# Edit a raster (DEM) to create a constant slope angle

I'm trying to edit a raster (DEM) that has square sides (90 degrees) to create a surface with a constant slope angle of 18 degrees.

I'm working in QGIS and have not been able to find any help for this issue.

• When you say "edit" do you mean "create"? "Edit" sounds like you already have a raster. Do you want it to slope in a constant direction? Because you can construct a cone with a constant 18 degree slope everywhere too (except arguably at the point...). – Spacedman Aug 21 '18 at 10:54
• Thanks for your response. I mean edit in the sense that I already have a raster. To provide a little more context, the raster (DEM) represents a the crest of a hill. I'm trying to reduce the height of the crest (which I'm able to do) and then 'reshape' the slope to a constant angle. And yes, i'd like the slope to be in a constant direction. Could you please provide further information on what you mean when you say that I can construct a cone? Any help is much appreciated. – r_olsen Aug 21 '18 at 11:17
• What I'm saying is that if you imagine a cone sitting in front of you, every point on that cone has the same slope angle, but with different slope direction as you go round the cone. – Spacedman Aug 21 '18 at 12:06
• A surface of constant slope and direction has equation `Z = aX + bY` - in QGIS you might have to do this by first creating a vector grid of points, then computing Z as a new attribute, then making a raster based on those points and that attribute. The values of `a` and `b` control the slope and direction. Formulae for that are in geometry/trigonometry texts. – Spacedman Aug 21 '18 at 12:08