Create a bare earth DTM from a classified LiDAR LAS file with SAGA GIS

A LiDAR LAS file can have its point clouds classified under a few standard classes e.g. Ground, Low Vegetation, Building, etc. For generating a bare earth digital terrain model (DTM) without vegetation and structures, only the points classified as Ground class are necessary. An example is shown below.

It is relatively straightforward to use SAGA GIS to import the LiDAR LAS file, to extract out a subset containing only the ground points, and then to interpolate the points into a bare earth surface model. I tried out the following steps to create an ESRI ArcGrid ASCII file of the bare earth ground surface.

Import the LiDAR LAS file

1. Start up SAGA GIS. Select Modules | File | Shapes | Import | Import LAS Files.
   
   The Import LAS Files dialog box appears.
   
2. Select the Input File row. Click the [...] button on the right. In the Open dialog box, browse and select the LAS file, e.g. lub_tile1.las. Click Open.
   
   The selected file name is displayed in the Import LAS Files dialog box.
   
3. In the Attributes to import besides x,y,z list, toggle on classification and any other attributes you want.
   
   
   
4. Click Okay.
   
   The LAS file is imported into SAGA GIS.
   

Extract out Ground points

1. Select Modules | Shapes | Point Clouds | Tools | Point Cloud Reclassifier / Subset Extractor.
   
   The Point Cloud Reclassifier / Subset Extractor dialog box appears.
   
   
2. Click the Point Cloud row. Choose the newly imported point cloud in the drop down list, e.g. 01.lub_tile1.
   
3. Click the Point Cloud Attribute row. Choose classification.
   
4. In the Mode of operation row, choose Extract Subset.
   
5. In the Method single old value row, type in 2 for Ground class.
   
6. In the Method single new value row, type in 2 to keep it the same value in the subset.
   
   
   
7. Click Okay.
   
   The point cloud subset of only Ground points is created. By default, the new layer will have the string subset_classification appended to the input name.
   

Interpolating the bare earth ground surface

1. Select Modules | Grid | Gridding | Interpolation from Points | Triangulation.
   
   The Triangulation dialog box appears.
   Note: Other interpolation algorithm is also fine depending on your requirements.
   
2. In the Points row, choose the newly created subset e.g. 02.lub_tile1_subset_classification.
   
3. In the Attribute row, choose Z.
   
   
   
4. Click Okay.
   
   The User Defined Grid dialog box appears.
   
5. Optional. In the Cellsize row, type in the desired output cell size e.g. 1.
   
   
   
6. Click Okay.
   
   The grid layer is created.
   

Exporting the bare earth surface in ESRI ArcGrid ASCII format

1. Select Modules | File | Grid | Export | Export ESRI Arc/Info Grid.
   
   The Export ESRI Arc/Info Grid dialog box appears.
   
   
2. In the Grid system row, choose the grid system of the bare earth surface grid layer, e.g. 1;684x 685y; 312479.7x 5195216.9y.
   
3. In the Grid row, choose the bare earth grid layer, e.g. 01.lub_tile1_subset_classification(Triangulation).
   
4. In the File row, click the [...] button. Browse and specify the output file name, e.g. C:\Temp\dtm.asc. Click Save.
   
   
   
5. Click Okay.
   
   The bare earth ground surface is exported out.