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Nick C

Shapefile Data conversion doesn't look right

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I had a question regarding my conversion of shapefile data to DEM. I am working on a project that requires using shapefile data at 10 ft. contours. However, in order to get cross-sections, I convert the shapefile data to Feature Objects, then a TIN, and lastly a DEM. I am then taking cross-sections out of the data.

My question is why do my converted contours looks so far off from my original contours? The picture attached shows my original contours (grey) and my new DEM contours (yellow/green). The black line is a cross-section of a bridge crossing. As you can see, the cross-section is extract from the TIN data (aka the DEM contours) is a horrible representation of the original data. I ave many cross-sections, and editing them all would be a pain, and would also not show a true view or my routed river if I run is it through HEC-RAS.

Can I get my DEM/TIN to match my original contours, and just interpolate between the original lines?

Bad Contours.zip

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First of all, you would probably have a better match if you contoured the TIN instead of the DEM. Depending on what resolution you chose when you created the DEM, you could potential lose some elevation detail during the resampling process.

Actually, I'm not sure why you needed to create a DEM at all. When extracting cross-sections for HEC-RAS in WMS, you do this using a TIN, not a DEM. So if your extracted cross-sections are a poor representation of reality, there must be something funny going on with the TIN. One thing you could do is turn on the display of TIN triangles. This will show you what the underlying TIN surface looks like and should explain a lot. One possibility is that when converting shapefile contours to feature objects, then to a TIN, if you do not "redistribute vertices" along the feature arcs, the TIN ends up creating long, thin triangles between the contour lines that result in a poor elevation surface. Redistributing vertices is key in order to space out the triangle vertices, but you also want to be careful not to spread them out too far so as not to lose horizontal detail.

I hope this helps.

Clark

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