Clip or filter point cloud

[Neilw_05]

I am working with a Lidar dataset that is over 34 million points. It covers an area far larger than I need for our project. I have experimented with point clouds and am able to process and generate a surface from the data by applying a clip boundary to the surface. The problem with this is the surface only has about 540k points out the total dataset and it appears that the points have been greatly reduced in density. I am thinkking this is due to the way C3D limits large data sets to display only 1.5 million points and thus my surface data is highly filtered since it is extracted from a small area of the dataset. I have tried setting pointclouddisplay to 100%.

My question is: can I somehow filter my point cloud so that I can reduce the area of coverage and thus increase the density that is extracted into my surface?

C3D 2012.

[ericcollins6932]

The point cloud density display affects only that - the display. Also ensure you are only using ground points (assuming your lidar is classified by using ASPRS class 2 pts only). A half million points out of 34 million for a small subset "sounds" right. I work with about 50-60K lidar ground pts per square mile typically.

Finally ensure you have a polygon around only your area of interest and when adding pts to a surface, use it to bound your extraction.

[Neilw_05]

So I'll assume that I am getting the full density of points in my surface. I was expecting a higher density. I'll run some experiments to verify.

Thanks Eric.

[Neilw_05]

Upon further investigation I am seeing a significant difference between the surface generated from point clouds vs. using the point files directly.

My approach has been as follows:

Point Clouds

1) Create point clouds from point files

2) Create emptysurface

3) Add point clouds to surface using site boundary to limit extents

4) Add breaklines from CAD graphics to surface

5) Add Site boundary to surface as outer boundary

Resulting Surface has 293,384 points

Surface from Point Files

1) Create empty surface

2) Add site boundary as Data Clip boundry

3) Add point files to surface definition

4) Add breaklines from CAD graphics to surface

5) Add Site Boundary as outer boundary

Resulting Surface has 504,256 points

I am in the process of investigating the cause of the difference

[Neilw_05]

I would like some clarification on this issue. Here is what I am seeing:

The statisics for one of my point cloud objects in the properties are as follows:

Total number of points: 7381118

Maximum number of points displayed 230810

When I select add point cloud to surface with Point Cloud Extents option the summary reports:

Region Option: Point Cloud Extents

Number of points to be added: 230810

Thus it appears that the Pointclouddensity variable is influencing the surface model.

I read in Mastering C3D 2012 that display density does not affect the surface model. Something seems amiss.

[mathewkol]

Thatv slider definitely affects the surface model. I see few reasons to use a point cloud if you already have a normal text fileand all you want is a surface. Fewer steps, fewer weird limitations.

[ericcollins6932]

I concur with your findings, although that is NOT what one would expect. As to using LAS files over point files, one must remember the point files were INTERPOLATED from the LAS files and

1) are a derived product - one should always revert to the original dataset for the best results depending on gridding performed.

2) Point files could result in more points in the surface due to their regular interpolation origin. Less is better when working with large datasets for operational efficiency.

[Neilw_05]

After consulting with Support I've learned that the density of points is dependent on view scale. In other words if you zoom in on a point cloud, C3D will adjust the point display density accordingly, up to the maximum of 1.5 million points, and this density is what will control how many points get added to a surface. Thus the suggestion was to zoom to my project site within the point cloud and then add the points to the surface, which should result in a higher density of points added. However in my experiments using this workflow I could not get C3D to add more points to the surface. At this point I am not going to spend more time trying to make use of point clouds and will instead use the point files along with data clip boundaries.

[chierchio]

Is there any resolution to this problem? Or are point clouds with more than 1.5 million points to be abandoned?

[ericcollins6932]

Not abandoned, simply broken down into more manageable sizes.

C3D apparently can't handle more than that at once.

[antoniovinci]

I concur with Mr.Ericcollins6932: you have to decimate that point cloud, e.g. by means of the italian opensource Meshlab...

[bhaugi]

I have just determined similar results the hard way before finding this thread.  I'm working with LiDAR files that are only 1m accuracy to start with, and only in the 1.5 million point range, but still finding the same results.  Even with density at 100%, for what I consider a relatively small corridor of under 400 acres, I had to zoom in and create 6 separate surfaces, then paste those into one.  The resulting surface was under 3 million points at 1m density.  (the increase in points was the result of pieces from several individual .LAS files.

I don't so much have a problem with why, but the lack of documentation, and fact that I wouldn't have known had I not been doing due diligence comparing resulting contours. 

[ericcollins6932]

There is a best practices document for Civil 3D on the autodesk website that talks about large surfaces and what to do about them.

[bhaugi]

Thanks!  I had the older version but need to revisit the updated.