LiDAR Data, Data Clip Boundary

[silvernich]

Quick sanity check question on surface boundaries,

So since I have a LiDAR surface, I added a Data Clip boundary to it and moved it to the top of the definitions so that there is now no longer data outside of that boundary.

However, the surface still triangulates on the concave portions along the boundary. Thus, I added the same polyline as an outer boundary as well. My question is, that additional data added outside of the boundary is still there but isn't displayed?

I had only ever used Outer boundaries to this point, but with LiDAR I would like to shrink these down with a Data Clip.

Thanks,

Nich

[sboon · Accepted solution]

Lidar data is just points.  If you import those to a surface then the software will build a TIN that connects them all together.   An outer boundary applied after that cuts away all of the excess TIN lines and points outside of it.  Of course you're wasting a lot of time creating all of this excess TIN just to get rid of it immediately after.

If you apply a Data Clip boundary first then all of those points outside the boundary are never imported.  The TIN process works just the same but with less data.  If you apply the same polyline as an outer boundary there will still be TIN lines across concave sections to remove, but many less than before.

Steve
Please use the Accept as Solution or Kudo buttons when appropriate

[ksorsby]

You can get rid of virtually all of the concave triangulations quite easily. One the surface is imported, under surface properties, definition tab, unfold build and in there you can set a maximum triangle length.

LiDAR data is usually a grid of a particular resolution so if the grid spacing is 1m, you can set the maximum length to be 1.5m so it will only ever triangulate across a 1m square (hypotenuse is around 1.4m). Similarly, for a 2m grid, use 2.9m.

If you want it to trianulate across 2 squares, for a 1m grid use 2.3m and so on.

Works pretty well actually.

Kevin

[Joe-Bouza]

The LIDAR I have is a TIFF. are there more than one kind of dataset?

[ksorsby]

Yes. All kinds of resolutions and file formats. I can get down to 12cm grid. ASC, XYZ, LAS files, GeoTIFF's and more I'm sure.

[Joe-Bouza]

so there are no points from the tiff, correct? so as suggested above the surface cannot be reduced by including lidar point in a tin surface, yes?

[ksorsby]

No. The GeoTIFF's should have elevations (as I understand it, each pixel has an elevation) so you can easily build the surface from the GeoTIFF and you will end up with a surface that is essentially made from a grid of points (check the triangulation with a style). You can then do the build reduction, clip as necessary etc.

Kevin

[Joe-Bouza]

Sorry, I may have mis led you. I know geo tiff has elevation points, but how do I access them? I created my surface from DEM and it is too large for my needs. If I could get some of the point data to make a smaller surface that would be great

[ksorsby]

Draw a polyline/feature line as a boundary then add it to the surface definition as a data clip boundary. Move to the front of the build order so the clip is above the TIFF import in the list and it should work.

Once the geoTIFF is imported, it's just a normal set of points defining the surface which can be clipped or modified accordingly. Change the style to something which shows the surface points and you'll see them.

Kevin

[silvernich]

Steve,

My research on this topic seems to state that with an Outer Boundary, the data is still there and is still used for calculations. This is my concern, especially when the triangulation happens on concave edges.

Are you saying this is not the case? That only the area within the Outer Boundary is used?

Thank you for your thoughts,

Nich

[sboon]

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@silvernich wrote:

Steve,

My research on this topic seems to state that with an Outer Boundary, the data is still there and is still used for calculations. This is my concern, especially when the triangulation happens on concave edges.

Are you saying this is not the case? That only the area within the Outer Boundary is used?

Thank you for your thoughts,

Nich

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Your description sounds more like a Surface Mask which only hides the display of the surface but doesn't remove any TIN lines.  Calculations will be performed on areas of the surface hidden by the mask.

I tried an experiment which may help to show what happens when an outer boundary is applied to a surface.  This is one of the tutorial drawings, created from contour data.

In this picture I have moved the outer boundary inward so that some of the TIN is cut away and removed.  Calculations cannot be performed on an area without TIN, but the source data still exists in the excluded area.

In the third pic I have added a show boundary surrounding the whole area.  This causes the surface to recreate the TIN in the area previously removed.  At this point every time the surface is rebuilt the entire TIN would be created, then cut away outside of the outer boundary, then recreated again by the show boundary.

Steve
Please use the Accept as Solution or Kudo buttons when appropriate

[silvernich]

Steve,

So basically you area saying, the Outer Boundary does indeed remove information with which any calculations would be done. This is what I have always assume. The data exists outside an Outer Boundary, but the TIN is created only within the Outer Boundary.

Thanks,

Nich

[AllenJessup]

You can Create a Cropped Surface

This will create a new smaller surface.

Allen Jessup

[sboon]

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@silvernich wrote:

Steve,

So basically you area saying, the Outer Boundary does indeed remove information with which any calculations would be done. This is what I have always assume. The data exists outside an Outer Boundary, but the TIN is created only within the Outer Boundary.

Thanks,

Nich

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If you look at the Definition tab for a surface you will see all of the pieces of data used to create that surface.  At each step in that list the TIN is updated.  Add a point group - the software builds the TIN.  Add breaklines and the software updates that TIN.  Add an outer boundary and the TIN is updated again, cutting away the excess as I described earlier.  Every time the surface rebuilds it goes through every step again.

This is why a Data clip boundary is important for large data sets - to prevent that TIN from being created and then immediately removed outside your area of interest.

[ksorsby]

An Outer Boundary just trims the surface at the boundary and so the data beyond the boundary is retained in the model, albeit not used. A Data Clip boundary disregards data beyond the boundary when building so it doesn't exist in the model, hence smaller models from larger data sets.

This is as I understand it. Might be wrong though.

Kevin