I am using Windows 7 enterprise and QGIS version 2.12.2 Lyon. Have found an unexpected result when I use QGIS to generate a topographic position index (TPI) raster from a 30m by 30m DEM. I find a pattern of what I believe to be aberrantly valued pixels forming a square grid pattern that are approximately 900 m on each side. I expected the resulting raster to not have this grided appearance.

UPDATE: I have looked at my hillshade, slope, aspect, terrain ruggedness index and roughness rasters generated using the same tool and they all have this same "grid" (although it is much less apparent than in the TPI raster).

Here's a screenshot of the original DEM I used: enter image description here

Here's a screenshot of the resulting TPI. Notice the grid pattern formed over what I suspect to be an accurate TPI raster layer. enter image description here

Here's how I created the TPI Raster: Raster – analysis – DEM:

  • What is the projection of the DEM data?
    – ahmadhanb
    Commented Apr 21, 2016 at 1:50
  • ahmadhanb: the projection is EPSG:32710, WGS 84/UTM zone 10S
    – trilobite
    Commented Apr 21, 2016 at 15:47

1 Answer 1


I think the problem is related to how you change the projection of your data. If the DEM data was originally in geographic projection (WGS84), and not projected to UTM zone 10S, then you need to reproject the original DEM image (the WGS84) again in a proper way. When you reproject (Warp) the DEM from geographic to UTM projection, it is better that you choose "Cubic" as a Resampling Method - sometimes the method called "Cubic Convolution" - especially for continuous data like DEM.

enter image description here

I think using proper resampling method when reprojecting the DEM will solve the problem.

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