2

I'm setting up a graphic modeler to handle three vector layers and one raster layer. In the end, I hope to vectorize the raster and merge the other three into one coverage. The raster file is a slope DEM, and I'm looking for anything steeper than a 25% grade.

I'm pulling my hair out because every time I use the 'select by expression' tool, the next step is to save the selection. Of course it tells me that nothing has been selected. I've tested my expression...

"Shape_Area">2000 AND "DN" = 1 (DN is the binary value attributed to slope < or > 25 degrees)

...outside of the graphic modeler and it works just fine. In debugging, I went back and saved the vectorized file with the $area field calculation included, and the 'select by expression' works just fine.

Thoughts?

3

Use "extract by expression" instead. That will create a new layer with just matching features - effectively the select & save operation in one step. This works well with models too.

  • Effectiveness aside, do you have any insight as to why my approach didn't work? And you we're taking about the 'v.extract' correct? Because it worked like a charm... – mgallotta Sep 2 '17 at 14:35
  • @mgallotta - The Extract by expression algorithm is a new tool which can be accessed from QGIS 2.99. Not sure why your expression isn't working but v.extract is a good tool to use :) – Joseph Sep 4 '17 at 9:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.