Editing DTMs, multiple DTMs?

[P Retondo]

I have a few questions about DTMs that don't seem to be addressed in the manual:

1. Is it true that you can create only one DTM per document?

2. I frequently modify my DTM by changing the shape of the 3D polygons and regenerating the DTM. Sometimes I run into trouble doing this (errors). I had to delete all DTM info the last time this happened (even "Check polygons" command created an error ("-21"), until I deleted DTM). Any advice?

3. If I could generate two DTMs (one before grading, the other after, both generated from 3D polygons) I could calculate a net cut/fill by comparing the volumes of each. Is this possible without creating two documents?

Is there a technical description of the DTM process that goes into a more "behind the scenes" description of what is going on than we can find in the VWA manual?

VWA 10.1

WinXP

[Robert Anderson]

Responses:

1. You can technically create (3) DTMs per document, the "existing", "proposed" and "cut and fill" site models, each of which has a DTM associated with it.

2. Avoid conditions that create perfectly vertical or "concave" conditions. (Concave conditions result when topo lines cross. Think "cave".)

3. Draw your existing DTM data on a layer and run the Site Model command. Instead of repeating your DTM data for the new site, use modifiers (pads and fences) to create your proposed site. You will want to review pages 6-16 through 6-20 of your VectorWorks Architect 10 manual. Then run the Cut and Fill... command to see your cuts and fills.

[P Retondo]

Robert, thanks very much for your reply. Regarding item 3), I was aware of how to use the cut and fill tool after creating pads. But pads are not the only way of grading a site! I was hoping it was possible to compare two volumes created from a different sets of 3D polygons (orignal "topo lines" vs. modified), but if not I can always create them in duplicate documents and compare numbers.

[Robert Anderson]

It's possible to do this: For your second set of data, put all the 3D topo polys in the 'Site-DTM-Modifier' class (This will make them "pads" or "breaklines") and surround them with a 2D poly in the same class (this will be a 'Fence'). Then you will be able to treat your second set of data as a "modifier". HTH.

[P Retondo]

Robert, I have been able to create a "Proposed DTM" with a pad and fence, per instructions in the manual. But I have been unable to do what you describe here. In fact, adding a second 3D polygon (open or closed) within the area of the fence causes the program to crash when calculating a "Proposed DTM" object.

Query: once I have a DTM, can I just modify my 3D poly source data, update the "Proposed DTM", and then calculate the cut and fill? Or will updating in such a manner automatically modify the "Existing DTM"?

It turns out that I am unable to make this calculation from two separately-calculated DTMs because no volume calculation is generated from a DTM created with 3D polygon data.

[ 03-30-2003, 08:40 PM: Message edited by: P Retondo ]