Tool DTM Filter (slope-based)
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
- Author: Volker Wichmann (c) 2010, LASERDATA GmbH
- Menu: Grid|Filter
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 | Grid System | Grid system | PARAMETERS_GRID_SYSTEM | - | - |
| Kernel Radius | Integer | 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 | 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 | STDDEV | The standard deviation used to calculate a 5% confidence interval applied to the height threshold [map units]. | Minimum: 0.000000
Default: 0.100000 |
| (*) optional |
Command-line
Usage: saga_cmd grid_filter 7 [-INPUT <str>] [-GROUND <str>] [-NONGROUND <str>] [-RADIUS <num>] [-TERRAINSLOPE <double>] [-FILTERMOD <str>] [-STDDEV <double>]
-INPUT:<str> DEM
Grid, input
-GROUND:<str> Bare Earth
Grid, output
-NONGROUND:<str> Removed Objects
Grid, output, optional
-RADIUS:<num> Kernel Radius
Integer
Minimum: 1
Default: 5
-TERRAINSLOPE:<double> Terrain Slope [%]
Floating point
Minimum: 0.000000
Default: 30.000000
-FILTERMOD:<str> Filter Modification
Choice
Available Choices:
[0] none
[1] relax filter
[2] amplify filter
Default: 0
-STDDEV:<double> Standard Deviation
Floating point
Minimum: 0.000000
Default: 0.100000