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dtm data sources

Mat Caird

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Hi all

In the VWA9.01 manual it says "The source data for the site can be 3d loci, 3d polygons, or surveyor data"

Is this a typo (the 3d polygon bit). The following paragraphs describe how to create the DTM with 2d polys, but not 3d polys.

Otherwise, how does one use 3d polygons to create a DTM?


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You start with 2d polys and then convert them to 3d polys.

2d objects are only flat. They don't have z settings. Therefore, the DTM can't create anything other than something flat, a piece of paper for example.

If you want depth to your DTM, which I imagine you do, then you need to use 3d polys that have various z heights.

If you start with 2d polys, there is a convert 2d poly to 3d poly command in the Sitework menu (VW 9) or the AEC menu in VW 10.

I prefer working with 3d polys rather than 3d locus points in case there is an instance where information sits on top of each other. If you have crossing polys, the dtm will fail. There is a check 3d polys command and filter 3d polys command. The first allows you to check the polys for crossing data. If there is crossing data, it will tell you how many instances and where the first one is based on x,y coordinates.

The second allows you to filter out some of the 3d polys in case there are too many in the drawing making it difficult for the modeler to complete it's task. There are some cases where too many polys (gernally large dtm's) will cause the modeler to fail and build nothing. If this is the case, the filter 3d polys will filter polys based on an interval you specify in the dialog box. It's very helpful and keeps the drawings a little more basic and easier to work with. The more complex the dtm is, the longer it's going to take to generate and work with. I've seen some DTMs take a few hours to generate. Of course, that was someone drawing an entire city in the mountains.

The grid entry method places a 3d poly based on the z height you specify. Of course, if you do it this way, you cannot overlap data. This is a case where you wouldn't even need the check data command.

If you import survey information and you get stake objects. You can ungroup those and copy only the 3d locus points and create a DTM with those or with the stake objects. This is a case where there may be bad information in the survey file, causing the dtm modeler to have difficulties. This is probably the main reason I prefer working with 3d polys instead of locus points. It's a personal preference I guess.

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Go to that coordinate. You will see two polygons that cross each other.

You need to either delete one of the polygons OR you can reshape one of them using the reshape tool.

If you don't see two polys that cross each other or touch, you might have two polys on top of each other. This is a common mistable by the surveyor if you imported survey data. Not a biggie. Just select one of the polys and delete it. Make sure you delete the right one because they may have different z heights.

It's not uncommon for a surveyor to take a reading on one elevation then retake the reading and get a different elevation. Then they usually forget to erase the first instance. It's a human mistake.

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