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

Chaitanya kumar CH chaitanya.ch at gmail.com
Wed Aug 31 08:59:17 EST 2011


Can you compare the two contours with the pixel values? Which contours are
FYI, you can read the algorithm rules in the docs of GDALContourGenerate()

[1]: http://www.gdal.org/gdal__alg_8h.html#ceaf98ad40f159cbfb626988c054c085

On Wed, Aug 31, 2011 at 6:42 PM, Rengifo Ortega <rengifoo at yahoo.de> wrote:

> Dear gdal users
> 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 am in Grass GIS  MS Windows environment (native using MinGW) combined
> with QGIS for visualisation purposes
> Using FWtools 2.4.7
> 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
> Comparison 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
> Rengifo
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Best regards,
Chaitanya kumar CH.

17.2416N 80.1426E
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