topic Re: Need Help with Generating Contour Lines in ArcGIS Spatial Analyst Questions

Need Help with Generating Contour Lines

PhilipSalter — Wed, 30 Mar 2011 22:13:20 GMT

I am having problems generating contour lines in ArcGIS 10. Please see the following steps that I have taken and let me know if I am missing something. (Also I have an attached screen shot of my results. The green lines are the newly created contour, and the blue lines are the existing contours that do not cover the entire area I need. The blue lines correspond precisely to a USGS Topo_DRG.)

1. Downloaded 10 Meter DEM from both the USGS Website and Geo Community
2. Created a Raster Data Set
3. Used Mosaic to combine the different sheets I needed
4. Created Contours for 20' intervals with a Z value of 3.280839895
5. Contours do not match the DRG/TOPO Lines (The ones in Green)

How do I create contours that are coincident to the 1:24k Topo DRG?

Thanks for your help!

Re: Need Help with Generating Contour Lines

GregoryLipps — Thu, 14 Apr 2011 12:30:04 GMT

The original USGS 7.5 topo maps were created by extracting an elevation point every 30 meters. Contour lines connect the same values and are shown wherever elevation changes every x units (x is larger in rugged terrain, smaller in flat terrain).

What you've created is more detailed, using a 10 m DEM. The jagged nature of your lines could probably be smoothed, but I think your contour lines look pretty good and are probably more precise than the old topo maps.

Greg

Re: Need Help with Generating Contour Lines

SteveLynch — Thu, 14 Apr 2011 14:36:46 GMT

A word of caution regarding post processing contour lines. You could end up with intersecting lines.

The recommended approach is to smooth the DEM and then contour it or experiment with a different contouring algorithm. Try the ContourWithBarriers tool and don't specify a barrier.

-Steve

Re: Need Help with Generating Contour Lines

JakubSisak — Fri, 15 Apr 2011 12:44:50 GMT

The jagged nature of your lines could probably be smoothed, but I think your contour lines look pretty good and are probably more precise than the old topo maps.
Greg

Unfortunately, there is no out-of-the-box smoothing algorythm in ArcGIS that is capable of doing a good job with this. Not enough smoothing factor will just round the corners, too much will cause the contours to cross each other.

Re: Need Help with Generating Contour Lines

JakubSisak — Fri, 15 Apr 2011 12:58:19 GMT

I often run into this problem with data from various sources. I once contacted a data provider here in Canada with a somewhat related issue and the response had to do with "...elevations readings are rounded to the nearest meter of elevation for the entire Canadian coverage..."
I tend to think this is a data issue.

I posted the reposnse from the data provider here: http://gis.stackexchange.com/questions/1795/where-can-i-download-1-arc-second-elevation-data-for-canada-other-than-geobase