[FWTools] shift in contours lines gdal vs. grass gis

Rengifo Ortega rengifoo at yahoo.de
Wed Aug 31 08:12:57 EST 2011

Dear gdal
I hope you
can help me with a question that is puzzling me, since a couple of days. So let
me explain my problem.
 2 years ago,
I downloaded a SRTM 90m .asc file into GRASS GIS. 
A week ago, I
decided to generate some contours lines using gdal_contour command (version FWTools2.4.7)
and compare the contours lines with contours already produced in GRASS GIS. I was quite
disturbed when in fact, I found a horizontal shift of the contours lines to the
south of about -/+35m or to north (unknown distance), with respect to the grass gis- generated contours

Technical aspect of my hardware

I amin Grass GIS  MS Windows environment (native using MinGW)
combined with QGIS for visualisation purposes
Using FWtools
So, here are
the steps I followed
Part I
-Produced 90m contours lines in GRASS
GIS out of the SRTM DEM 90 by using the r.contour command
-Visualised the contours lines in QGIS
and saved them as shp format
Part II
1)   -Export SRTM 90 rasterfile as  an ESRI ASCII GRID fomat and converted it to .tif
format using FWTools2.4.7
2)       Using gdal_contours utility I generated 90
meters contour lines
 Results 1

between the GRASS-generated and the GDAL- generated contours lines exhibited a horizontal
shift of -/+ 35m to the south in relation to GRASS GIS generated contours
Part III
1--Export the raster file SRTM90m from
GRASS GIS; as a Geotiff using gdal

-Generate 90m contours using gdal

Result 2 
Comparing the GRASS GIS contours with the geotiff- generated contours a
displacement to the north was observed, no distance measured.

I hope you can give some hints..
best regards 

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