Individual Tree Detection (ITC)
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# Open the '''Point Sampling Tool''' clicking [[File:Qgis_psample_button.png]]. | # Open the '''Point Sampling Tool''' clicking [[File:Qgis_psample_button.png]]. | ||
# Specify the output of the resulting vector file marking column with '''CTRL + left click'''. | # Specify the output of the resulting vector file marking column with '''CTRL + left click'''. | ||
− | # Enter an output shapefile name '' | + | # Enter an output shapefile name ''seed_points_chm.shp'' and path by clicking {{button|text=Browse}}. |
# Confirm with {{button|text=OK}}. The new layer is added to the Layer Panel . | # Confirm with {{button|text=OK}}. The new layer is added to the Layer Panel . | ||
[[File:Qgis_point_sample2.png|300px]] | [[File:Qgis_point_sample2.png|300px]] | ||
+ | * Right click and open the Attribute table of the new vector layer. | ||
+ | * Toggle editing on. | ||
+ | * Select features using an expression. | ||
+ | * Type in the expression editor: {{typed|text="chm_gauss" <= 3}} and click {{button|text=Select}}. All point with height below 3 m are now selected. | ||
+ | [[File:Qgis_select_espression.png|300px]] | ||
+ | * Click '''Delete selected features'''. | ||
+ | * Toggle editing off. | ||
=Generate a seed grid= | =Generate a seed grid= |
Revision as of 00:10, 7 January 2018
Contents |
Filter the CHM derived from LiDAR data
We use a Canopy Height Model (CHM) derived from LiDAR data as decribed here to detect Individual Tree Crowns (ITC). Two preprocessing steps prepare a watershed segmentation approach: (1) Gaussian filtering and (2) inversion of a CHM.
- In the search engine of the Processing Toolbox, type smooth and select Smoothing (gaussian) under Image filtering of the Orfeo Toolbox.
- Select the CHM raster data file in GeoTiff format as input layer.
- The smoothing type is gaussian.
- The circular structuring element has a radius of 2 pixels.
- Enter name and path for an output file.
- Click on Run.
Invert the CHM
Now the smoothed CHM will be inverted.
- In the search engine of the Processing Toolbox, type invert and select Invert grid under Raster tools of SAGA.
- Select the smoothed CHM raster data file from previous step as input layer.
- Enter name and path for an output file.
- Click on Run.
Watershed segmentation
- In the search engine of the Processing Toolbox, type watershed and select Watershed segementation under Image Analysis of SAGA.
- Select the inverted and smoothed CHM raster data file as input Grid.
- The Output is Segment ID
- Select as Method the flow accumulation of Minima
- Seed points: enter name and path for a vector point output file.
- Click on Run.
Extracting tree heights
We extract normalized heights from the original CHM using the QGIS point sampling plugin.
- Click Plugins --> Manage and Install Plugins.
- Type in the search bar Point sampling tool, click on the plugin name and then on Install plugin.
- Load the single band raster file chm.tif.
- Load the vector point file Seed points.shp'.
- Make sure that both layers are ticked in the TOC.
- Open the Point Sampling Tool clicking .
- Specify the output of the resulting vector file marking column with CTRL + left click.
- Enter an output shapefile name seed_points_chm.shp and path by clicking Browse.
- Confirm with OK. The new layer is added to the Layer Panel .
- Right click and open the Attribute table of the new vector layer.
- Toggle editing on.
- Select features using an expression.
- Type in the expression editor: "chm_gauss" <= 3 and click Select. All point with height below 3 m are now selected.
- Click Delete selected features.
- Toggle editing off.