How can I extract a Digital Elevation Model from a 3D model of a building, made with SketchUp, using Blender,qgis or any other software?
[GIS] How to extract DEM (Digital Elevation Model) from 3D model
3ddemqgissketchup
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There are several ways to accomplish what you want. You've left out some specifics such as what extensions you have available and what license level you're using, as well as how thoroughly you want to sample the raster (ie how fine is your grid). I'll make some assumptions.
The simplest, if you want a point for every cell in the DEM, is the Raster to Point tool. A point will be created at the center of every cell with an attribute that is the value of that cell. There's also the Raster to ASCII or Raster to Float tools if those would produce formats more readily imported into ACAD than a shapefile (ACAD links in the last paragraph would indicate they would work just as well as the original DEM).
If you don't want every cell, you first need to use Create Fishnet to generate the grid. There's an option called Create Label Points, which will create a separate point feature class with points at the center of every grid cell. It's simplest to use these rather than all the extra steps needed to convert your grid lines to points at the intersections (which can be done, and is discussed in other questions). Since the points are at the center of the cells, you'll need to shift your grid origin -x and -y one half of the cell x/y value to get the points where the intersections would be. For example, 10m square grid, origin at -5,-5 will put the label point at 0,0. The grid can be any dimensions you like, including identical to the DEM cell size though that would just duplicate the first method. Once you have the points, you can use the Extract Values to Points tool from Spatial Analyst as you mention to get the elevation values. You could also use the Sample tool to generate a table (not shapefile) with x and y coordinate columns as well as an elevation column.
You can also convert the raster directly to a TIN using Raster to TIN if you have 3D Analyst, however I don't know how compatible Arc's TIN format is with ACAD (if at all). That said, as Aaron mentions you should be able to add a raster DEM directly to ACAD. I found this ACAD KB article that suggests you can. There's also this blog post and a helpfile / user's guide page that covers it. In fact a quick Google search turned up a lot of results on the subject.
No need to convert your raster; I see what you're getting at, but there's an easier way - the Point Sampling Tool plugin.
Once you install this plugin, go to Plugins > Analyses > Point Sampling tool
Choose your point layer, and the raster band(s) you want to sample from at each point. It will also allow you to sample from vector layers. It'll create a new layer with the values added as attributes to each point.
If you don't want it as a separate layer, you can use join to pull the values back from this new layer into your original layer.
Best Answer
Export your terrain object from SketchUp as a .dae file (or .kmz if you're using the free version - rename the .kmz extension to .zip and extract it find the .dae file inside)
Open the .dae mesh in Meshlab (free) and export mesh to .xyz points format, ignoring normals. Now, you can add the .xyz as a layer in QGIS using 'add delimited text layer'. Specify your x and y column, 'space' as custom delimiter, and that you have no header row. Looking at the data table, column 3 will contain your Z values.
Remember that unless your model was already georefenced, you will probably need to translate, rotate and scale the resulting layer, as required.
You can now use the raster > interpolation module to create a raster based DEM.