Kearns and Breman 2010). Point decimation is the process of setting a defined grid size and using the point values that are contained within those grids to calculate summary statistics from those points.
Figure 4 from Brasington et al. (2012) illustrates this workflow (at left). On a cell by cell basis this operation is computationally expensive and is not a standard tool in popular GIS. ToPCAT uses the point values contained within the defined grid size to calculate the minimum, maximum, mean, detrended mean, range, standard deviation, detrended standard deviation, skew, detrended skew, kurtosis, detrended kurtosis, and point density. A minimum number of points is necessary, generally 4, to calculate the detrended standard deviation; the necessity to have more than one point within the cell resolution of analysis is in support of many of the concepts presented in Cell Resolution. Having these statistics in a regularly spaced grid is invaluable for developing a detrended standard deviation raster to represent surface roughness.
In the MBES Toolkit, you can run ToPCAT to export just the locally detrended standard deviation as a proxy roughness surface using theDetermining Grid Resolution. Having these statistics in a regularly spaced grid is invaluable for developing a detrended standard deviation raster to represent surface roughness.
Simple ToPCAT Roughness Tool.
The command-line version of ToPCAT is straight forward to use, but requires a fair degree of pre and post processing. The MBES Toolkit, now makes these pre and post processing steps seamless and allows you to run ToPCAT from an easy to use user interface. The two main commands are
ToPCAT Point Cloud Decimation.
- Brasington, J., D. Vericat, and I. Rychkov. 2012. Modeling river bed morphology, roughness, and surface sedimentology using high resolution terrestrial laser scanning. Water Resources Research 48. DOI: 10.1029/2012wr012223.
- Rychkov, I., J. Brasington, and D. Vericat. 2010. Processing and Modelling on Terrestrial Point Clouds. Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, Wales.
- Rychkov, I., J. Brasington, and D. Vericat. 2012. Computational and methodological aspects of terrestrial surface analysis based on point clouds. Computers & Geosciences 42:64-70. DOI: 10.1016/j.cageo.2012.02.011.
- PCTools 32 Bit Explanation on Joewheaton.org (former name of ToPCAT was PCTools)
- PCTools (Point Cloud Tools - the documented alternative to TLS Decimation Utility)