Contour tool creating several parallel lines for each contour

Question

Hi all,

I run into this issue every time I try to generate contours into a different unit - usually from meters into feet - where the contour tool (Spatial Analyst and 3D Analyst) outputs the contours correctly but generates 2 sometimes 4 sets of parallel lines for each contour. I have tried setting the contour interval to .3048 and I've tried setting the interval to 1 with a z-factor of 3.28084. Both achieve the same result. I have also tried smoothing the DEM before but that does nothing as well.  The DEMs I use claim to have pixel values of both meters and feet and I have made sure that the map's vertical units are set to feet when not using a z-factor.

I am at my wit's end here so any tips or advice would be greatly appreciated!

Duplicated_Parallel_Contours.jpg

JacobLyons · Answer

Yeah, I think it's because the raster resolution is at 1-meter. I just assumed it would evenly distribute the contours in between instead of clustering them together. Thanks for your help!

DavidPike · Answer

Also re-read and saw your DEM is a long integer in metres, but the tool is trying to calculate a precision of 0.3m when it uses 1 ft intervals.  I think that's very likely the cause of the strange output.  the only way i can immediately think of is to interpolate a higher resolution DEM, but it may be misleading and probably best avoided.

DavidPike · Answer

Ah I see there's very little variation in the elevation and you want contours of 1ft intervals?  I presumed these were big hills and saddles from the first picture! I wonder if this could be due to effectively tripling the precision, but the underlying DEM not having the resolution to enable it - causing this weird effect.

What's the post-spacing/cell-size of the DEM?

Also just to try it out - I wonder if you raster calculator a subset of the DEM * 3.208 then try the tool again with no Z factor and interval of 1.