Friday, August 04, 2006

Which Aerial Topo Drawing do I use to build my TIN?

When you receive aerial topography, you are often presented with two drawings.

1) Contours and Features (trees, buildings, etc)
2) DTM (softbreaklines, hardbreaklines, obscured breaklines, points)

Most people ignore the DTM drawing because it looks unfamiliar. Land Desktop made it a little bit difficult to build surfaces from those little bits of text or acad points that come in the drawings (it was possible to use them, but a little digging to find out how).

Most people build their surface from the contour drawing. We all understand contours and building a surface from contours is very straightforward.

Depending on your aerial topography company, the DTM drawing is actually the raw material from which they create the contours. So the DTM is the better source for surface data. (If your aerial topo company does not provide you this drawing, ask them what they might be able to give you instead, like perhaps LandXML, a TIN or a grid surface)

Also note that I have been in many offices in this region and asked the engineer for the DTM drawing- and they say they don't get one. But when we look on the disk provided by the aerial topo company, there is actually a DTM drawing in there. They just didn't ever go looking for it or notice it.

I wanted to see if there were other reasons why a person should consider using the DTM drawing.

So I did a little experiment. I took the contour drawing and DTM drawing from a site, and built surfaces from each to compare.

I had some problems uploading images to blogger over the weekend, so click on these images actually takes you out to flickr. It didn't work quite as well as I had hoped, so I hope you can still make out what I am trying to show.

Surface built from DTM drawing

Close up of surface built from DTM drawing

Surface Built from Contour Drawing

TIN8

Then, I applied the minimize flat areas tools to the contour surfaces to see if that would make a difference.
Contour Flat Triangle Elimination Tools

The results?

Using the DTM Created Surface as the "baseline", it is pretty clear that using a surface created from contours is certainly different. Minimizing flat areas didn't really make a difference, and weeding made the difference larger for sure. But what i found interesting is that the biggest jump is from DTM to Contours. Weeding the contours certainly make the gap larger, but not as big a gap as from DTM to Contours.

Spreadsheet showing differences between different surface builds
Why does this matter? I don't know. Have a talk with your aerial topography company and find out what they recommend. Perhaps they can give you a DTM drawing. Other acceptable data sources would include: 3D Grid, 3D Faces, Polyface, .TIN file, LandXML and similar.

1 Comment:

jpostlewait said...

I think we have identified why we were talking apples and oranges on the large surface thread.
Long time ago when Aerial photo companies started to deliver electronic packages, we always required the DTM and a hard copy of the contour map.
I didn't see the we use derived data, the electronic contours, bus coming.
I hope Laurie and Connie B have helped you get your "arms around" this.
Got to get the model not the map.

Your buddy
John P.