Gap Fraction

Summary

Gap Fraction is a crucial variable that governs interactions between light and vegetation and requires accurate modelling to predict light climate in the canopy, photosynthetic activity or canopy reflectance.

Principle

The Gap Fraction (GF) is calculated using the following equation:

where nground is the number of ground points and n is the total number of vegetation and ground points. Note that all LiDAR points with a normalized height lower than the user-defined height break (2m is commonly used) are treated as ground points in gap fraction calculation (Richardson et al.,2009).

Usage

To generate the gap fraction, navigate to ALS Forest > Forest Metrics > Gap Fraction.

Gap Fraction

Settings

  • Input Data: Ensure that each input point cloud data is Normalize by DEM or Normalize by Ground Points. The input file can be a single file or multiple data files. Point cloud data should be opened in LiDAR360 before being processed.
  • XSize (meter)(default value is "15"): The length of a grid size should be greater than an individual tree crown width. For most forest types, the grid size should be greater than 15 meters.
  • YSize (meter)(default value is "15"): The width of a grid size should be greater than an individual tree crown width. For most forest types, the grid size should be greater than 15 meters.
  • HeightBreak (meter)(default value is "2"): Users can define a height break to treat all airborne LiDAR points as ground points in gap fraction analysis. A value of 2 m is commonly used.
  • Output Path: Path of the output file. A corresponding raster file (*.tif format) will be generated for each input point cloud data.
  • DefaultValue: Restore the default parameters.

Note: Only when the point cloud data is loaded in the software can you use the Gap Fraction function; otherwise, the message "There is no point cloud data meeting the conditions of calculation!" will pop up. If the maximum Z value of the point cloud is greater than 200 m or the maximum Z minus the minimum Z is greater than 200 m, the data is not considered to have been normalized, and the prompt information shown in the figure below will pop up. Click “YES” to keep using this type of data in the operation; otherwise, click "NO" and reselect the input data file.

Gap Fraction
    @inproceedings{
        author={Richardson J J, Moskal L M and Kim S H},
        title={Modeling approaches to estimate effective leaf area index from aerial discrete-return LIDAR},
        booktitle={Agricultural and Forest Meteorology,149: 1152-1160},
        year={2009}
    }

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