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

GRASS GRASS user
,
1

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 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?

Thanks
Hanlie

2

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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