Tutorial - Classification of DTM
Tutorial - Classification of DTM
Tutorial - Classification of DTM
Generally 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
lays 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).
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 choose
Reset Classification option in Tools Dense Cloud menu and then select point classes
you wish to be discarded.
Ground points will be colored with brown color, low-points (noise) class points will be colored by
pink and unclassified points will remain white.
In the Assign Class dialog specify source and destination class points. If you need to reset the
classification results for selected points, choose Any Class in From field and Created (never classified) in
To field.
Build Mesh
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.
Set the following recommended values for the parameters in the Build Mesh dialog:
Surface type: Height Field
Source data: Dense cloud
Polygon count: High (maximum number of faces in the resulting model)
Interpolation: Enabled
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):
Export DEM
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 specified in the Reference pane settings dialog is used,
Crop invalid DEM: check this option to crop the regions with unreliable elevation data, since
they are visible on less than two source photos,
Pixel size: you can increase the effective resolution compared to the default value,
Split in blocks: 10000 x 10000 (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: 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.