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Author Topic: Filtering Ground Points on sloped surfaces  (Read 1801 times)

BobvdMeij

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Filtering Ground Points on sloped surfaces
« on: August 10, 2018, 12:06:25 PM »
How does one go about filtering non-ground points on sloped surfaces, such as small lumps of vegetation on water barriers (dikes)? The Classify Ground Points-tool works quiet okay on horizontal surfaces, but its methodology seems to fail when surfaces are increasingly angled.

Which makes sense as the tool supposedly splits the pointcloud into cells of a user-defined size, followed by detecting the lowest point in each of these cells to arrive at an approximated DTM. Consequently all other points within each cell are validated with respect to meeting or exceeding the user-specified values for distance and angle (relative to the approximated DTM).

The first of the above two steps, however, becomes flawed when the surface represents a slope. The lowest detected point will always lie downwards of the slope within each cell. This point can be far from representing ground level. In fact it may even represent vegetation if the upward section of the surface within the cell is situated higher, athough being bare soil.

I have literally ran countless iterations, each time varying one or more of the three user-defined criteria. I have tried cell sizes between 0.1m and 20m, angles between 2 deg. and 40 deg. and distance-values as low as 0.01m or as high as 1m. Strangely enough, the output is hardly any different no matter what I try. Furthermore, lumps of vegetation that are easily discernible as non-ground by the naked eye are still classified as ground point regardless of the parameters set.

Attached is a screendump of the pointcloud displaying a subsection of the water barrier, please note how some sections of the vegetated cover clearly stand out from the surrounding area. This contrast is particularly well discernible when the pointcloud is visualized by lasses as this enhances shaded relief. As you can see, however, the far majority of the clearly outstanding vegetation is still classified as ground (brown color), and NOT as non-ground points (white).

jbueno6

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Re: Filtering Ground Points on sloped surfaces
« Reply #1 on: October 30, 2018, 06:01:34 PM »
Are you reset classification between all your different parameters?
I have used 0 degrees, 0.05 max distance, and 0.5 cell size and seems to classify

toxicmag

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Re: Filtering Ground Points on sloped surfaces
« Reply #2 on: March 15, 2019, 08:47:04 AM »
Strange Result.
Maybe an external tool can help in your situation. Cloudcompare offers interesting filter plugins and classification possibilities.  You will find them easily by searching the web.

Take a look at the CSF plugin for example: https://www.cloudcompare.org/doc/wiki/index.php?title=CSF_(plugin)
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