Contour line density thinning

Question

Would anyone know how to thin contour lines in high gradient areas such as a cliff face? Using Esri ArcGIS 10.1 with Maplex labeling engine.

Answer 1

With some manual work and a Spatial Analyst extension. I assume you have access to Spatial Analyst since you are able to generate contours from DEM?

I use a different approach - I show only major contours in areas with slope more that say 30 degrees but it will work the same for thinning contours with some tweaks.

Create a slope raster and generalize if need be

identify slope beyond which you want to thin the contours

Reclassify the slope raster to only 2 values (slope at which you want thick contours and slope at which you want to thin contours)

Once reclassified - export raster to polygons (GRIDCODE field should have only 2 values from the reclassified raster)

Depending on how much you generalized you may need to manually tweak this result in some areas, remove artifacts, etc.

Seperate the polygon layer which now contains only areas of "high gradient" and "low gradient" into 2 different layers.

Add 2 of the same contour feature classes to map and symbolize one layer with thick line and the other with thin.

Right click on Dataframe and select properties - go to Advanced Drawing Options

Mask Thick Contours with High Slope Gradient layer

Alternatively mask thin Contours with Low SLope Gradient Layer (not necessary if symbology is the same colour and if you position thin contours below thick contours)

For better map drawing performance you may want to actually CLIP the contours by the High and Low gradient layers since masking may result in slower drawing performance.

I run through the entire exercise as I was writing this... Below is a capture of the result. I generalized too much and my high slope areas could use some clean up but this is just a demonstration: