Create Canopy Height Models from Lidar in Global Mapper Pro

Global Mapper Pro has the tools to create Canopy Height Models from lidar, with no extensions required. 

A Canopy Height Model (CHM) is a raster that maps tree heights as a continuous surface. It represents the height of the tallest point above the ground. They can be easily made in Global Mapper Pro by creating a DTM (digital terrain model) representing the ground, a DSM (digital surface model) representing the trees, and subtracting them: 

Canopy Height Model = Digital Surface Model – Digital Terrain Model 

Measuring forests with remote sensing methods such as lidar saves time and manpower. Lidar can penetrate forest canopy and undergrowth, allowing the ground elevation to be precisely determined – even in dense or inaccessible terrain. For more information on how lidar “sees” the ground, check out this blog on point cloud types.

A quick way to view an estimate of tree height without using rasters is to change the Lidar Draw tool to Color by Height Above Ground. This tool measures the height for each point above an estimate of the ground surface. For more accurate results, classify the ground points first. 

NOTE: Individual tree location, crown size, and tree height can be extracted directly from the lidar. Check out this youtube video on Extracting Tree Features to learn more. 

Create a Model of the Ground 

First, classify the lidar to differentiate ground points apart from vegetation. Global Mapper has automatic lidar classification tools that make this a straightforward process. More information can be found in this video on Lidar Classification and Grid Creation. 

A Digital Terrain Model (DTM) is a raster that represents the bare ground without any vegetation. It is a solid surface made up of the lowest points in each area. Use the Create Elevation Grid tool to create the DTM from the lidar. In the settings, choose the Binning: Minimum option, and use the Filter Lidar Points… button to only use the Ground points.

Create a Model of the Forest Canopy 

A Digital Surface Model (DSM) represents the highest points in the area, which, in this case, is the top of the forest canopy. Use the Create Elevation Grid tool to create the DSM from lidar. In the settings, choose the Binning: Minimum option, and use the Filter Lidar Points… button to only use vegetation; otherwise, the height model will also represent buildings and other structures in the data.

Subtract Them to Create a CHM

The canopy height model is the difference in height between the DTM and the DSM. You can see this in the image below, which displays the two stacked layers from a perpendicular perspective. After subtraction, the resulting raster will show values of tree height instead of elevation, which is typically associated with rasters.

Subtract the layers using the Combine/ Compare Terrain Layers tool. Choose the Subtraction – Unsigned option because we know that all of the values will be positive; all of the trees are above ground. It also makes layer input order irrelevant.

The calculation creates a new layer, the CHM, where you can see the patterns in tree heights across your study area. You can compare this to other data to assist in determining why the heights are distributed as they are. For example, assess the slope and aspect by changing the shader for your DTM. Other data can also be brought in, such as rainfall, soil, and other site index indices.

To style the data using color, you can use the shader options to visually differentiate “tall” trees from “short” ones. The image above uses a built-in shader, the Magma Shader, but you can create custom ones as well.

The lidar in these images was downloaded for free from the USGS National Map. Try Global Mapper Pro by downloading a free, 14-day trial today!