This blog post is written by Clayton Crawford, a Product Engineer in the Software Products Group's 3D Team in Redlands.
Estimating Forest Canopy Density and Height
Canopy density and height are used as variables in a number of applications. These include the estimation of biomass, forest extent and condition, and biodiversity. Canopy density, or canopy cover, is the ratio of vegetation to ground as seen from the air. Canopy height measures how far above the ground the top of the canopy is. Lidar can be used to determine both.
What follows are steps to calculate canopy density and height from lidar points. First, you need lidar that's been classified into ground hits (bare earth) vs. non-ground hits. This type of classification is usually performed by your data provider. Secondly, you need to consider when the lidar was collected and the type of vegetation in the study area. If there are a lot of deciduous trees and the data collection was performed during leaf off, then the density calculation is not going to work.
Loading points into the Geodatabase
To calculate canopy density load the ground, or bare earth, lidar points into one multipoint feature class and above ground points into another. Assuming your data are in LAS format, you do this with the LAS To Multipoint tool. Specify the proper class codes to filter on. Here are the LAS class codes as defined in the LAS 1.1 Standard:
Any LAS file made in the last few years should use these codes if the points have been classified. Unfortunately, there’s still some ambiguity in the standard. For example, we know class ‘2’ is ground but class ‘8’ is ground as well. Class ‘8’, or model key, points are a special set of ground points used for contouring or other application requiring a thinned set of ground points. Whether you have them depends on how the data was processed. If you don’t know specify both classes. If it turns out there aren’t any model key points it won’t hurt. Vegetation has a similar issue. Sometimes vendors place everything that’s above ground into class ‘1’ because they haven’t performed a more detailed classification on them. So, if you’re unsure of the specifics of your data’s classification, load non-ground points using classes 1, 3, 4, and 5; that’s a reasonable catch-all to get your vegetation points. Note: If buildings or other manmade non-ground features are in class ‘1’ you’ll get them too and they’ll skew the results somewhat.
Calculating the density
The most effective way to determine the canopy density is to divide the study area into many small equal sized units. Do this through rasterization. In each raster cell you compare the number of above ground hits to total hits. The trick is to figure out an appropriate cell size. It needs to be at least 4 times the average point spacing. You can go larger but not smaller.
1. Use the Point To Raster tool on the above ground points with the COUNT option.
2. Convert any resulting NoData cells to 0 so that subsequent operations treat zero points in a cell as 0. This is accomplished using the Is Null tool followed by Con.
Next use the Con Tool
3. Repeat steps 1 and 2 with the ground points.
4. Add the above ground and ground rasters to get a total count per cell using the Plus tool.
5. All the rasters we’ve made so far are longs. We need one to be floating point in order to get floating point output from the Divide function that we’ll use in step 6. Do this by sending the output from Plus through the Float tool.
6. Now use the Divide tool between the above ground count raster and the floating point total count raster. This gives us the ratio from 0.0 to 1.0 where 0.0 represents no canopy and 1.0 very dense canopy.
The following image represents canopy density. The lightest areas have little to no vegetation. These are areas where a large percentage of lidar shots could ‘see’ the ground. The dark green areas, where lidar could not penetrate to ground as well, indicate denser vegetation canopy.
Calculating the height
To determine canopy height you'll need to subtract the bare earth surface (DEM) from the 1st return surface (DSM). Take a look at a previous blog to learn how to make these surfaces. Find it at this link: Creating raster DEMs and DSMs from large lidar point collections.
Once you have your first return and bare earth rasters the Minus tool gives you the difference which, over forest, represents the canopy height.
The following image represents canopy height above ground. It ranges from blue for little to no height, to orange which is the tallest.
Lidar can be used to calculate the density and height of vegetation. This is useful for a variety of purposes including biomass and carbon estimates as well as forest management.
That concludes part four of Lidar Solutions in ArcGIS. Subscribe to this blog or check back in a month or so for a discussion on the creation of intensity images from lidar.