I generated random points on a map. Around each point I would like to draw a circular buffer which considers the slope in that area, and within these circular buffers I would monitor tree height in the field.

It is very important for the polygons to consider the slope because I would always like to have the same surface area in each polygon. So I assume that when drawing a circular buffer it will turn out ellipsoidal on a area with a slope.

  • 5
    I suppose you could create ellipsoids around your points, using the direction of the slope for orientation, and its steepness as a factor for calculating the minor extent of the ellipsoid. Applied trigonometrics.
    – Erik
    Feb 7, 2022 at 10:48
  • 3
    in most cases the vertical variation will be negligible for the area, unless you are looking at very steep slopes over very short distances
    – Ian Turton
    Feb 7, 2022 at 11:00
  • 1
    The monitoring plots will have a radius of just 8-15meters and the terrains that we are working in have steep slopes. So unfortunately we found out that we cannot neglect the slopes.
    – N_LLC
    Feb 7, 2022 at 11:22
  • @Erik is there a way to do this automatically. It seems like such an easy and basic necessity. I really only want to draw a polygon of a specified radius considering the slope.
    – N_LLC
    Feb 7, 2022 at 11:26
  • I told you how to do it automatically? What else do you need?
    – Erik
    Feb 7, 2022 at 11:34

1 Answer 1


Not sure about the math, but the steps before it should work. I'm using 50 m buffer radius.

  1. Calculate a aspect raster
  2. Calculate a slope raster
  3. Sample raster values twice to get aspect and slope as attributes on each point
  4. Field calculate the ellipse major axis: cos(radians("slope1"))*50
  5. Geometry by expression: make_ellipse($geometry, "major", 50, "aspect1")

For some reason I cant get Geometry by expression to work without the Field Calculator step.

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