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Digital terrain model techniques


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Does any have advice for producing an all singing dancing DTM for the attached site plan? It's slopes about 9 m in elevation across the site.


I've used DTMs on occasion but nothing substantial. This is the first DTM that I've done which is reasonably large and complex and which we will rely on heavily for documentation.


1. I started by laying out roads using the Roadway tool but I soon ran into the limitations of this tool. Unless roads intersect nicely at rights angles etc it seems pretty useless. Is there a more flexible way? Model my own modifiers and roads?


2. I've also found in the past that it's easier to use the 3D Polygon tool to form site modifiers—placing them on the modifier Class—than it is to use the Site Modifiers tool. I find it easier to create the shapes I need. However there doesn't seem to be any way to use 3D Polygons and also produce a retaining edge. Am I missing something?


3. How best to group modifiers? Should I create large pads forming the mains steps in the site (which will include building sites, gardens, roads, etc.) or should I start off straight away modelling the roads and separate pads for gardens, building sites etc.?


Screen Shot 2017-01-12 at 12.36.00.png

Edited by Christiaan
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Christian - 
These are just opinions without actually doing any of the work. Here are some things to consider:


1. Roads - check out the NURBS roadway tool.  It might help for tricky parts of the roads.
• Otherwise, note that the Road tool basically “links” a bunch of pads. Each adjacent pair has a  coincident edge, or near coincident. The individual pads usually have slightly different slopes. More pads means smoother curves and slope transitions. The whole bunch of pads is surrounded by a grade limit. Use the Road tool or NURBS road tool for the easy parts, then draw your own pads for the non standard sections of road, eg the roundabout or places where the road width changes. And model the roads/walks with extrudes or other 3d tools.
• Modifiers can be made from 3d loci.  So, for instance, if you draw NURBS curves or 3d polys to represent the excavation edges of your roads (z value for each point is at bottom of paving), Duplicate Along Path a bunch of 3d loci. Insert those new points as site mods (I think as Pads) into the proposed model and surround with a grade limit. A second set of 3d loci can represent top of curb if needed, slightly offset outboard of the bottom of pavement.
• Road & walks would be modeled with 3d tools, eg extrudes or NURBS surfaces with shell, or lofts, etc 

2. 3d polys
In case you haven’t found it already explore the Create Objects from Shapes command. Draw a 2d poly (eg footprint of a house pad), Right Click and choose Create Object from Shapes, then choose Site Modifier and just accept the default.  In the OIP, choose Pad or, Pad with retaining edge, or Contour or whatever mod type is appropriate.


3. Strategy
• Is any of this site already developed with roads (even if they change?), buildings, etc? Watch my video about modifying with loci. Video is aimed at swales, but applies also to roads, paths, or any desired interruption of the site model surface:




Good luck, this looks like a lot of work




Edited by Benson Shaw
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  • 3 weeks later...

@J. Wallace - The only reason I convert to NURBS is to limit the number of vertices. And the only reason I do that is to prevent laggy behavior. Converting a curved 2d poly to a 3d poly produces way more vertices than converting curved 2d to NURBS then to 3d poly. It's my opinion that all the extra vertices do nothing to make a better site model, but they do slow down screen redraws, renders, etc.


But, your sample appears to be made from 2d polys with all straight segments. If that's the case, probably not much advantage to the intermediate NURBS conversion step.


Also, just a thought: Select one of those final, composed contours with the Reshape Tool (or dbl click) and examine the vertices to make sure that the composed segment end points are not stacks.  Drag one or two of the corner points sideways with the Reshape Tool and see if another vertex was hidden beneath (then UnDo). If stacks are present, try a direct conversion to 3d polys and test again. It would definitely be better if the converted 3d polys are continuous rather than collections of segments or rather than polys with stacks at the corners.  Stacked points can cause problems in VWX.



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How in god's name did I miss this!?!


Much love Jonathan, that's exactly what I needed! 🙏🏼🙏🏼🙏🏼


Edit: I remember now, I tried this tool but didn't go on to select the Reshape Tool. I wonder, for an object like this, if the Reshape Tool should be selected by default once the object is created?

Edited by Christiaan
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I haven't tried this yet my self but thinking about vertical road alignments and the approach we took when I was a young lad would be to define the slope of the centerline. Perhaps you could create the road centerline and offset for curb and gutter. Use as site modifier and assign a slope for each polygon. The last time I did a complex grading plan for a develop larger then this was i used pads for the road assigning the slope and or cross slope as they changed from, lets say 2% to 5%. The resulting solution didn't give me the changes at the curbs but it did give me accurate cut and fill volumes.

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