[GRASS-user] "v.transform" reports erroneous values in the transformation matrix

GRASS GRASS user
,
1

Hi all!

I've found that values of transformation matrix reported by
v.transform are erroneous after using this values in ST_Affine (a
PostGIS's function).

The values that seems to be erroneous are x_offset and y_offset. The
rotation values seems to are fine.

The transformed elements are ok (it is, their coordinates are in the
right place).

Values reported by v.transform:

xoff = -139153.031250
yoff = 8352534.450195

The correct values must be (aproximately):

xoff = 5461371
yoff = 6321181

The complete session's log with one test element is there (same
results with a whole layer):

http://pastebin.com/FUtnePvb

Local map: tmp
Transformed map: tmp_geo

Take a look at the map region for tmp/tmp_geo (reported by v.info),
and compare these values with those of x_off / y_off (reported by
v.transform).

Maybe I'm not interpreting this correctly.

Thanks all!
Jose

2

On Tue, Feb 26, 2013 at 2:17 PM, José María Michia Roberts
<jose.maria.michia@gmail.com> wrote:

Hi all!

I've found that values of transformation matrix reported by
v.transform are erroneous after using this values in ST_Affine (a
PostGIS's function).

The values that seems to be erroneous are x_offset and y_offset. The
rotation values seems to are fine.

The transformed elements are ok (it is, their coordinates are in the
right place).

Values reported by v.transform:

xoff = -139153.031250
yoff = 8352534.450195

The correct values must be (aproximately):

xoff = 5461371
yoff = 6321181

The complete session's log with one test element is there (same
results with a whole layer):

http://pastebin.com/FUtnePvb

Local map: tmp
Transformed map: tmp_geo

Take a look at the map region for tmp/tmp_geo (reported by v.info),
and compare these values with those of x_off / y_off (reported by
v.transform).

Note that there are two transformation matrices, one for forward
transformation, one for backward transformation. Can the PostGIS
ST_Affine function print both matrices?

If possible, rather use v.rectify in GRASS 7 instead of v.transform in GRASS 6.

With a recent version of GDAL/OGR, you can also add GCPs to a
datasource with ogr2ogr -gcp which should then automatically transform
the coordinates.

Markus M

Maybe I'm not interpreting this correctly.

Thanks all!
Jose
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3

2013/2/26 Markus Metz <markus.metz.giswork@gmail.com>:

Note that there are two transformation matrices, one for forward
transformation, one for backward transformation. Can the PostGIS
ST_Affine function print both matrices?

AFAIK, PostGIS cannot print neither calculate transformation matrices.
User must obtain matrices values from somewhere. [sorry if this thread
became off-topic]

If possible, rather use v.rectify in GRASS 7 instead of v.transform in GRASS 6.
With a recent version of GDAL/OGR, you can also add GCPs to a
datasource with ogr2ogr -gcp which should then automatically transform
the coordinates.

I will try it. Thank you for update me!

Salud!
Jose