¶ Drape
Draping is one of the most critical aspects of point cloud extraction, so a separate article is dedicated to this topic to help the user understand the concepts and how they work. Draping can help a lot but can also cause many issues if misused. With a better understanding of the tool, these issues can be avoided.
Drape, generally, takes one or more vertexes, inspects the vertex's environment and searches for point cloud points within a search radius (an infinite elevation tube). As it finds many point cloud points, it can be set to find the lowest, highest or average from these points, and the tool adjusts the initial vertex's Z value to this found value. See the following figure for a better understanding.
It seems pretty straightforward, but it's also very easy to make errors with drape, as the point clouds are not always the best. If there is water, the drape might catch the reflection, or if there is a gully or water drainage pit, and the laser could shoot inside the shaft, it can drag the vertex to the shaft, or if there are leaves in the road the bottom of the kerb might catch the leaves instead of the asphalt. It may also happen that the user uses a drape radius that is too big or too small.
Cross-checking all draped vertexes in 3D is strongly recommended to ensure no false vertex elevation remains. An experienced operator can spot a 2 cm mismatch in vector-cloud alignment in a 3D view. It is always the operator's responsibility to cross-check the drape results, and we highly recommend doing so. If classification is in use, it is also recommended to pay attention to the turned on and off classes in 2D OR(!) in 3D view and make sure the Drape inspects the correct view when searching for the point cloud points.
The drape tool always starts with the following setting panel:
The definition of the settings are the following:
Lowest/Highest/Average/Weighted Average radio button - The user can select the designated elevation for the vertexes. Select the appropriate.
Interpolation radius - The search radius in meter dimension.
Delta Z - A shift value compared to the found value. If the drape drapes a vertex to X elevation, the user can set this elevation to X+Delta Z.
Area—Select which view's classification settings you would like to use. If the 2D and 3D turned-on classes are different, this setting should be paid attention to.
Bidirectional—By default, the drape can drag a vertex only downwards. If the Bidirectional checkbox is selected, it can also pull the vertex upwards.
Drape connected vertices - If a draped object is connected to another object in the same SHP file, it will be draped as well to avoid topology errors.
¶ Recommendations
It is common for users to misuse the tools, which can cause financial disadvantages. The developers cannot take any responsibility for the user's loss because of misuse of PCS tools. Still, we would like to share some best practices to help our users develop their own experience or highlight potential weak spots.
- If the user wants to drape an object to the ground level, DO NOT USE HIGHEST SETTING!!! Even if it seems that there is no noise or obscuring object in the cloud, it is highly unrecommended to do so.
- Pay attention to the connecting element vertexes. As of 2024.06.25, the PCS offers the checkbox to drape the connected vertices as well. Pay attention to the connecting vertices and crosscheck the drape results in 3D.
- If there is water or the scan has been taken after rain, it is highly recommended to cross-check the results, as water surfaces can "mirror" the objects, which means that the drape will drag the vertex lower than it is in real life. The same applies to other challenging weather conditions like snow and fog.
- Every user shall experience the perfect drape radius, as it depends on many factors. We recommend experimenting with a 10 cm search radius for general use. This works fine for TLS and MLS uses. If a TLS cloud is used, which is denser, 5 cm can be good for a starter, but this would not fit further areas from the scan position. Talking about ULS scans, we can go up to 20-50 cm; for ALS, even 1-2 meters might needed. Try out different settings which would work.
- Use the Average, Weighted Average, and Highest settings only if a specific task requires them. The lowest setting can adequately cover 95% of general use cases.
- If the user modifies the Delta Z value, please note that this setting remains after you restart the software, which can cause issues in other projects. Do not forget to set it back to 0 if it is being used.