# DTM Filter (slope-based)

Volker Wichmann (c) 2010, LASERDATA GmbH
The module can be used to filter a digital surface model (DSM), i.e. to classify its cells into bare earth and object cells (ground and nonground cells).
The module uses concepts described by VOSSELMAN (2000) and is based on the assumption that a large height difference between two nearby cells is unlikely to be caused by a steep slope in the terrain. The probability that the higher cell could be a ground point decreases if the distance between the two cells decreases. Therefore the filter defines the acceptable height difference between two cells as a function of the distance between the cells. A cell is classified as terrain if there is no other cell within the kernel search radius such that the height difference between these cells is larger than the allowed maximum height difference at the distance between these cells.
The approximate terrain slope parameter is used to modify the filter function to match the overall slope in the study area. A confidence interval may be used to reject outliers.
Reference:
VOSSELMAN, G. (2000): Slope based filtering of laser altimetry data. IAPRS, Vol. XXXIII, Part B3, Amsterdam, The Netherlands. pp. 935-942

## Parameters

Grid system
Grid system
  Grid system
Grid to filter
Input Grid
  The grid to filter.
Search Radius
Integer
  Search radius of kernel in cells.
  Minimum: 1.0
Approx. Terrain Slope
Floating point
  Approximate Terrain Slope [%]
Use Confidence Interval
Boolean
  Use 5 percent confidence interval
Bare Earth
Output Grid
  Filtered DTM
Removed Objects
Output Grid
  Objects removed from input grid