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 how I created the TPI Raster: Raster – analysis – DEM: