everest_India_Nepal",DATUM["D_Everest_India_Nepal",SPHEROID["Everest_Definition_1962",6377301.243,300.8017255]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]]
>
>
>The corresponding proj.4 definition (as defined in the esri epsg-codes 
>file in /usr/share/proj/)
>
>+proj=longlat +a=6377301.243 +b=6356100.230165384 +no_defs
>
>And when I create a location based on this shapefile, the projection 
>information is:
>
>-PROJ_INFO-------------------------------------------------
>name       : GCS_Everest_India_Nepal
>a          : 6377301.243
>es         : 0.006637846068423442
>proj       : ll
>no_defs    : defined
>-PROJ_UNITS------------------------------------------------
>unit       : degree
>units      : degrees
>meters     : 1.0
>
>Question 1: Now, I can import a shapefile or raster layer in 
>latlon/WGS84 (EPGS: 4326) without reprojection. GRASS thus assumes the 
>projection is identical, even if they are not? Why is that?
>
>The proj.4 definition above is missing the shift w.r.t. the WGS84. 

That would be why: if no towgs84 info is given, the projection is assumed as using wgs84 as datum.


>The 
>correct proj.4 definition for EPGS 37203 would be [2]
>
>+proj=longlat +a=6377301.243 +b=6356100.2284 
>+towgs84=295,736,257,0,0,0,0 +no_defs
>
>Question 2: In QGIS it is possible to define a custom CRS using the 
>above. Can I do this in GRASS as well, and if so, how? 

The location creation wizard allows you to define a location using a proj.4 string.

>Or can / should
>I 
>add this to the datum information files in $GISBASE/etc/proj/ogr_csv?

Not sure about that file, but you could also add it directly in the /use/share/project/epsg or equivalent.

Moritz

>
>
>Regards,
>
>Paulo
>
>
>[1] https://epsg.io/37203
>[2] 
>https://disqus.com/home/discussion/qgistutorials/georeferencing_topo_sheets_and_scanned_maps/#comment-1426454689
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