For QGIS3.0:

Using DEMs, I am drawing polygons around depressions. Inside that polygon, I need to generate not only the minimum elevation value but the location that value was extracted from; this location needs to be a point by the end.

The DEMs in use have near centimeter resolution over features that are a kilometer or so in diameter, therefore generating random points, or sampling all points methods are computationally intense; the PC is capable but future usage of this technique may be on lower-end computers.

Things tried:

Maximum altitude in polygons

How to plot a point on the highest elevation of a raster on a polygon. QGIS

Is there a way to find the minimum Z point feature inside of each polygon in a polygon layer?

The above-mentioned methods were modified from maximum they were written for into minimum, they return a value but not the precise location or require iteration over randomly derived samples of the feature.

I have been performing a similar technique in ArcGIS but trying to commit to QGIS once and for all. This is the last process in my conversion I have not been able to figure out.

  • 2
    "Generate points (pixel centroids) inside polygons", select lowest value per polygon
    – BERA
    Jul 20 '20 at 20:01
  • 1
    Thank you. This led me down the path I needed to solve my problem. Much appreciated it. I Answered the question below. Jul 22 '20 at 20:12
  • I tried applying your method, but in my case, I have many (about 2 thousand) polygons, so how could I select the lowest elevation point for each of the polygons? (Apologies I don't have enough reputation to add a comment, so had to add an answer instead).
    – Eddy
    Aug 6 at 3:45

Surprisingly, after @BERA's comment, this was not a computationally taxing as I thought it would have been.

I used the Generate points (Pixel Centroids) inside polygons as you suggested; though it only made a list of points and with no values. I then used the Sample Raster Values to sample the DEM, using the points generated from the previous step. Made the layer editable selected the lowest elevation pixel by toggling the field order, inverted my selection to highlight the other points, and simply deleted. As I was working with scratch files I made sure to export my points to a .shp file.

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