[GRASS-user] Datum <unknown> in v.in.ogr

[Micha Silver]

Hanlie Pretorius wrote:

> Hi,
>
> I'm using QGIS to export shapefiles from WGS84 GCS to a local
> projected coordinate system so that I can import the projected
> shapefile into GRASS.
>
> The PROJ4 definition that I use to export the shapefile (and that I
> used to create the GRASS location) is:
>
> +proj=tmerc +lat_0=0 +lon_0=29 +k=1 +x_0=0 +y_0=0 +ellps=WGS84
> +units=m +no_defs
>
>
> When I use v.in.ogr to get the shapfile into GRASS, I get the message:
>
> 'Datum <unknown> not recognised by GRASS and no parameters found
> Projection of input dataset and current location appear to match'
>
> Should I worry about the first part of the message?
>
>   
The warning indicates that the shapefile has no *.prj attached so GRASS 
has no way to be sure that the CRS is correct. If you're sure that the 
shapefile is projected into the CRS that you specified above then you 
can ignore the warning.

> The imported vector data does look a bit skew. It consists of
> rectangles that should match raster DEM rectangles, but the outlines
> now seem to be leaning a few degrees towards the left. Is this just a
> result of the errors inherent in a projected coordinate system?
>
>   
Typically, projecting features from a geographic (Lon-Lat) CRS into a 
planar CRS will cause changes to distances, shapes or areas. So you 
would expect that a rectangle in one CRS becomes some "skewed" shape in 
another CRS.
> Thanks
> Hanlie
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>   


-- 
Micha Silver
Arava Development Co. +972-52-3665918
http://www.surfaces.co.il