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I am attempting to contour a DEM raster using gdal.ContourGenerate().

This is my code:

in_data_set = gdal.Open(TEMP_CROP_DEM_PATH, gdal.GA_ReadOnly)
in1 = in_data_set.GetRasterBand(1)

ogr_ds = ogr.GetDriverByName("ESRI Shapefile").CreateDataSource(TEMP_CONTOURS_PATH)
contour_shp = ogr_ds.CreateLayer('contour', dest_srs)

field_defn = ogr.FieldDefn("ID", ogr.OFTInteger)
contour_shp.CreateField(field_defn)
field_defn = ogr.FieldDefn("elev", ogr.OFTReal)
contour_shp.CreateField(field_defn)

gdal.ContourGenerate(in1, contour_offset, 0, [], 0, 0, contour_shp, 0, 1)

This code has worked for me on several tests, however it does not work when there is an area of no data in the DEM. When contouring a DEM with no data areas, QGIS freezes on the last line (ContourGenerate) and crashes.

I have looked through the GDAL API and made sure all my arguments are correct. I have tried changing the 5th argument useNoData to 1 and saw no change.

Running in1.GetNoDataValue() also causes QGIS to crash.

I am running out of ideas of where to look to solve my problem. Has anyone come across something similar? Are there other ways to achieve my goal, create vector contours from a DEM in Python?

1 Answer 1

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I was able to solve my problem by setting the no data value. I used QgsRasterLayer class to grab the no_data_value. I then changed my arguments on the gdal.ContourGenerate to set useNoData to 1 and set noDataValue to the no_data_value.

The docstring for gdal.ContourGenerate is:

"""ContourGenerate(Band srcBand, double contourInterval, double contourBase,
int fixedLevelCount, int useNoData, double noDataValue, Layer dstLayer,
int idField, int elevField, GDALProgressFunc callback=0,
void * callback_data=None) -> int"""

my final code additions:

layer = QgsRasterLayer(TEMP_CROP_DEM_PATH, 'terrain_temp')
block = layer.dataProvider().block(
    1,
    layer.extent(),
    layer.rasterUnitsPerPixelY(),
    layer.rasterUnitsPerPixelX()
    )
no_data_value = block.noDataValue()

...
...
...

gdal.ContourGenerate(in1, contour_offset, 0, [], 1, no_data_value, contour_shp, 0, 1)

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