Surface Simplify

[kartografs]

when i read about Surface Simplification in books and posts it seems to be straight forward function. But it seems i have missed something..

For example in this post https://www.augi.com/articles/detail/the-civil-side-managing-large-surface-data/

it says

:

Now i try to simplify my LIDAR surface using Point Removal Method and choosing Reduction method as Maximum Change in elevation. In this example i wanted to remove points that would not affect surface by +/- 2.5cm (so together 5cm margin). That is how i understand it. But when i tried to compare simplified surface with original surface i was getting lots of areas where that was exceeded. By much more than i expected.. See image below where i created elevation banding where yellow is from -0.05cm to +0.05cm (10cm margin - double the size i was looking for) and red and green areas are respectively with higher cuts (like in image 10cm cut) of fills (greens).

Could anyone please shed some light for me on this function? I have obviously misunderstood something. What is that? And how to achieve 5cm margin for simplification process?

I am using latest and greatest C3D 2018.

Thank You!

Zig

[kartografs]

See also attached example file.

It is very simple.

Surface one - 5000 points. Surface 2 - copy of surface 1 and simplified by +/-0.025 cm.

Comparison surface showing differences.

Thank you!

[maximilian.kroner]

Hi @kartografs,

I did some tests with surfaces and compared them.

In my opinion those differences occur because the surface has to be triangulated again. Frankly I don't know how the algorithm that is doing the triangulation works but if it chooses to triangulate in a different way in some places this may cause elevation differences that exceed the threshold you input when simplifying the surface.

[Anonymous]

Do you have access to Infraworks360?  The latest versions of that has some new point cloud surface extraction tools which should work well with dense Lidar data

[kartografs]

Interesting.. C3D help file indicates that i am specifying maximum allowed difference in surface this function can make, but it does not honor that and can make the difference multiple times bigger than specified. How am i to use such function if it does not do what it says it is doing? Is this considered normal?

[kartografs]

Unfortunately i do not have Infraworks. Do you offer to use it because you know that there are issues with this Simplify function? Just trying to figure it out..

[Anonymous]

just a better and more efficient workflow

[kartografs]

Hello @maximilian.kroner,

Have you been able to find out more about simplify functions algorithm? So is this considered as normal behavior?

I am very weary on using this function since i have found out that it alters my surface much more than i would like to! Personally i have stopped using it since i found this issue, but i would very much like it to be functional. Has no one noticed it?

Thanks

Zig

[udo.huebner]

Hi Zig,

Since Max has taken other tasks, I would like to continue here instead.

I could follow your investigation. In some situations points are removed and the resulting difference in elevation is much higher than the given maximum elevation. I will ask the developers, if there is a more detailed description on how  Simplify Surface > Point Removal > Maximum Change in Elevation works.

Cheers Udo

[kartografs]

Hello Udo,

Have you been successful with finding out more details about this function?