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I need your help regarding extraction of contour lines in QGIS. I am new to QGIS (in GIS actually).

I followed the tutorials on extracting contour lines from DEM (which I downloaded from cgiar.srtm) using qgis. Everything went well except that there is no contour line with 0 elevation. The contour lines started with the elevation of 10. I checked the DEM file and the minimum value is 0.

How do I set the QGIS so that the resulting contour lines starts with 0 elevation (and that there is a contour line with 0 elevation)?

Thank you very much for your help.

Jake

  • Link the tutorials please. Also Qgis warps around gdal_contour command, so you might want to give gdal.org/gdal_contour.html a look. – nickves May 14 '13 at 16:08
  • Hi nickves. Thank you for the advice. I added "-off 0.0" to offset at 0 elevation but nothing is happening. It still started at 10. Is there other way of doing it without going through the code thing? Iam also trying to teach it to someone who is not really familiar with GIS just like me. Thank you again.. – user18131 May 14 '13 at 18:42
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I'm assuming that 0 elevation = sea level?

Given that the minimum elevation of the DEM = 0, I don't think that a contour for this elevation could possibly be drawn; any attempt to interpolate this would be meaningless (it would effectively be trying to join every point with an elevation of 0).

If there were negative height values then a meaningful elevation=0 line could be drawn.

A coarse workaround to define the coast a bit closer might be to set the contour distance to 1m and then use the 1m contour as a closer approximation of the coast line...

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Download the high resolution GSHHG supplied by NOAA.

GSHHG is a high-resolution geography data set, amalgamated from two databases in the public domain: World Vector Shorelines (WVS) and CIA World Data Bank II (WDBII).

It's available either in bin and shp formats at: https://www.ngdc.noaa.gov/mgg/shorelines/data/gshhg/latest/

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