View source for Canopy Height Model based on Airborne Laserscanning using LAStools

=Download of LAStools=
[https://rapidlasso.com/lastools/ LASTools] is software for efficient processing of LiDAR data (Laser scanning data) programmed by [https://rapidlasso.com rapidlasso GmbH]. There are two parts of LAStools: one part is Open Source (LGPL 2.1), the  other part is Closed Source and requires licensing for most commercial or government uses. The closed source software may be used without charge for non-profit personal, non-military educational, or non-profit humanitarian purposes. Note, that the output of the unlicensed version can be slightly distorted after certain point limits (1 million points) are exceeded.
Download the recent version as [http://lastools.org/download/LAStools.zip LAStools.zip file]. Extract the zip file to a folder without space (e.g. C:\LAStools).

=Configure Tools for LiDAR data in QGIS Processing Toolbox= 
Start QGIS.
Open  {{mitem|text=Processing --> Options --> Providers --> Tools for LiDAR data}}. 
Check the  {{button|text=Activate}} button and fill in the path to your local LAStools folder as shown below.
[[File:Qgis_lidar_tools.png|400px]]

=Point cloud visualization: lasview=
lasview is an OpenGL-based viewer for LiDAR in LAS/LAZ/ASCII format that can also edit or delete points as well as compute/display
a [https://en.wikipedia.org/wiki/Triangulated_irregular_network TIN] computed from (a selection of) points.
* In the search engine of the Processing Toolbox, type {{typed|text=lasview}} and select '''lasview''' under LAStools of the Tools for LiDAR data.
* Select a LiDAR data file in LAZ format as the input layer.
* Select color by {{button|text=elevation2}} from the drop down list.
* Click on {{button|text=Run}} to start the viewer.
[[File:Qgis_lasview.png|400px]]
* The viever will open in '''PAN''' mode. Left click and hold moving the cursor to rotate the point cloud. Press the space bar to switch to '''TRANSLATE''' or '''ZOOM''' mode and change the perspective interactively. Open the context menu with a right click to see other functions of lasview as seen below.
[[File:Qgis_lasviewer1.png|600px]]
* Type '''<c>''' to try other color maps.
[[File:Qgis_lasviewer2.png|600px]]
* Type '''<t>''' to create a TIN.
* Type '''<h>''' to try different shadings of the TIN.
[[File:Qgis_lasviewer3.png|600px]]
* Type '''<T>''' to return back to the point cloud view.
* For quality checking and detail views you can press <x> and select a cross section or smaller area of interest. Move it around with the arrow key. Holding down '''<SHIFT>''' or '''<ALT>''' is  taking larger or smaller steps.
[[File:Qgis_lasviewer4.png|600px]]
* Press <x> again and you see  only the selected area.
[[File:Qgis_lasviewer5.png|600px]]
* Jump back between cross section view and the full extend with '''<x>'''.
* Type '''<X>''' to switch the green bounding box off or on.
* You can also edit the classification of the points as well as delete them. For more details see the '''README''' file in the {{typed|text=C:\LAStools\bin}} folder or open the context menu with a right mouse click on the viewer.

=Point cloud metadata: lasinfo=
lasinfo reports the contents of the file header and a short summary of the points such as point density and spacing.
* In the search engine of the Processing Toolbox, type {{typed|text=lasinfo}} and select '''lasinfo''' under LAStools of the Tools for LiDAR data.
* Select a LiDAR data file in LAZ format as the input layer.
* Check the {{button|text=Compute density}} button.
* Enter name and path for the output text file.
* Click on {{button|text=Run}}.
* Open the output file with an text editor an report the most important LiDAR data characteristics:
# Which Coordinate Reference System is defined?
# What is the point density?
# What is the point spacing?
[[File:Qgis_lasinfo.png|300px]]

=Classify ground and non-ground points: lasground=
lasground is a tool for bare-earth extraction: it classifies LIDAR points into ground points (class = 2) and non-ground points (class = 1).
* In the search engine of the Processing Toolbox, type {{typed|text=lasground}} and select '''lasground''' under LAStools of the Tools for LiDAR data.
* Select a LiDAR data file in LAZ format as the input layer.
* Select as terrain type {{button|text=wilderness}}.
* Select {{button|text=default}} preprocessing.
* Enter name and path for the classified output file.
* Click on {{button|text=Run}}.
[[File:Qgis_lasground.png|300px]]


=Remove low and high outliers: lasheight=
lasheight removes low and high outliers that are often just noise (e.g. clouds or birds). By default lasheight uses the points classified as ground to construct a TIN and then calculates the height of all other points in respect to this ground surface TIN. with '-drop_above 50 -drop_below -3' all points that are 50 meters above the ground or 3 meters below the ground are removed from the output LAZ file.
* In the search engine of the Processing Toolbox, type {{typed|text=lasheight}} and select '''lasheight''' under LAStools of the Tools for LiDAR data.
* Select the LiDAR data file in LAZ format with the classifed ground points as the input layer.
* Check {{button|text=drop above}} and as drop-above height type {{typed|text=50}}.
* Check {{button|text=drop below}} and As drop-below height type {{typed|text=-3}}.
* Enter name and path for the denoised output file.
* Click on {{button|text=Run}}.
[[File:Qgis_lasheight.png|300px]]

=Identify tree layer in the denoised file: lasclassify=
lasclassify identifies buildings and trees (high vegetation, class = 5) in a denoised file.
* In the search engine of the Processing Toolbox, type {{typed|text=lasclassify}} and select '''lasclassify''' under LAStools of the Tools for LiDAR data.
* Select the denoised LiDAR data file in LAZ format as the input layer.
* Enter name and path for the classified output file.
* Click on {{button|text=Run}}.
[[File:Qgis_lasclassify.png|300px]]

=Create a normalized point cloud: lasheight=
lasheight creates a point cloud with normalized heights with the option '-replace_z'.
* In the search engine of the Processing Toolbox, type {{typed|text=lasheight}} and select '''lasheight''' under LAStools of the Tools for LiDAR data.
* Select the LiDAR data file in LAZ format with the classifed ground points as input layer.
* Check {{button|text=replace z}}.
* Check {{button|text=drop above}} and as drop-above height type {{typed|text=50}}.
* Enter name and path for the normalized output file.
* Click on {{button|text=Run}}.
[[File:Qgis_lasheight2.png|300px]]

=Create a CHM directly from height-normalized points: lasthin and las2dem=
A Canopy Height Model (CHM) can be computed as a difference between DSM and DTM. At first A DTM is created from the ground returns and a DSM from the first returns. The two rasters are subtracted from one another.
A different approach computes the CHM directly from height-normalized LiDAR points in a two step approach:
(1) keep only one highest return per grid cell with lasthin (2) interpolate all these highest returns with las2dem.
 
* In the search engine of the Processing Toolbox, type {{typed|text=lasthin}} and select '''lasthin''' under LAStools of the Tools for LiDAR data.
* Select the LiDAR data file in LAZ format with the height-normalized ground points as input layer.
* The size of grid is {{typed|text=0.25}} which is about rhe mean point spacing of the input LiDAR file.
* We keep the {{button|text=highest}} points per cell. 
* As additional option we define a circular footprint with a radius of 10cm typing {{typed|text=-subcircle 0.1}}.
* Enter name and path for a temporary output file.
* Click on {{button|text=Run}}.
[[File:Qgis_lastin.png|400px]]

* In the search engine of the Processing Toolbox, type {{typed|text=las2dem}} and select '''las2dem''' under LAStools of the Tools for LiDAR data.
* Select the temporary LiDAR data file as input layer.
* The step or pixel size is {{typed|text=0.25}}m.
* Select the Attribute {{button|text=elevation}} and the Product {{button|text=actual values}}.
* The additional command line parameters{{typed|text=-epsg 25832}} defines the output Coordinate reference system (ETRS/UTM32) and {{typed|text=-spike_free 0.9}} defines an additional filtering.
* Enter name and path for an output raster file in format GeoTiff.
[[File:Qgis_las2dem.png|300px]]

=Create a DSM with abolute heights: las2dem=
A digital surface model (DSM) in raster format can be used as basis for a precise orthorectification ("True Orthos") of aerial images.
* In the search engine of the Processing Toolbox, type {{typed|text=las2dem}} and select '''las2dem''' under LAStools of the Tools for LiDAR data.
* Select the classified LiDAR data file (see above) as input layer.
* The step or pixel size is {{typed|text=0.25}}m.
* Select the Attribute {{button|text=elevation}} and the Product {{button|text=actual values}}.
* The additional command line parameter {{typed|text=-epsg 25832}} defines the output Coordinate reference system (ETRS/UTM32) and takes only the {{typed|text=first_only}} returns.
* Enter name and path for an output raster file in format GeoTiff.
[[File:Qgis_las2dem2.png|300px]]
If the Product type is changed to {{button|text=hillshade}} and a shaded relief is produced as shown below. 
[[File:Qgis_las2dem3.png|300px]]

=Create a DTM with abolute heights: las2dem=
A Digital Terrain Model can be produced in raster format.
* Use again. '''las2dem''' under LAStools.
* Select the classified LiDAR data file (see above) as input layer.
* The step or pixel size is {{typed|text=1.0}}m.
* Select the Attribute {{button|text=elevation}} and the Product {{button|text=actual values}}.
* The additional command line parameter {{typed|text=-epsg 25832}} defines the output Coordinate reference system (ETRS/UTM32) and only the {{typed|text=first_only}} returns.
* Enter name and path for an output raster file in format GeoTiff.
[[File:Qgis_las2dem4.png|300px]]
Change the Product type to {{button|text=hillshade}} and a shaded relief is produced as shown below. 
[[File:Qgis_las2dem5.png|300px]]

[[Category:QGIS Tutorial]]