LiDAR Indices Documentation


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Indices

Index Description Tags
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Concepts

Digital Terrain Model (DTM)

A raster of 1x1 meter pixel resolution is generated based on LiDAR ground points (of bounding box).

Digital Surface Model (DSM)

A raster of 1x1 meter pixel resolution is generated based on LiDAR top points (of bounding box).

Canopy Height Model (CHM)

A raster of 1x1 meter pixel resolution is generated by subtracting DTM from DSM (of bounding box).

CHM := DSM - DTM

Point height above ground

Height of LiDAR points is calculated by subtracting DTM value.
All points are included. An exact ground point leads to point height of zero.
The DTM resolution is 1x1 meter pixels. As LiDAR points may be more dense ground points may be a small fraction above and below zero height at rough ground.

height_value := z_value - dtm_value

Vertical aggregation levels / horizontal layering

LiDAR metrics that characterize the vertical distribution of vegetation by analysing the distribution of return heights often aggregate the returns into height strata or horizontal layers. From the literature we found that in LiDAR based studies commonly three layers are differentiated in temperate forests. For the standard metrics we define three layers for the analysis:

1. Regeneration: 0m < x ≤ 2m
2. Understory: 2m < x ≤ 5m
3. Canopy: 5m < x ≤ hmax

Return density

Return Densities (RD) quantify the number of returns in each layer. Here we define RD as the proportion of returns in a given height layer relative to the total number of returns.

Penetration Rate (pass through rate)

The RD-values need to be interpreted with caution as the number of returns in the sub-layers is depending on the vegetation density in the target layer and on the density of the above layers. In other words, if we find a low RD value from sub-canopy layers we cannot conclude that there is no vegetation as a low RD-value could also indicate a dense canopy layer that already reflects most of the beam energies. An alternative to RD is to calculate penetration rates (PR) (pass through rate) as the proportion of returns that pass through a vertical layer (strata) defined by an upper and lower elevation boundary. Thus, the denominator is always adapted to the number of returns that enter the layer.

Vegetation Coverage

Vegetation Coverage is based on CHM. It is the ratio of pixels above a given height:

vegetation_coverage := count_of_pixels_greater_equal_min / all_valid_pixels

Pulse

One LiDAR laser pulse may result to several returned signals. So one pulse may generate several points in data.

Surface Area

general rule: Surface Area of rough terrain is higher than area of flat terrain.

Area ratio is the ratio of surface area value to area of hypothetical flat area value of the examined region.

reference of algorithm:
Jenness, Jeff S. 2004. Calculating landscape surface area from digital elevation models. Wildlife Society Bulletin 32 (3): 829-839


Details


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