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Working with LIDAR

LiDAR (which stands for Light Detection and Ranging), sometimes called laser scanning or 3D scanning, is a remote sensing technology that uses a pulsed laser to generate a 3D spatial representation of the surveyed environment in the form of highly accurate 3D point cloud data.

Lidar data is stored in one or more LAS or LAZ (compressed form of the LAS) files—the industry-standard binary format for storing lidar data. Lidar data files can be quite large, containing billions of 3D points. These massive data files can affect the processing speed and responsiveness of software that loads it.

Therefore, sometimes as per the project’s requirements, lidar data needs to be filtered to reduce the size of the overall data. CivilGEO’s software can perform this filtering process using the Process LIDAR command. In addition, this command provides many other functions, including point cloud classification filtering, extraction of buildings and other structures, terrain surface generation, and much more. Some of the command’s functions include:

  • Processing of point cloud files
  • Advanced filtering options to efficiently remove erroneous or unneeded points
  • Merging multiple LIDAR data sets
  • Trimming LIDAR data
  • Thinning LIDAR data

To use the Process LIDAR command, the user must have already loaded the LIDAR data into the project. Refer to this article on how to load a LIDAR elevation layer.

Follow the steps below to process the LIDAR data:

  1. From the Terrain ribbon menu, select the Process LIDAR command.
    Process LIDAR Terrain ribbon menu command
  2. The Process LIDAR dialog box will be displayed.
    Process LIDAR dialog box

The following sections describe the Process LIDAR command and how to interact with the above dialog box.

Select LIDAR Layer to Process

This section controls the selection of the LIDAR layer to be processed from loaded LIDAR layers.

From the Existing LIDAR layer entry, select the LIDAR elevation layer that needs to be processed. Upon selecting the LIDAR elevation layer, the software will display the number of points contained in the LIDAR layer as well as the layer’s corresponding coordinate reference system (CRS).

Merge LIDAR Data

This section allows the user to select additional LIDAR layers to include and merge into the selected LIDAR layer, and then export into the final resultant LIDAR file.

Trim LIDAR Data

This section allows the user to trim down the extents of the exported LIDAR file. There are multiple methods for defining the data extents from the Map View.

  • Selected LIDAR data extents: This default option does not trim the extents of the exported LIDAR file.
  • User-defined limits: This option allows the user to define the limits on the Map View for trimming the exported LIDAR file. Click the [Pick] button and the dialog box will temporarily disappear. An informational prompt will be displayed on the status line explaining how to define the rectangular clipping region. Click and drag a rectangular region to define the limits of the exported LIDAR data. After releasing the mouse button, the user will be returned to the dialog box. A layer will be created with a rectangular box representing the user-defined region.
  • Clipping polygons: This option allows the user can select polygon shape regions for trimming the exported LIDAR file. Click the [Pick] button. The Process LIDAR dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user what to do next. Select the clipping polygons on the Map View and press the [Enter] key or right-click and select Done from the displayed context menu. The user will be returned to the dialog box. The area underneath the selected clipping polygons will be considered by the software for processing the terrain.
  • Model extents: If a project model has been defined, such as a HEC-RAS model, this option will create a bounding rectangular region to the extents of the defined model, plus an additional buffer around this region.

Filter LIDAR Data

This section is used to filter the LIDAR data based upon each LIDAR’s point classification. By default, the filter is set to filter everything out except for Ground (i.e., Bare Earth). The American Society for Photogrammetry and Remote Sensing (ASPRS) has defined a list of classification codes for LIDAR. Example classes include:

  • Ground
  • Vegetation (low, medium and high)
  • Buildings
  • Water
  • Unassigned

A point can be classified into more than one category.

Thin LIDAR Data

This section is used for thinning large LIDAR data sets.

Point clouds and LIDAR data sets can contain billions and billions of points. Many of the points are redundant and do not add any additional definition to the surface or object that they are describing. Also, loading large data sets can severely slow down the computer as it labors to load the data into memory. These thinning options function to remove redundant LIDAR points.

Processed LIDAR Specifications

This section describes how to save LIDAR data after it has been processed.

Click the […] button for the Processed LIDAR file entry to specify the directory location to save the processed LIDAR file.

Processing the LIDAR Data

After the options have been defined, click the [OK] button, and the software will process the LIDAR data and save it as a new LIDAR file.

In addition, after the LIDAR data has been processed, the software can then load this LIDAR data as a new layer in the Map Data Layers panel.

Display Options for LIDAR Data

The user can change the visualization of the lidar data on the Map View by styling the point cloud objects of the LiDAR 3D point cloud. To view or modify the display properties of the LiDAR data, follow these steps:

  1. In the Map Data Layers panel, click on the […] button next to the LiDAR data layer.
    Current Layer properties
  2. The LIDAR Properties dialog box is displayed.
    LIDAR Properties dialog box
  3. The General Information section contains read-only information about the location of the lidar data file, the minimum and maximum elevation, and the total number of points in the point cloud.
  4. The Default Point Stylization section defines the shape and size of the point cloud object.
    • Select the shape (Circle or Square) of the point cloud object from the Symbol drop-down list.
    • Select the size of the point cloud object using the Size spin control. The size can range from 1 to 11 units.
      Default Point Stylization panel
  5. The color scheme for the lidar 3D point cloud can be defined using either the Single Color section or the Graduated Color Enabling one section disables the other.
  6. Enable the Single Color section to apply a single color to all points in the cloud.
    • Select the preferred color square for the point cloud object from the Symbol color drop-down color palette.
      Single Color panel
  7. Enable the Graduate Color section to assign colors to point cloud objects based on their Z (elevation) value.
    • Select the preferred color ramp for point cloud objects from the Elevation color ramp drop-down list.
    • In the Color ramp min elev and Color ramp max elev input fields, enter the minimum and maximum Z values for the color ramp.
    • Define the transparency value of point cloud objects using the bar slider and spin control in the Apply Transparency section. By default, this section is enabled. If disabled, all the points in the cloud will have zero transparency.
      Graduated Color panel
  8. Once finished, click the [OK] button to apply the stylization of the point cloud objects to all the points in that cloud.

About the Author Chris Maeder

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