Dense Cloud Classification and DTM Generation
with Agisoft PhotoScan Professional
This tutorial illustrates how to perform dense point cloud classification in manual and automatic mode and how to produce Digital Terrain Model (DTM).
There are two options of dense point cloud classification: automatic division of all the points into two classes – ground points and the rest, and manual selection of a group of points to be placed in a certain class from the standard list known for LIDAR data. Dense cloud points classification opens way to customize Build Mesh step: you can choose what type of objects within the scene you would like to be reconstructed and indicate the corresponding point class as a source data for mesh generation. For example, mesh reconstruction based on ground points only, allows to export DTM (as opposed to DSM – Digital Surface Model, based on the complete dense point cloud).
Note: If you need guidance on how to build dense point cloud with PhotoScan refer to Tutorial (Intermediate level): Orthophoto and DEM Generation with Agisoft PhotoScan and follow all steps up to Build Mesh. It is also important to georeference your model prior to proceeding to the steps mentioned below.
Classify Ground Points
PhotoScan provides instrument for automatic ground point classification tool.
Select Classify Ground Points... command from the Tools menu -> Dense Cloud submenu:
Automatic classification procedure consists of two steps. At the first step the dense cloud is divided into cells of a certain size. In each cell the lowest point is detected. Triangulation of these points gives the first approximation of the terrain model.
At the second step new point is added to the ground class, providing that it satisfies two conditions: it lies within a certain distance from the terrain model and that the angle between terrain model and the line to connect this new point with a point from a ground class is less than a certain angle. The second step is repeated while there still are points to be checked.
The following parameters control automatic ground points classification procedure:
Max angle (deg)
Determines one of the conditions to be checked while testing a point as a ground one, i.e. sets limitation for an angle between terrain model and the line to connect the point in question with a point from a ground class. In fact, this parameter determines the assumption for the maximum slope of the ground within the scene.
Max distance (m)
Determines one of the conditions to be checked while testing a point as a ground one, i.e. sets limitation for a distance between the point in question and terrain model. In fact, this parameter determines the assumption for the maximum variation of the ground elevation at a time.
Cell size (m)
Determines the size of the cells for point cloud to be divided into as a preparatory step in ground points classification procedure. Cell size should be indicated with respect to the size of the largest area within the scene that does not contain any ground points, e. g. building or close forest.
In case the result of automatic dense cloud classification is not acceptable the procedure can be re-run using adjusted parameters (for example, if some on-ground objects like stones and small bushes were classified as ground points it is reasonable to reduce Max angle and Max distance parameter values).
Ground points will be colored with brown color, low-points (noise) class points will be colored by pink and unclassified points will remain white.
Note: It is important to reset the assigned classes for the dense cloud prior to starting the procedure again with the new set of parameters. To Reset classification results use Reset Classification option from Tools Menu -> Dense Cloud submenu.
Classify Points Manually
Dense cloud points may be classified manually, also the same workflow allows to reset the classification results for the selected dense cloud points.
Switch to the Dense Cloud or Dense Cloud Classes view mode using the corresponding buttons located on the Toolbar. Then select points in the Model view to be placed to a certain class using Rectangle Selection, Circle Selection or Free-Form Selection tools.
To add new points to the current selection hold the Ctrl key during selection of additional points. To remove some points from the current selection hold the Shift key during selection of points to be removed.
When all the points to be assigned to any class are in selection, choose Assign Class... command from the Tools menu:
In the Assign Class dialog specify source and destination class points. If you need to reset the classification results, choose Any Class in From field and Created (never classified) in To field.
New when the dense point cloud has been generated it is possible to generate polygonal mesh model based on the certain point classes. To produce Digital Terrain Model (DTM) on ground point class should be used for mesh reconstruction.
Select Build Mesh command from the Workflow menu:
- Object type: Height Field
- Source data: Dense cloud
- Polygon count: High (maximum number of faces in the resulting model. The values indicated next to High/Medium/Low preset labels are based on the number of points in the dense cloud)
- Interpolation: Enabled or Extrapolated
- Point classes: Ground (use Select... button to choose the point classes to be used for mesh generation)
Uncheck all classes except Ground class only and click OK button to start building geometry.
Here are results of mesh generation based on Ground point class only (DTM) and based on all the dense cloud points (DSM):
After the polygonal model reconstruction to export DTM use the standard export instruments.
Select Export DEM -> Export GeoTIFF/BIL/XYZ command from File menu.
Set the following recommended values for the parameters in the Export DEM dialog:
- Projection type: Geographic
- Projection: by default the projection set in the Reference Settings is used
- Crop invalid DEM: enabled (check this option to crop the regions with unreliable elevation data, since they are visible on less than two source photos)
- No-data value: Default value
- Pixel size: Default value (you can increase the effective resolution compared to the default value)
- Split in blocks: disabled (if the exported area is large it is recommended to enable Split in blocks feature, since the memory consumption is rather high at exporting stage)
- Region: disabled (set the boundaries of the model's part that should be presented as DEM.)
Click Export... button and then specify target file name and select type of the exported file (e.g. GeoTIFF). Click Save button to start DEM generation.