How do you trim or split a planar 3D polygon???

[Mia Nelson]

I have a huge contour map comprised of hundreds of planar 3D polygons. I need to cut out a piece in the middle of the map, to be able to plug in more accurate contours that I have from another source. How can I do this? The 2D trimming tools don't work on them, and all the 3D stuff seems to require a solid to work with. I need to maintain the continuity of the polygons, so busting them up into loci isn't an option...

Thanks in advance...

[Petri]

1. It does not matter whether you have 3D-polys or loci.

2. The "more accurate" contours may just make things worse, without providing any useful information. They may just be produced with another DTM-engine.

3. It is always best if you can get the original spot level data.

4. DTM is not an exact science.

[propstuff]

Whether or not your new data will be better that the old, I don't know, but here's a couple of (laborious) ways of doing what you want.

1/

-Draw the shape you want to cut out over the map.

-Extrude it to thicker than the polys

-Duplicate it (because it will be removed by the next action)

-Select it and the first poly and use Model>Subtract solids.

-Repeat the last 2 steps till finished........

2/

- Draw the shape you want to cut out

-Duplicate it (ditto)

-Select the first poly, use Model>3D Power Pack> Create Surface from Curves

-Use the Project and Trim Tool from the 3D tool set (second mode)

- repeat, repeat, repeat..........

-Select Modify>Convert>Convert to 3D polys if you want to turn the Trimmed NURBS Surfaces back to a Polys

not exactly fun, but it'll work.

HTH

[Mia Nelson]

I do need to end up with 3D polys in the final product...can those be created en masse from 3D loci if I converted everything to loci?

[Mia Nelson]

I tried both of these ideas, but the Model>Subtract solids is grayed, so I can't do that, and when I tried to use Model>3D Power Pack> Create Surface from Curves, an error comes up, "Surface could not be created from curve network"

I think the problem may be, that they are just linear polygons, just a line with a Z value. So it isn't a solid.

Any other ideas??

[mike m oz]

In your first post you indicate you are trying to use the 2D Trim tool to split the 3D polygons. This will not work on 3D objects, and hence this is why it will not "split" your 3D polygons. You need to use the Split tool.

I have just tried splitting a 3D polygon and it splits every time when I use the Split tool in Split by Point mode. (I created the 3D polygon by converting a 2D polygon into a 3D polygon.) Unless there is something really strange about your 3D polygon it should work for you as well.

An alternative to what you are trying to do would be to Reshape the existing 3D Polygons in the required area.

- Place your new contours in the correct location in a layer below the existing one.

- Make them a different colour so you can easily distinguish them.

- Select the existing 3D polygons one by one and Reshape them using the 3D Reshape tool until they are the same as the ones in the layer below. (The 3D Reshape tool will allow you to move, add and delete vertices as necessary.)

- When you have finished delete the contours on the layer below.

Edited February 3, 2007 by mike m oz

[Petri]

I do need to end up with 3D polys in the final product...can those be created en masse from 3D loci if I converted everything to loci?

Yes. The DTM generates the 3D-polygons; triangles, do be exact: the process is called TIN (triangulated irregular node or something like that).

[Petri]

I think the problem may be, that they are just linear polygons, just a line with a Z value.

I sense some potential problems here. At least in the past, the DTM engine has not been able to work if there is duplicated data. In your situation, it will find three coinciding coordinate triplet points where the "mesh" lines meet.