5 replies to this topic

[batence1986]

Hi! I have digitized contours from a map. After that I made the terrain model. But the problem is that when I try to create contours from the terrain model, they don't coincide with the digitized contours I've made earlier.

On the picture below: the green contours are these, that I digitized, and the red ones - the contours from the terrain model.

I'd be very happy if anyone can help me!

Attached Files

•  Corel505.jpg   60.28K   38 downloads

[Charles Syrett]

New vector contours from a DEM will never match the original contours. Actually, the sample you attached looks pretty good! The contour re-entries (the pointed tips in the valleys) especially tend to become clipped off.

This is because the new contours are created from a grid of elevation points, which in turn is created from the original contours using one of any number of algorithms. Some algorithms are better than others at creating grids that are close to the landscape that the original contours portray. The DEST algorithm used by Manifold is touted as being very good at this, though my experience is that there are still some clipped valleys here and there.

What to do? Depending on what software you're using, you may be able to go into the problem areas and manually add extra elevation points, and then recreate the grid. It may even be as simple as making sure there are extra vertices within the contours themselves. Again, the exact how-to depends on your software.

Charles Syrett
Map Graphics
http://www.mapgraphics.com

[batence1986]

But another problem is that the area between the contours (digitized and those from the DTM) have one and the same elevation.

[Charles Syrett]

I don't know how that could happen. Can you recontour the DTM with a smaller contour interval and post the result? If what you're saying is true, there should be dense bands of parallel contour lines.

What software are you using?

Charles Syrett
Map Graphics
http://www.mapgraphics.com

[batence1986]

I made it with Autodesk Land Desktop 2006 and Bentley Microstation V8i. In the both cases the result is the same.

[Charles Syrett]

I'm not familiar with either of those apps; I use GIS rather than CAD to work with elevations. You may want to try online forums for those apps to find out why the gridding process isn't working properly.

Charles Syrett
Map Graphics
http://www.mapgraphics.com

I made it with Autodesk Land Desktop 2006 and Bentley Microstation V8i. In the both cases the result is the same.