[GRASS-user] Raster and shape files miss aligned.

GRASS GRASS user
,
1

Kirk,

My guess is that the projection information for one of the original files is incorrect. My bet is on the shapefile. You can test by importing a 3rd file where you know the projection info is good (e.g., something from USGS or an SRTM) and seeing which one matches and which one doesn't.

Michael
____________________
C. Michael Barton
Director, Center for Social Dynamics & Complexity
Professor of Anthropology, School of Human Evolution & Social Change
Arizona State University

voice: 480-965-6262 (SHESC), 480-727-9746 (CSDC)
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www: http://www.public.asu.edu/~cmbarton, http://csdc.asu.edu

On Nov 9, 2011, at 10:00 AM, <grass-user-request@lists.osgeo.org> wrote:

Date: Tue, 8 Nov 2011 12:38:24 -0600
From: Kirk Wythers <kirk.wythers@gmail.com>
Subject: [GRASS-user] Raster and shape files miss aligned.
To: "grass-user@lists.osgeo.org" <grass-user@lists.osgeo.org>
Message-ID: <B15E5500-88C1-4B97-9685-25947F68BA47@gmail.com>
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I have imported two files (one raster with r.in.gdal, and one shape file with v.in.ogr). Both file are in the same projection, but do not align properly. There is a North South shift that I can not explain. Any ideas what I might have missed in the import?

Thanks in advance

Here are the results from g.region on each file:

GRASS 6.4.1 (northcentralus_albersequalarea):~ > g.region -p vect=nw_prov212
projection: 99 (Albers Equal Area)
zone: 0
datum: nad83
ellipsoid: grs80
north: 1322833.8573
south: 647305.3646
west: 6150.4319
east: 1003361.9406
nsres: 250.01054504
ewres: 249.99035064
rows: 2702
cols: 3989
cells: 10778278

GRASS 6.4.1 (northcentralus_albersequalarea):~ > g.region -p rast=mnwimifnfftgk7wt175v3_250m
projection: 99 (Albers Equal Area)
zone: 0
datum: nad83
ellipsoid: grs80
north: 2960848
south: 2087348
west: -122110
east: 1141890
nsres: 250
ewres: 250
rows: 3494
cols: 5056
cells: 17665664

2

Michael wrote:

My guess is that the projection information for one of the
original files is incorrect. My bet is on the shapefile. You
can test by importing a 3rd file where you know the
projection info is good (e.g., something from USGS or an
SRTM) and seeing which one matches and which one doesn't.

specifically check that the datum is set correctly.

a correct projection but wrong datum typically shows up as a
roughly 100m north-south misalignment, as the ellipsoid is less
or more squashed from the poles to the equator in one vs the
other.
(here I use "datum" and "ellipsoid" interchangably, while
technically in grass a given datum implies its given ellipsoid)

Hamish