Elevation Grid Creation in Global Mapper: Creating a DTM
The Elevation Grid Creation tool in Global Mapper uses loaded 3D data, data with x, y, and z values, to create a raster gridded elevation layer. This layer can then be exported in one of the supported elevation formats, or used for further analysis or to create a map.
The first method Global Mapper offers to generate elevation grid layers is the Triangulated Irregular Network or TIN method. This method connects 3D point features or the vertices of 3D line and area features into a network of triangles. From there, the program interpolates over the triangular faces using the feature elevation and slope values to generate an elevation grid layer.
With the Lidar Module, Global Mapper not only provides point cloud classification and processing tools, but the program also provides additional methods for generating an elevation grid. These additional options are all variations on the binning method. This method is better suited for point cloud processing because not every single point in the point cloud is used to generate the output grid.
Typically point clouds are quite dense and you don’t need to use every single value to generate an accurate output. In fact, using every point often results in an elevation grid layer that contains lots of noise and appears rougher than the actual study area. The binning methods help to reduce this noise by spatially binning the data into areas corresponding with the size of the output grid cells. One value from each of the spatial bins is then used to generate the gridded layer. The elevation value from each bin that is used to generate the grid is determined by the specific binning method that is selected. For example, the Binning Minimum Value method uses the minimum elevation value from each bin to generate the grid. The Lidar Module currently offers three variations on the binning method, with two additional variations coming soon.
A digital terrain model, commonly referred to as a DTM, is an elevation model that describes the terrain or ground of an area as opposed to the structures and features on top of the ground, such as buildings and vegetation. Conversely, a digital surface model, or DSM, aims to show the structures and features on top of the ground.
When creating a DTM, you will likely want to use the binning minimum value method. Since lidar is not ground-penetrating, the minimum values detected in the point cloud are most likely to be true ground measurements.
Another option you have in your workflow is to further identify ground points by classifying your point cloud using the classification tools available in the Lidar Module. The automatic classification tools allow you to perform rough classifications that you can then clean up and fine-tune with manual classification.
When generating the elevation grid layer, there is the option to further filter the points of your point cloud to use only points within a specific class, with specific flags, or in a designated elevation range. This filtering will help to further narrow down the points available to consider when Global Mapper is building the elevation grid layer.
Water bodies such as ponds, lakes, and rivers may not provide consistent point cloud data. When generating an elevation grid that contains water-covered areas, you may want to flatten those areas to a specified elevation value. This can be done by including a 3D area feature in the data used to create the grid, and using the grid creation option to ‘Use 3D Area/Line Features as Breaklines’. This will burn the area feature into the output grid at the elevation designated by the area feature, thus flattening the noise within the area. This can be used for road features, building footprints, or any other area features as well.
To compare a few different elevation grid creation methods, the path profile tool can be used. Below is a path profile over three elevation grids all using different methods. You can see that the binning method grid appears smooth compared to the triangulation method grid.
With an elevation grid layer created to show the elevation as a surface, you can continue your analysis in Global Mapper to generate contour lines, generate watershed areas, perform volume calculations, or any other analysis function. To see more of what Global Mapper can do for you, please visit the Tips & Tricks page or request a demo or a trial today.