Manage Point Clouds in AutoCAD Map 3D21 Jul, 2010 By: Michael Schlosser
Access, stylize, and filter LiDAR data with the new tools in AutoCAD Map 3D 2011.
AutoCAD Map 3D 2011 is the latest version of Autodesk's software for creating and managing CAD and spatial data. My favorite features of this release include the new point cloud tools for viewing, querying, and analyzing 3D laser scanning and LiDAR (light detection and ranging) data sets.
LiDAR is a system for collecting topographic information whereby laser-equipped aircraft collect points in three dimensions, creating data sets known as point clouds. These point clouds are then used to locate topographic features, define feature geometries, and create 3D surfaces.
Point clouds often contain hundreds of millions of points and can be difficult to work with using traditional AutoCAD methods. However, the new Point Cloud Manager in AutoCAD Map 3D 2011 is designed to handle large numbers of points and enhances the ability to use these large LiDAR data sets. Plus, with AutoCAD Map 3D we have the additional benefit of being able to assign a geographic coordinate system so that we can use these point clouds in a geographic context with design information and other geospatial data sources.
In this article, I'll explain how to use the new point cloud tools in AutoCAD Map 3D 2011 to access, stylize, and filter a LiDAR point cloud corresponding to a municipal downtown area. I will also show how to use point clouds to first create a 3D surface, and then analyze the results. A video demonstrating the techniques discussed below is available via my blog.
Adding a Point Cloud
Point cloud data sets are large, often containing many millions (and sometimes billions) of points. To work effectively with such a large number of points, we can use the new Point Cloud Manager to index the point cloud data. The most common formats for LiDAR data include the LAS (LiDAR Aerial Survey) and XYZ formats. The LAS format is particularly useful because it includes 3D coordinate data, as well as point classification information that helps distinguish points corresponding to ground, vegetation, building, and other types of surfaces.
Point cloud data representing buildings, vegetation, and ground.
Let's begin by creating an index for the LAS file. In the Map Task pane, activate the Display Manager tab, then click Data > Add Point Cloud Data > Create New Index to reveal the Point Cloud Manager. The Point Cloud Manager displays summary information about the point clouds that we are currently working with. This information includes source file name, corresponding coordinate system, current filters being applied, output file name, and creation date. (Note: This area will be empty if no point clouds have been added to the Point Cloud Manager yet.)
The Point Cloud Manager in AutoCAD Map 3D 2011.
Now, click on the Add File option and specify the LAS file containing the point cloud. To set the geographic coordinate system for this point cloud, click the area under the Coordinate System heading and select the appropriate coordinate system code from the list.
If needed, enter the name of the index file under the Output header, or accept the default. The extension of the output file will be ISD.
Click on Generate Index to create the index file, then click on Add to Map to display the results. If needed, zoom to the extents of the point cloud by right-clicking on the point cloud layer in Display Manager and choosing the Zoom to Extents option.
To control the number of points displayed at any one time, click on the AutoCAD Point Cloud option from the Insert menu. Use the slider bar to choose a value from 1 to 100; the higher the number, the greater the density of the points displayed.
LiDAR point clouds with point display densities of 2 (left) and 10 (right).
Stylizing a Point Cloud
The LAS file format may include elevation, intensity, and classification information that can be used to stylize (e.g., color-code) 3D points.
Let's stylize a point cloud based on the classification information. In the Map Task Pane, click on the point cloud layer to activate the Point Cloud menu. Note the various predefined stylization options including classification, elevation, and intensity.
Select the Classification option from the Style area to categorize the points based on ground, vegetation, building, and other surface types. Note that we can also define our own stylization colors via the Set Style option.
Points can be color-coded to represent various surface types.
Filtering a Point Cloud
Filtering is used to show a subset of the point cloud whereby only those points meeting certain conditions are displayed. You can filter by classification, elevation, intensity, or geographic location.
Let's define a filter to show only those points that are classified as "ground." Ensure that the Point Cloud menu is active, then select the Filter Point Cloud option from the View area to display the Filter Point Cloud dialog. Set the Filter by: value to "Classification." Now, select the classification values in the Define filter area. In our example, we need to select the "ground" value. Click Apply Filter to show the results.
Filtering is used to show a subset of the point cloud; in this case, only points classified as "ground" are displayed.
Creating and Analyzing a 3D Surface
The ability to create and analyze surfaces from point cloud data is another useful planning and conceptual design feature in AutoCAD Map 3D 2011. Let's use our point cloud filter results from the previous steps to create a surface representing natural ground. In the Point Cloud menu, select the 3D Surface option from the Create area to display the Surface Manager dialog.
Select the entry containing the source points for the surface. Set the type of surface to GeoTIFF. (ESRI ASC is another surface-type option.) Note that the Filter value is already set to "Classification" from the previous step. Specify the output file name, then click Generate Surface. Once the surface has been created, click on Add to Map to show the resulting surface.
At this point we can use the standard stylization tools in AutoCAD Map 3D to theme our surface by elevation, slope, and aspect. Let's theme the surface by elevation. In the Map Task pane, select the surface layer and click on the Style icon to display the Style Editor dialog.
Users can access Theme options through the Style Editor dialog.
Click on Theme to display the Theme dialog. Set the value for Property to "Height." Activate the Palette option and set its value to "USGS DEM palette."
A 3D surface generated from LiDAR point cloud data corresponding to points classified as "ground" without theming (left) and themed by height (right).
Click OK to accept the remaining defaults, then click Apply to show the results.