[Qgis-user] projection confused during import from Grass to QGIS

[Paul Paris]

Hi Tyler,

>From the gdalinfo output it would appear that you have no coordinate system
defined for your layer(s). Note the Coordinate System line. Here you
should--but do not--find datum, ellipsoid, projection type and its
associated parameters, units of measure, etc. Since your data are not
actually projected, you should see listed here something like:

PROJCS["WGS84",
    GEOGCS["WGS84",
        DATUM["World Geodetic_Datum_1984",
            SPHEROID["WGS1984"],
        UNIT["degree"]],
    PROJECTION["Geographic"],
    PARAMETER["scale_factor",0.0],
    PARAMETER["false_easting",0.0],
    PARAMETER["false_northing",0.0]


 Your output lists only the affine coordinate geo-reference info (e.g.,
Origin, Pixel Size, Corners).

With the addition of the negative signs in front of the (East/West)
longitudes, as Randy pointed out, you get the correct E-W orientation, but
you mention that a slight mis-alignment remains. About how much
mis-alignment do you measure?

Try setting the layer CRS explicitly in QGIS; I think that WGS84 is
EPSG=4326. That might help. Too, if the mis-alignment is small (small is
hard to define, I realize, when dealing with data in GPs), perhaps it has
something to do with floating point round-off in GRASS (FCELL versus
DCELL). Or, it could be related to the region, where the region set during
export differed from that native to the layer(s) itself/themselves. These
would account for only small differences. If there is larger mis-alignment
(for instance two or more layers do not even overlap geographically), then
something bigger is afoot.

Keep us posted on progress.

Paul


On Tue, May 12, 2015 at 8:44 AM, Tyler Smith <tyler at plantarum.ca> wrote:

> On Mon, May 11, 2015, at 05:20 PM, Randal Hale wrote:
> > > The header appears correct:
> > >
> > > tws:~$ head -6 tri_mean.asc
> > > north: 46:53:04.633938N
> > > south: 34:44:13.536462N
> > > east: 68:50:28.1616W
> > > west: 94:51:40.938811W
> > > rows: 1458
> > > cols: 3122
> > >
> > Shouldn't your east and west Lattitudes be negative? That would make it
> > appear reversed as they stand now. At least I think - but I've not done
> > what you are doing.
>
> I don't think so. The east and west lattitudes have a 'W' appended to
> indicate west. Although I have tried manually deleting the W and adding
> '-' in front of the digits. This does indeed flip the maps east to west,
> but they are still somewhat misaligned with the actual projection.
>
> Regardless, I can export and import the layers to share them across
> Grass installations without problem. The mis-projection only shows up
> with gdal and QGis.
>
> Thanks,
>
> Tyler
>
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