Runtime problem gdal:polygonize processing algorithm

Question

I have a c. 60 MB file containing the GTOPO30 30-arc second (c. 1 km) digital elevation model of Africa. The dataset is described here: https://databasin.org/datasets/2965da954b114ff3b47621e99e3b29ba.

When manually running the gdal:polygonize processing algorithm to vectorize the raster layer from the QGIS processing toolbox, even after many hours of waiting, QGIS is still working on the transformation and showing the "Processing algorithm ..." message (exact command in English might differ due to translation from my native language).

My input parameters are the following:

{
'BAND' : 1,
'EIGHT_CONNECTEDNESS' : FALSE,
'EXTRA': '',
'FIELD': 'elevation',
'INPUT' : '<raster vector file path>',
'OUTPUT' : '<desired output file path>'
}

Can community members more familiar with QGIS help me understand if this behaviour is to be expected or if there is maybe any way to speed up this process?

I run QGIS version 3.12 on a computer with Windows 10 (AMD Ryzen 7 3700 CPU / 8 GB RAM).

Answer 1

The algorithm is trying to create polygon geometries for each elevation value.

The common case is that there are many single pixels that must be transformed into polygons with the size of the pixel.

Also, adjacent and edge pixels may form invalid geometries.

In general, a polygonization of a DEM is not what we want, and usually the solution is to make another process to get the desired result, that may be get contour lines.