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I am working with 2 shapefiles. One contains the boundaries of a city, the other contains polylines representing the topographic features of the area. Essentially, the polylines make a topographic map.

I want to calculate the average elevation of the city. I have some basic GIS knowledge, so I can select out just the polylines in the city... but I'm not sure how to determine the city's average elevation using this data? I could average the polylines together controlling for their length, but that's not quite accurate - a long line that goes around a lake contains a lot more area at the same elevation than a long line that goes around a big hill, for example.

Suggestions on how to do this?

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    You would be a lot better off using a DEM and using Zonal Statistics if you have Spatial Analyst. – jbchurchill Oct 9 '14 at 19:21
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    As @jbchurchill remarks, you need a digital elevation model (DEM) to do this, period. Although it's possible to create one from the contour lines, that is a time-consuming and problematic exercise. Freely available DEMs at moderate to high resolution (100 meters would work just fine in most urban areas) now exist for every city in the world. – whuber Oct 9 '14 at 19:37
  • Thanks guys. Ended up finding a LIDAR raster file and using that. It was pretty simple at that point. – John Chrysostom Oct 10 '14 at 11:54

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