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I've been trying to calculate slope in percentage for a project I'm working on but keep running into this issue

  1. Whenever I calculate slope using the designated Slope tool or the r.slope.aspect tool I get these lines all over my output (see below). This happens when I try to calculate either percentage or degrees. I have seen similar problems to this on StackExchange and they all suggested mosaicking the raster using a bilinear interpolation. The image below is the output after doing that, so it doesn't seem to have worked. enter image description here

Extra information: I'm using the USGS 3DEP raster images for California. I mosaicked three images using build vrt with bilinear interpolation. I then reprojected the images to EPSG:26910 (units are in meters).

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The herringbone pattern in your image is a classic indication that a Nearest Neighbor resampling occurred somewhere in your workflow. I suggest that you go back through each processing step and closely review each tool's Resampling options and make sure that you select either Bilinear or Cubic.

For example, you mention that you conducted a reprojection, but you did not state if you selected the Reproject tool's appropriate resampling option. I suspect that you allowed Reproject to operate with the default Nearest Neighbor setting, which may be the source of your problem.

Finally, when symbolizing your raster data, you must also select Bilinear or Cubic resampling within Layer Styling > Resampling.

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