Add Lidar and Trajectory Data

In this article:

• Loading Lidar Data
• Adding Trajectory Data
• Showing a Lidar Point Cloud on the Terrain
• Showing Photos on the Lidar
• Adding Photos to Your Project
• Adding Ground Control Points (GCP)
• GCP Scattering
• Verifying the Accuracy and Quality of Your Data
• Completing the Build

Lidar (light imaging, detection and ranging) is a remote sensing method that uses pulsed laser light to measure terrain and objects. This data together with other information recorded by the sensor system is used to generate a densely spaced point cloud of highly accurate georeferenced points. Lidar data can be loaded into a PhotoMesh project and integrated with project photos. Integration of Lidar data refines and enhances the 3D model, producing cleaner results and preventing artifacts, while increasing the level of accuracy of the complete PhotoMesh process:

• Lidar improves the precision of the mesh geometry, since Lidar has less noise-to-signal ratio than image-based correlation. This means smoother and cleaner roads, facades, and other planar surfaces.
• Lidar helps in areas that are very homogenous and don’t have a lot of texture to produce image correlation points (wide roadways with no markings, white rooftops, repetitive vegetation like crops or grass, etc.).
• Lidar makes it easier to get into tight angles in urban areas, since Lidar only needs a single ray to reach the bottom of a building or inside an alley in order to have it measured, whereas with image correlation two images with enough angular disparity are required to reach a feature and to be able to correlate depth in that point.
• Lidar enhances street level data, providing points underneath vegetation and where trees get close to buildings.
• Lidar improves fine details of complicated features such as scaffolding, tops of towers, draw bridges, cranes, etc.
• Lidar provides a geographic control to achieve a high level of absolute accuracy in the final 3D mesh and additional geospatial products.

PhotoMesh supports Lidar data in .las, .laz or .e57 formats. To improve the reconstruction of the model, Lidar data can be loaded together with a trajectory file that provides information about the scanner’s position while capturing the point cloud. Some .e57 files include internal trajectory information and therefore do not require an external one.

Loading Lidar Data

1. Load a Lidar file.
2. Set the Lidar’s coordinate system including its vertical datum. 
   Note: If incorrect coordinate system information is entered, the *.CPT created for Lidar will need to be manually deleted from the project directory: PM_project\cpts\

Adding Trajectory Data

To improve the reconstruction of the model, Lidar data can be loaded together with a trajectory file that provides information about the scanner’s position while capturing the point cloud. Trajectory data can only be imported in a tab delimited .trjt (text format) file. Some .e57 files include internal trajectory information and therefore do not require an external one. The file format should match that of the example below.

Showing a Lidar Point Cloud on the Terrain

A Lidar point cloud can be shown on the terrain to see if it fits the terrain imagery or to better understand the position of the Lidar.

To show Lidar on the terrain:

• In the Project Tree, right click the Lidar and select Show on Terrain. The Lidar is projected on the terrain. If the Lidar file is large, this may take a while.

Note: If you want to show more than one Lidar on the terrain, the conversion to CPT performed via “Show on Terrain” may take too long. So instead build the project, performing only the Data Preparation step. It is recommended to work with a single Lidar. More about: Setting build steps >

Showing Photos on the Lidar

Select photos with orientation information, and select Project on Terrain to project them on top of the Lidar. Check that the photo is positioned correctly on the Lidar.

Note: If the Lidar is under the terrain, turn on underground mode (see screenshots below). If the Lidar is partially under the terrain, use Modify Terrain to lower the elevation of the terrain below the Lidar. More about: Modifying the terrain >

Before and After Underground Mode was Activated

Adding Photos to Your Project

1. Add photos to your PM project.
2. If your photos have orientation information, skip to the next step. Otherwise, perform an “AT Only” build to add position and orientation information. More about: Setting build steps >

Adding Ground Control Points (GCP)

Ground control points are used to align your Lidar data with the project’s photos.

1. On the Home tab, select Create Ground Control Point from Terrain. Then click the Lidar feature in the 3D Window that you want to sample.
   Note: You can enlarge the Lidar point size to find a clearly visible location on the Lidar ground level.
   Note: It is generally recommended to pick points in open areas that can be seen from all different photo combinations and properly referenced to the nadir camera and several oblique cameras.
2. The Control Point Editor lists all the photos in which the selected Lidar feature appears. Mark the control point in a few photos from each collection or photo direction. It is recommended to have at least 10 sampled photos for each GCP. The Control Point Editor calculates the sampling error. For best results, make sure the sampling error is small (1-2 pixel range).

GCP Scattering

It is recommended to add at least five control points evenly distributed around each AT tile (four in the corners and one in the center).

Verifying the Accuracy and Quality of Your Data

Perform a build on a single reconstruction tile, and examine the accuracy and overall quality of your point cloud, model, textured model build before building the full project.

Completing the Build

Select the reconstruction settings that best suit your Lidar data:

• The direction of Lidar collection assists in calculating surfaces from the point cloud. When there is no trajectory file and the Lidar collection is known to be airborne, select Lidar Collection Mode = Aerial. Otherwise, select Auto.
• The Use Lidar Colors setting enables you to select the data to use in texturing the model: only color from Lidar points, only texturing from photos, or Auto – supplement photo textures with color from Lidar points where photo texturing information is insufficient. Select Only Photos.

More about: Managing Lidar data >