Point Clouds, Part 2: Working with Huge Point Clouds
This blog post was originally written for the Managed Design blog. Managed Design is now a U.S. CAD company, so please check back here at www.uscad.com/blog for more written articles from Bill Neuhauser and our new U.S. CAD Minnesota team.
In Part 2 of the Point Cloud series, I will be sharing a quick way of getting what you need from a huge point cloud, plus a lot more. Let’s create what I call a Hybrid Point Cloud. We’ll take a huge 2500 acre LIDAR created point cloud that is nearly 37 million points, and import it into Civil 3D. We will do this by eventually creating three separate DWGs; each with one surface. Using data shortcuts, we will reference two other surfaces into a third DWG, and paste them together to make a HYBRID surface that includes both.
Now DWG #1 we will call TOPO-existing ground – LD.dwg. In this file, we will import the .LAS files, from the LIDAR, into one point cloud called EG-LD. Next we’ll add this point cloud to a new surface also called EG-LD. By default, Civil 3D only uses 10% of the points from our original LIDAR data. This is fine, but we just went from a density of about one point per meter to about one point per 10 meters. This is acceptable for the areas outside and away from our proposed alignments, but within our ROW, this can be a bit troubling when you look at the EG profile along the alignment in some areas. Creating the low definition (LD) surface is pretty straightforward. Below is a point cloud with about 2500 acres that was flown with LIDAR:
As we zoom in you will start to see the resulting contours (level of detail is turned on).
Now, if we had upped the density of the used point data for the surface to 90% or maybe 100%, the issue we would now have is that the computer wouldn’t be able to process the data and we might get a fatal error or it might just be plain slow!
So the workaround is to bump the density up near the proposed alignments and keep the 10% density everywhere else. We do this by now taking this LD DWG and doing a simple “Save As” and creating a new HD DWG. We will call it Grnd-HD.dwg. I had previously laid out the proposed alignments in another DWG where I created a closed poly-line that is approximately 100 ft. offsets from the design alignments to give me a buffer zone around my design. This will be used as the boundary when we create the HD surface. The bottom red arrow points at a part of this closed poly-line that I purposely stretched to be near or crossing the outer edge of the point cloud box. This is to make it easier to grip the point cloud and then select the new poly-line.
Now as an FYI on how to change the density of the point cloud from low def to high def all you do is grip the point cloud and on the ribbon you’ll see the current density.
Just drag this value to the right or manually change the 10 value to 90 or 100. Now be careful here. When you change this, 100 percent of the resources being used will change somewhat, but what I have discovered (and reported to Autodesk) is that after I change the density to 100% and then save and leave the DWG, AutoCAD can’t now open the file and will fatal error out every time – essentially making the DWG locked or corrupted.
Here’s the workaround:
Go ahead and change the density to 100% then make your new HD surface and use the closed polyline as an outer boundary.
Now rename the surface as shown below, then select Next.
Here we will toggle the Object option and then choose the Define Region in Drawing. This is where you choose the closed polyline.
When the surface is complete you will get something like this.
Now depending on your surface you may need to go into the surface properties and toggle on the Use Max triangle length as shown below and set this value. The value will all depend on your surface. I was able to use 150 ft with no undesired results. This will clean up the boundary area within your curved areas.
Now change the density of the point cloud back to 10%. Now save and create your new data shortcut. You should now be able to leave the DWG and reopen it with no issues. The surface will still use the HD data but without the fatal error.
Last let’s create a third DWG and call this one Hybrid.dwg. All we need to do now is data shortcut in the two above surfaces – LD and HD. With these now in this new DWG we will create a new surface called Grnd-HYBRID. In this new surface under the Definition and Edits we will first paste the EG-LD surface and then the EG-HD surface into this new HYBRID surface.
If you should now turn on the TIN lines you will be able to see the HD areas of the surface in contrast with the LD areas. The result is now a usable HYBRID surface that is giving you the best of both worlds!