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I downloaded several USGS GeoTIFFs from the USGS Earth Explorer site (https://earthexplorer.usgs.gov/). These were then converted to contours using the qgis toolbox (GDAL -> Raster Extraction -> Contour). The command was of the form:

gdal_contour -b 1 -a ELEV -i 5.0 -f "ESRI Shapefile" n33_w086_1arc_v3.tif n33_w086_1arc_v3.shp

At the boundaries of my tiles, there is a slight overlap which leaves a "ladder" effect:

contours extend past tile boundary

If I had to guess, these are artifacts resulting from GDAL hitting the edge of the GeoTIFF while processing a contour line. These are very distracting visually. How can they be trimmed or removed?

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    First: get more DEM data outside your focus area than you need, you can trim off the contours outside your AOI later. Second: Merge your GeoTIFF into one file, that way there won't be internal edges. Third: smooth the DEM gis.stackexchange.com/questions/12833/smoothing-dem-using-grass then make your contours. This assumes you're only dealing with a relatively small area, for a larger area you'll hit the 2 GiB shapefile size limit depending on the ruggedness of the terrain and the contour interval.
    – Michael Stimson
    Commented Dec 5, 2023 at 5:59
  • Excellent advice. Could you post this as an answer so I can accept it?
    – Jeff W
    Commented Dec 5, 2023 at 12:14
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    Jeff W, please answer your own question expanding on my previous comment illuminating how you performed each of the tasks.
    – Michael Stimson
    Commented Dec 6, 2023 at 4:45

1 Answer 1

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Following the advice of Michael Stimson (see the response to the original question), this was quite easy.

First, add your GeoTIFF tiles:

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Open the "Processing Toolbox", and access GDAL-> Raster Miscellaneous -> Merge:

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Select the GeoTIFF tiles you want to merge:

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A new "Merged" layer is added to your Layers panel. Note that the tiles are normalized and appear cohesive:

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Back in the Processing Toolbox, select GDAL -> Raster Extraction -> Contour:
enter image description here

Select your merged layer, adjust the settings as required, and press "Run":

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Done!

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