8

I am generating 2' contours made from a smoothed DEM that is 3.2808 foot (1m) horizontal resolution. The result shows these odd squares that are about 80' on a side. The smoothing was done using 8 neighbors (EDIT: I thought I used focal majority but I checked my notes and I actually did use a Focal Mean. I was also wrong about the 8 neighbors. It was a 9x9 neighborhood.). The DEM doesn't show these squares (see second screenshot below).

I drew one of the squares with the drawing tool to demonstrate, but I think they are pretty obvious.

Q) Can anyone provide any explanation for this, or a potential solution?

This result is from the "Contour" tool in "Spatial Analyst Tools -> Surface -> Contour" and I'm using 10.3.1

Contours showing rectangular pattern

Second screenshot showing the smoothed DEM (same general area but zoomed in a little) enter image description here

  • It just does, especially in flat areas, it has something to do with yes/no cell logic and reading blocks. I found a good smoothing makes them less evident (resample and/or focal mean). You could try using GDAL_Contour or if you have a Terrain, LAS dataset or TIN work from those - but from memory these methods are the same or worse. – Michael Stimson May 31 '16 at 22:30
  • I'd love to be supportive, but the only action I can suggest is sending DEM back to provider. Something terribly wrong with tiling/handling overlaps (?) they applied – FelixIP Jun 1 '16 at 1:42
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By default any interpolation (projecting/resampling) will be nearest neighbour and this is the issue most of the time (meant for categorical data NOT continuous). I would say focal majority is a poor choice as I would imagine the cells all have unique float values. Better off with the mean. You could also change the interpolation to bilinear and just use resample (Data Management>Raster>Raster Processing>Resample) as opposed to Focal Mean.

  • I'm fairly certain I used bilinear for the projection. I'm going to have to revisit my notes. Also I may have misremembered which filter I used (the logic of using focal mean is apparent to me and that may have been what I actually used. Thanks for the ideas. – jbchurchill Jun 1 '16 at 3:10
  • You might try going to C:\Program Files (x86)\ArcGIS\Desktop10.1\Utilities\AdvancedArcMapSettings.exe (or wherever it is for you) and just changing the default to bilinear in the Raster tab until you get this to work. Just in case there is something going on in the background. I vaguely remember a folder of dems provided as integer value in a geographic system that I needed to convert to float before projecting or else it wanted to output an integer...which made the stepping more prominent... could be related to pixel type maybe as well – Mike Jun 1 '16 at 18:35

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