Global Mapper can automate the process of locating features on
the ground by using Lidar classified data with the automated feature extraction tools
available with the Lidar Module. The default parameter values can be changed
by the user for improved results.
The Extract Vector Features tool can be used on classified ground points, either on an entire point cloud or on a user-defined subsection. This lidar feature extraction tool lets the user derive features such as building footprints, building roof structures, power lines, and other structures from classified Lidar ground points.
This tool requires a Lidar Module license.
Select which features to extract, choosing from Buildings, Trees, Powerlines, or Poles.
Specify the appropriate settings under each extraction type using the guidelines below.
Press Ok to perform the extraction. See also 3D Point Symbols for options representing the extracted point features.
Select Point Cloud(s) to Extract Features From – If more than one lidar data set is loaded into the workspace, specific lidar layers may be selected or unselected for automated feature extraction. Check the option to Only Extract From Lidar Points Selected in Digitizer Tool to run the classification tool only on points selected by the Digitizer.
Only Extract from Lidar Points Selected in Digitizer Tool – Pre-select a subset of points, and then check this option to only process the selected points.
Building Footprint Extraction Settings
Check Extract Buildings to create 3D building vectors or mesh features based on points in the Structure/ Building Point group.
Resolution to Extract at – This may be set to Point Spacings or Meters. For building points, the resolution should be small enough to capture unique planes within the building structure. Smaller distances will result in higher accuracy but requires higher resolution lidar data. Those working with low resolution data can use a higher value.
The average Point Spacings comes from the calculated point cloud statistics. For consistent and repeatable results when working with multiple lidar files, it is recommended to specify an exact resolution for the extraction in meters, rather than using the calculated Point Spacings.
Building Footprints - Select this option to create polygons at the base of the buildings representing the footprint.
Pin Footprints to Height - Use this option to set the elevation for all generated building footprints features to a single value.
Simplify - Check this option to simplify buildings. Extra vertices will be removed given that they do not move the original boundary further than the specified horizontal threshold.
Regularize - Check this option to regularize building footprints so that corners become 90 degrees and opposite walls are parallel. This option can only be enabled with simplify is disabled. Footprints will be regularized based on the horizontal threshold. The horizontal threshold defines the maximum distance a point can be moved to make line segments parallel or perpendicular.
Buildings extracted using the regularize setting with roof planes and sidewalls enabled
Minimum Footprint Area - Specify the minimum size of a building footprint in square meters. Generated footprints smaller than the entered value will be discarded.
Roof Planes - Select this option to identify and create roof features for extracted buildings.
Side Walls - Select this to generate side wall features extracted buildings.
Sharpen edges and stitch planes by adding points at planar intersections - This option will generate point features along intersections between planes. The points will be placed between pairs of nearby points that are contained in different planes.
Buildings as Mesh - If selected, the building roofs will each be turned into a single 3D mesh feature.
Color Vertices by Lidar Intensity - Use this option to color the mesh vertices by the lidar intensity value at that location.
Reconstruct Surface _ Percent of Initial Points - check this option and enter a value to use only a certain percentage of the building points to generate the mesh. This will create less complicated mesh features. The density of vertices will be the specified percentage of the original mesh vertex density.
Maximum Distance to Plane - Set the maximum distance in meters from an identified plane a point can be to still be considered part of the plane.
Minimum Number of Points in Plane - Use this field to set the minimum number of points needed in an area to identify a plane and create an extracted building feature.
Tree Extraction Settings
Extract Tree Points – Select this for extracting trees from classified ground points. This option creates point features located at individual treetops.
Resolution to Extract at – This may be set to Point Spacings or Meters. Smaller distances will result in higher accuracy but requires higher resolution lidar data. Those working with low resolution data can use a higher value.
The average Point Spacings comes from the calculated point cloud statistics. For consistent and repeatable results when working with multiple lidar files, it is recommended to specify an exact resolution for the extraction in meters, rather than using the calculated Point Spacings.
Minimum Tree Height –The minimum height above ground for a tree.
Minimum Tree Spread – The minimum width (edge-to-edge) of a single tree.
Maximum Tree Spread– Specify the maximum tree spread (canopy width expected per tree) in meters.
Select Create Approximate Tree Coverage Polygons to extract tree top polygons.
Powerline Extraction Settings
Extract Powerline Features – Select this option to extract powerline features from loaded or selected lidar points.
Maximum Distance from Best Fit Line – Define the maximum allowable distance in meters away from the calculated best fit line for the classified powerline points.
Maximum Angle Delta Allowed – Maximum angle between points to define a co-linear powerline feature.
Minimum Powerline Length to Keep – Defines the shortest allowable powerline segment.
Pole Extraction Settings
Extract Pole Points –Select this option to extract poles from points classified as 'transmission tower' lidar class 15.
Minimum Pole Height – Specify the minimum height required to create an extracted pole from a cluster of pole points.
Minimum Pole Point Count – Specify the minimum count of points that must be contained in a cluster of points to create an extracted pole point.
Maximum Pole Spread – Specify the maximum width of a cluster of pole points to create a pole point.
Mark Top of Pole – Check this option to create a point at the top of the pole. When unchecked, the point will be created at the bottom of the pole.
Specify Bounds...– Manually specify
the bounds for extraction.
Filter Points... – Contains additional settings for filtering points for classification by elevation and color values, Source ID, and existing classification. The user may also exclude all points outside of a specified scan angle.
Restore Defaults – Restores the default settings for the Auto-Classify Ground Points tool.