Point cloud classification's effect on contour lines and DTM

PIX4Dmapper PIX4Dmapper Questions/Troubleshooting
1

Good day,

I have two construction sites where the point cloud classification is really mixed up between road surface and ground. I.e road is classified as ground and ground is classified as road. Does this mix up between the two groups have any negative impact on the accuracy or ‘truthfulness’ of the contour lines or DTM? 

It would be a painstaking job to reclassify everything manually so if it does not negatively impact the DTM or contour lines then I guess it’s not worth the trouble to reclassify. Unless it is really necessary.

Thanks,

Johann

2

Hi Johann,

DTM is generated based only on the points assigned to the “Ground” class. If some elements that belong to the ground that are not assigned to this class, they will not be taken into account during DTM generation and would therefore affect the accuracy of the DTM. Contour lines can be generated based either on the DTM or the DSM.

So yes, it will affect contour lines and DTM.
However, reassignment to all points from one class to another should not be that painful if you uncheck all classes except the one you want to reassign, you can then select all points and then reassign them to the desired class.

Hope this will help,

Best,

3

So the DTM generation is only based on points from the class “ground”? It does not take any of the other point groups into consideration when being generated? For example the “road surface” class?  

So if I’m wanting to generate accurate contours for my project, I need to make sure I have everything classified as “ground” only and not “road surface” because they won’t be used in the generation of the DTM and therefore the contours? 

4

Hi Brett,

You are right. I have done a mistake in my previous comment.
DTM generation is based on the Ground and also the Road Surface point class as it is written in this article:

How to automatically generate a Digital Terrain Model (DTM).

"The DTM interpolation:

  • Takes into account all of the points in the densified point cloud except points that you assign to the Disabled point class.
  • Applies little to no smoothing to points that you assign to the Ground or Road Surface point class.
  • Applies smoothing to points that you assign to the Unclassified, High Vegetation, Building, Human-Made Objects, or any other point class."

Therefore, any points assigned to the point class “Ground” or “Road Surface” will not be smooth during DTM generation.
So if you generate contour lines from DTM, only these classes will be taken into account.

Best,

5

Marco-

If I have already generated the DTM in step 3 when processing, after I edit the point cloud do I need to re-generate the DTM if I want contour lines? - or in other words - if I generate new contour lines will it include the changes/edits I made to the point cloud without generating a new DTM?

Thanks,

Adrian

6

Adrian,

If you make changes to your point cloud after the DTM has been created in running step 3, then you will need to re-run that step in order for the DTM to reflect those changes and in turn produce the contours with those changes as well. You should always run steps 1 and 2, then edit your point cloud and then run step 3, when possible. That way your outputs from step 3 reflect any changes you made to your point cloud. Otherwise you’ll need to run step 3 again after making changes to the point cloud. 

 

hope this helps. 

7

Hello all!

I found this post and it is the closest one to the issue I have.

In one project I am working on, I run step 1 to 3 straight. After that, I needed some new contour lines, so I edited the point cloud and assigned everything I don’t want to see, to the group “Disable”. Basically my Point Cloud classification has been divided into two: Ground and Disable.

I run step 3 again to generate new DTM and Contour lines, BUT as a result, I have the same as before editing the Point Cloud so I got the same results.

Any idea why this happens? TIA.

Miguel.

8

Hello @MiguelToril, The steps you are following are good. The other thing you could do to save your time is the first process steps 1 & 2 and then edit the point cloud and then process step 3 only.
Regarding your issue, DTM utilizes the point cloud from the Ground and Road Surface. So, make sure all required points are defined in these classes. I don’t see any issue with your process, so make sure to follow the steps explained in the below support article and change the setting for contours lines so that you might see the changes more clearly.