Ok, so this may be a more advanced topic than normally found on this forum. But I am struggling to classify the point clouds generated with the UAV. After Pix4D, after Agisoft or others you are left with an unclassified point cloud If I want to classify the buildings, vegetation, and ground points so I can then generate a DTM. How is everyone doing this? What software have you used, what settings?
The main problem I have found is that most point cloud manipulation software is designed for LiDAR, and LiDAR has one thing we do not have, multiple returns. So for them clasifying the ground is a matter of getting the last return and then just cleaning up the buildings. But most of these don't work so well with the point clouds from Pix4D, Agisoft, etc... I have tried LASTools, ESRI, and recently DTMaster. Any other suggestions? or suggestions on the settings to use?
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I agree Tazmoe.
Nonetheless, people here want fast results with limited knowledge of the photogrammetry limits. You essentially create an artificial DTM out of your imagination, not a real data behind. As for a vegetation, you have no idea where a ground is, just a best guess. For any man made structures or objects, you can at least assume. But at the end you always end up with a just non-real terrain model. Unless there is some better technology coming, we end up with virtual models, not real ones. Lidar has also limitations as you said, but it's the best we got so far.
Btw. very good topic and discussion here ! I will try my tests with Photoscan and DTM extraction too. But I assume I will end up with some Lidar tool at the end anyway.
Hi Mike
I do not completely agree with you. I have done several tests and found that the models I got were so close to reality. Several random point were picked from the Model and used dGPS to measure them on the ground and I had within 5-15cm. For earthworks this is excellent in my opinion. If was designing high rise, yeah this is not as accurate. I think the key is that you need to have the data referenced accurately with GCP spread well enough over the area. The main challenge I have seen when you have dense vegetation cover, it is simply not possible getting the data cleaned and have it accurate. This is where LiDAR has upper hand.
Yes it is a good discussion!
Not exactly "fast results with limited knowledge" I think is more like sharing your experience in DTM extraction using photogrammetry only. Due to the lack of specialized software (most of the software is design for Lidar data) this process is pretty complicated.
Have you tried Global Mapper's Lidar module yet? I've got it... Tried it, but with limited results. Just about every engineer I've talk to says get it done in India.
Robert
Filter ground points in Photoscan then export point cloud (ground only) to LAS format.
Then from Whitebox GAT tools run the next sequence Bare-Earth DEM (usually with 0.5m resolution) > Remove Off-Terrain Objects > Fill Missing Data Holes.
I'm getting good results using this method.
Of course you have to change parameters according with terrain type and DSM resolution to get the best of it.
I'm using Recap360 as well with a GoPro for now to learn how to do this and learn how to fly the drone w/out too much damage to a DSLR. Last week we put the XYZ coordinates in the upload & downloaded it to an RCS. I don't have that file on this computer, I gave it to one of the engineers at my husband's soils engineering company.
Here's at least the rendering of a little less than 200 photos. I'm hoping to upgrade equipment in the next few weeks since the flying is going well & I'm getting very close with the photogrammetry. These are of a landslide repair
I thought I would share my experience with Agisoft. Here is what Agisoft looks like when you are classifying the dense point cloud. The software takes all the points generated from the dense point cloud and breaks them into cell size which the user defined as Cell size. Then specify from the lowest point, z, in that cell how high to come up from that point (max distance) and what the angle (max angle) should be from the lowest point to any point included in the DTM surface. Everything that is brown is now in the newly classified ground surface, and everything that is white is separated out.
As you can see from the screen shot I am not filtering out the cars properly. I am getting the roofs separated but not the hood and lower parts. I had to end up manually removing them.
Bill Neuhauser P.E.
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