Tool Name	DTM Filter (slope-based)
Tool ID	7
Library ID	grid_filter
Version	1.0
Author(s)	Volker Wichmann (c) 2010, LASERDATA GmbH

Description

The tool can be used to filter a digital elevation model in order to classify its cells into bare earth and object cells (ground and non-ground cells).
The tool uses concepts as 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 might be non-ground increases when the distance between the two cells decreases. Therefore the filter defines a maximum height difference (dz_max) between two cells as a function of the distance (d) between the cells (dz_max(d) = d). A cell is classified as terrain if there is no cell within the kernel radius to which the height difference is larger than the allowed maximum height difference at the distance between these two cells.
The approximate terrain slope (s) parameter is used to modify the filter function to match the overall slope in the study area (dz_max(d) = d * s).
A 5% confidence interval (ci = 1.65 * sqrt(2 * stddev)) may be used to modify the filter function even further by either relaxing (dz_max(d) = d * s + ci) or amplifying (dz_max(d) = d * s - ci) the filter criterium.

References

Vosselman, G. (2000): Slope based filtering of laser altimetry data. IAPRS, Vol. XXXIII, Part B3, Amsterdam, The Netherlands, 935-942.

Parameters

Name	Type	Identifier	Description	Constraints
Input
DEM	grid, input	INPUT	The grid to filter.
Output
Bare Earth	grid, output	GROUND	The filtered DEM containing only cells classified as ground.
Removed Objects	grid, output, optional	NONGROUND	The non-ground objects removed by the filter.
Options
Kernel Radius	integer number	RADIUS	The radius of the filter kernel [grid cells]. Must be large enough to reach ground cells next to non-ground objects.	Minimum: 1 Default: 5
Terrain Slope [%]	floating point number	TERRAINSLOPE	The approximate terrain slope [%]. Used to relax the filter criterium in steeper terrain.	Minimum: 0.000000 Default: 30.000000
Filter Modification	choice	FILTERMOD	Choose whether to apply the filter kernel without modification or to use a confidence interval to relax or amplify the height criterium.	Available Choices: [0] none [1] relax filter [2] amplify filter Default: 0
Standard Deviation	floating point number	STDDEV	The standard deviation used to calculate a 5% confidence interval applied to the height threshold [map units].	Minimum: 0.000000 Default: 0.100000