Hi,
I am using gdal_contour to generate iso-polygons from a hillshade raster
created via the gdal_dem tool. In certain situations, I am seeing a lot of
tiny/narrow polygons and large polygons with long, skinny *whiskers*
protruding out (and sometimes in) . In these cases, the vertices in the
*skinny* areas are less than 1e-07 away from their nearest edge/vertex.
Here is an example of how the tool is being called (GDAL version 3.4.1):
> gdal_contour -p -f GPKG -amin level -nln hillshade -fl 231
295-hillshade-polygon.tif ./295-hillshade-polygons.gpkg
For reference, the hillshade raster has EPSG:4326 defined as its CRS with a
geo-pixel size of (0.044031311154586,-0.044031311154586).
The diagram below depicts a simple example when working through the
marching-squares algorithm by hand with a single threshold level of 231
(prior to the interpolation step):
229 229 229
x----------------x----------------x
| | |
| | |
| case 2 * case 1 |
| /|\ |
| / | \ |
| / | \ |
230 x------------*---o---*------------x 230
| | 231 | |
| | | | |
| | | | |
| case 6 | | | case 9 |
| | | | |
| | 231 | |
230 x------------*---o---*------------x 230
| \ | / |
| \ | / |
| \|/ |
| case 4 * case 8 |
| | |
| | |
x----------------x----------------x
229 227 226
I would expect the vertices surrounding each of the 231 intensity values to
be coincident, but the final result has the vertices in each set ~8e-08
apart.This leads me to believe there might be some precision issue in
gdal_contour during bipolar edge interpolation?
I ran this same configuration through another isoline generator for
comparison (see https://github.com/rveciana/raster-marching-squares) and
got what I would expect: a degenerate line with two sets of coincident
vertices around each 231 intensity pixel location.
If necessary, I can provide a complete example with the hillshade raster
and the generated polygons.
Thanks!
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