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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Processing/classifying Point Cloud data requires knowing how to use the software and experience regardless of the software being used. Vegetation is why people use LiDAR because you will likely get points on the ground. LiDAR is an active sensor which emits a pulse and then receives it with the time and angle used to determine where the point is in real space. Creating point clouds from photos is not LiDAR that is why vegetation is a problem and why Photogrammetry (an older technology) is being supplanted by LiDAR. Using photos requires the same point on the "ground" to appear in at least two photos shot from different angles so that a stereo image can be created to determine the Z value of the point. Orthorectification of a single photo is required to determine the point's X and Y position.

Multiple returns do provide more information that can be derived from the data but is not required to determine the ground surface. Even with multiple returns the last return is not always on the actual ground. So it is not as simple as saying if I have all last returns I have the ground surface, You would have every solid object in that surface as well including roof tops, water towers, heavy vegetation, etc.

Point cloud classification software regardless of the vendor takes an investment of time to learn how to use them. No vendor has the "one button push" and produce a ground surface solution. They all have some semi automatic tools to help classify points but they all require manual cleanup for good results.   

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 Jaime, I used CANUPO recently to classify a fiber optic cable in a lidar scene and it worked great. Haven't tried it on UAV SfM yet but I imagine it would do pretty well.

Another vote for Global Mapper

Not only for lidar classification, but also great imagery rectification options and much much more.

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.

Hi Jaime

Have you had any luck with this? Global mapper as suggested by few, is quite useful in some cases, but some cases it has issues. Like you said, there are a few parameters to set, and I did this by try and error finally I got an excellent DTM. So I assumed my parameters would work for every situation, it turns out to be wrong. Right now I have some data from another project once again I'm going through try and error to see what I come up with. I have tried without success using LASTools, there is very clear explanation on how to use it and since it is sort of "free" application you can't get much support or be hard on getting support. I tried to contact the developer Dr. Martin and his reply wasn't so clear to me so I gave up. The one problem with these developers, they tend to think users will have same level of understanding as them hence they may provide with explanation on how to do things, but may not be clear to you as an end user who is trying to learn it as new product.

 

See images from my DSM and finally the DTM I processed with Global Mapper. I have included the Orthophoto to give you a clue of how the area looks like.

Follow this link to my Public Dropbox folder.

https://www.dropbox.com/sh/rxhrgaznnup3ioh/AAB3qDvhBOIq3PgGGyja6sHv...

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!

Kudos mate!

do you have tutorial or ways how to make dtm from UAV data? thanks

IronDOME said:

Hi Jaime

Have you had any luck with this? Global mapper as suggested by few, is quite useful in some cases, but some cases it has issues. Like you said, there are a few parameters to set, and I did this by try and error finally I got an excellent DTM. So I assumed my parameters would work for every situation, it turns out to be wrong. Right now I have some data from another project once again I'm going through try and error to see what I come up with. I have tried without success using LASTools, there is very clear explanation on how to use it and since it is sort of "free" application you can't get much support or be hard on getting support. I tried to contact the developer Dr. Martin and his reply wasn't so clear to me so I gave up. The one problem with these developers, they tend to think users will have same level of understanding as them hence they may provide with explanation on how to do things, but may not be clear to you as an end user who is trying to learn it as new product.

 

See images from my DSM and finally the DTM I processed with Global Mapper. I have included the Orthophoto to give you a clue of how the area looks like.

Follow this link to my Public Dropbox folder.

https://www.dropbox.com/sh/rxhrgaznnup3ioh/AAB3qDvhBOIq3PgGGyja6sHv...

hello, do you have tutorial this method? thanks

IronDOME said:

Hi Jaime

Have you had any luck with this? Global mapper as suggested by few, is quite useful in some cases, but some cases it has issues. Like you said, there are a few parameters to set, and I did this by try and error finally I got an excellent DTM. So I assumed my parameters would work for every situation, it turns out to be wrong. Right now I have some data from another project once again I'm going through try and error to see what I come up with. I have tried without success using LASTools, there is very clear explanation on how to use it and since it is sort of "free" application you can't get much support or be hard on getting support. I tried to contact the developer Dr. Martin and his reply wasn't so clear to me so I gave up. The one problem with these developers, they tend to think users will have same level of understanding as them hence they may provide with explanation on how to do things, but may not be clear to you as an end user who is trying to learn it as new product.

 

See images from my DSM and finally the DTM I processed with Global Mapper. I have included the Orthophoto to give you a clue of how the area looks like.

Follow this link to my Public Dropbox folder.

https://www.dropbox.com/sh/rxhrgaznnup3ioh/AAB3qDvhBOIq3PgGGyja6sHv...

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