Contour tool creating several parallel lines for each contour

[JacobLyons]

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!

[DavidPike]

d be interested to see the actual terrain profile, hillshade and slope maybe (on a subset of the data).  Can you zoom in and maybe set a hyposmetric tint or choropleth colour banding for the elevation raster?  and add labels with the contour values, query some raster cells at those contour locations etc.

What's the coordinate system/projection?

[JacobLyons]

Here's a zoomed in screenshot with hillshade shaded relief and labeled contours. The pixels in the DEM raster are Long Integers so I can't tell if they exactly line up with the contours. It almost looks like it tightly clusters increments of .3048ft  instead of spreading them out evenly.

Coordinate system is: NAD 1983 (2011) UTM Zone 18N

[DavidPike]

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.

[DavidPike]

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.

[JacobLyons]

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!