Select Scale Options When Importing a Point Cloud File
When you import a point cloud contained in an LAS/LAZ (.las/.laz) or e57 (.e57) file, the file may not include enough information for TBC to determine the best way to ensure that appropriate scaling and positioning are applied to the cloud. In this case, the Point Cloud Scale dialog displays, enabling you to select from options specifying how the point cloud was created. Based on your response, TBC determines the best method for importing the point cloud. This allows you to import and work with point clouds that were created in different manners in the same TBC project.
Selecting the right option is easier if you understand the following terms:
Ground-scaled point cloud
A point cloudin which the distance between any two points is the same distance that you would obtain if you physically measured it yourself, such as with a tape measure. Most scanning instruments (terrestrial scanners) create point clouds this way. Typically these point clouds have positions near 0,0,0 and will need to be registered or georeferenced after import.
Grid-scaled point cloud
A point cloud in which the distance between any two points is the same distance that you would obtain if you inversed between them. The Inverse command measures grid distances in a specific coordinate system and therefore takes into account all of the distortions associated with the coordinate system’s projection. A grid-scaled point cloud is stretched or compressed when compared to a ground-scaled point cloud. Because the scale factor depends on both the horizontal position and vertical position, the degree of stretching and compressing is position-dependent. Most photogrammetry software packages create point clouds this way because they use grid positions for camera and/or ground control points (GCPs); such point clouds are typically also georeferenced (not near 0,0,0).
Georeferenced point cloud
A point cloud that has been scaled, moved, and rotated so as to make certain points in the point cloud match up with georeferencing points. Because the georeferencing points are expressed in a coordinate system, each point in a georeferenced point cloud is now in the same coordinate system. Mathematically speaking, the point cloud should be scaled as it is moved and rotated, but not all georeferencing software does that. If it is scaled in the georeferencing process, it becomes a grid-scaledgeoreferenced point cloud. If it is not, it becomes a ground-scaled georeferenced point cloud.
Registered point cloud
A point cloud (as would be contained in a TZF (.tzf) file) that has been georeferenced to another point cloud instead of georeferenced to georeferencing points (that is, it is relative instead of absolute).
Station-based point cloud
A point cloud that has an associated scan station, which is represented by an associated Point node and Scan station node in the Project Explorer. The position of the scan station can be thought of as a reference position (that is, the position that is used to compute the scale factor. As a station-based point cloud is moved horizontally and vertically, the scale factor is automatically changed. Generally speaking, station-based point clouds are desirable because various automatic functions are enabled for them.
Prerequisite:
Select to import a point cloud of unknown origin contained in an LAS/LAZ (.las/.laz) or e57 (.e57) file.
To select scale options for the imported point cloud:
- In the Point Cloud Scale dialog (displays automatically when you import the LAS/LAZ or E57 file), do one of the following as appropriate:
If the point cloud is: | | Do this: |
Ground-scaled and not registered or georeferenced (for example, from a terrestrial laser scanner) More... This option supports the import of data collected with a terrestrial scanner and contained in an LAS (.las) file, with no accompanying JOB/JXL file. TBC will use the first scan point in the point cloud (typically close to 0,0,0) to establish the cloud's station position, similar to an imported scan station. If there are any station-based point clouds already in the project, the new cloud's station, along with the cloud itself, will be automatically moved near the other point clouds on import. This makes it easier to view, register, and georeference the point cloud after import. No scale factor will be applied to the point cloud on import because it is assumed it is already scaled to ground. After import, the station position for the new point cloud, when averaged along with the positions of other scan stations in the project, will influence the project scale factor (that is, the single scale factor that is applied to all point clouds in the project). This means that the imported point cloud, when viewed in TBC, will be automatically scaled from ground to grid for display purposes based on the average position of all scan stations in the project. It is up to you to register and/or georeference the point cloud in TBC after import. | | Select the Non-georeferenced, ground-scaled point cloud option. |
Grid-scaled and georeferenced in the same coordinate system as the project (for example, from an unmanned aerial system (UAS)) More... This option supports the type of georeferencing typically performed on a point cloud that was created in a photogrammetry software package (for example, Trimble UASMaster) using photogrammetry data captured with a ground-based or aerial photo station. Under these circumstances, the position of each point in the point cloud is a valid grid coordinate. As such, the distance between any two points is a grid distance, not a ground distance. In order to make grid-scaled point clouds coexist with ground-scaled point clouds in the same project, it is necessary to scale the point cloud from grid to ground as it is being imported. In order to properly scale the point cloud—and to keep it georeferenced—a scan station is created with a grid position near the center of the cloud. The new scan station will not be automatically moved to be near the other point clouds in the project. The scale factor to be used to scale from grid to ground will be based on two key assumptions: - The point cloud file was created in the same coordinate system as the project into which it is being imported.
- The center of the point cloud is typically the best position to use when computing the combined scale factor that represents the cloud.
After import, the station position for the new point cloud, when averaged along with the positions of other scan stations in the project, will influence the project scale factor. This means that the imported point cloud, when viewed in TBC, will be automatically scaled from ground back to grid for display purposes based on the average position of all scan stations in the project. Notes on point clouds from aerial photogrammetry solutions... Although it’s expected that the automatic determination of the center position and combined scale factor will suffice in most cases, there will still be cases where it is not accurate enough. For example, the altitude of the UAS could be large enough to significantly influence the scale factor of the imported point cloud. In a photogrammetry bundle adjustment, the UAS camera positions are typically supplied by RTK vectors and/or trajectory event positions that are expressed in grid coordinates. However, the combined scale factor at flying elevation will be different from the combined scale factor at ground elevation. This means that the combined scale factor of the point cloud may actually be at the same elevation as the UAS, not at ground level. If ground control points (GCPs) were used, the combined scale factor for the GCPs will be at ground level and will help to bring the combined scale factor for the point cloud nearer to the ground. The elevation at which the combined scale factor is representative of this type of point cloud is therefore difficult to determine. The best center position probably has an elevation somewhere between ground elevation and flying elevation. The automatic method of reading the file and averaging the position will always obtain an elevation close to ground level; this, in turn, will lead to estimating the wrong combined scale factor for the import scaling. Because photogrammetry projects are typically larger than terrestrial projects, it’s even more important to apply the correct scaling. If you determine that the scaling is wrong, you should check the More options check box, select the Grid-scaled in project's coordinate system option, check the Advance check box, and either enter an Elevation value that is larger than what is logged in the file or slightly change the Combined Scale Factor. | | Select the Georeferenced, grid-scaled point cloud option. |
Ground-scaled and georeferenced in the same coordinate system as the project More... The position of each point in this type of point cloud can be thought of as a grid coordinate with a small amount of error that grows with distance from the center. To keep the data georeferenced, a scan station will be automatically created with a grid position near the center of the point cloud. The new scan station will not be automatically moved near other point clouds in the project, and no scaling will be applied because it is assumed that the point cloud is already scaled to ground. After import, the station position for the new point cloud, when averaged along with the positions of other scan stations in the project, will influence the project scale factor. This means that the imported point cloud, when viewed in TBC, will be automatically scaled from ground to grid for display purposes based on the average position of all scan stations in the project. Although it is expected that the automatic determination of the center position will suffice in most cases, there will still be situations where it is not accurate enough. Perhaps the best example is where the georeferencing was done primarily in a corner or near one side of the point cloud instead of being done symmetrically around the center. In this case, the best center position is the center of the georeferencing points, not the center of the point cloud. When these two positions vary greatly, the automatic scaling from ground to grid for display purposes will introduce a position shift that may be unacceptable. To avoid this position shift, you can check the More options check box, select the Ground-scaled in project's coordinate system option, check the Advance check box, and enter the Point Cloud Center coordinates for a point near the center of the georeferencing points. Optionally, you can click the Estimate from File button to view the automatically computed Point Cloud Center coordinates. | | a. Check the More options check box. b. Select the Georeferenced point cloud option. c. Select the Ground-scaled in the project's coordinate system option. |
Grid-scaled in the same coordinate system as the project More... This option supports the type of georeferencing typically performed on a point cloud that was created in a photogrammetry software package (for example, Trimble UASMaster) using photogrammetry data captured with a ground-based or aerial photo station. Under these circumstances, the position of each point in the point cloud is a valid grid coordinate. As such, the distance between any two points is a grid distance, not a ground distance. In order to make grid-scaled point clouds coexist with ground-scaled point clouds in the same project, it is necessary to scale the point cloud from grid to ground as it is being imported. In order to properly scale the point cloud—and to keep it georeferenced—a scan station is created with a grid position near the center of the cloud. The new scan station will not be automatically moved to be near the other point clouds in the project. The scale factor to be used to scale from grid to ground will be based on the assumption that the point cloud file was created in the same coordinate system as the project into which it is being imported. The center of the point cloud is typically the best position to use when computing the combined scale factor that represents the cloud. If this is not the case, you can check the More options check box, select the Grid-scaled in project's coordinate system option, check the Advance check box, and enter the Point Cloud Center (optional) coordinates for a point near the center of the georeferencing points. Optionally, you can click the Estimate from File button to view the automatically computed Point Cloud Center coordinates. See step 2 below. Optionally, you can click the Estimate from Center button to display the computed Combined Scale Factor. If you know the combined scale factor you want to use for the point cloud, you can manually enter it. See step 3 below. After import, the station position for the new point cloud, when averaged along with the positions of other scan stations in the project, will influence the project scale factor. This means that the imported point cloud, when viewed in TBC, will be automatically scaled from ground back to grid for display purposes based on the average position of all scan stations in the project. Notes on point clouds from aerial photogrammetry solutions... Although it’s expected that the automatic determination of the center position and combined scale factor will suffice in most cases, there will still be cases where it is not accurate enough. For example, the altitude of the UAS could be large enough to significantly influence the scale factor of the imported point cloud. In a photogrammetry bundle adjustment, the UAS camera positions are typically supplied by RTK vectors and/or trajectory event positions that are expressed in grid coordinates. However, the combined scale factor at flying elevation will be different from the combined scale factor at ground elevation. This means that the combined scale factor of the point cloud may actually be at the same elevation as the UAS, not at ground level. If ground control points (GCPs) were used, the combined scale factor for the GCPs will be at ground level and will help to bring the combined scale factor for the point cloud nearer to the ground. The elevation at which the combined scale factor is representative of this type of point cloud is therefore difficult to determine. The best center position probably has an elevation somewhere between ground elevation and flying elevation. The automatic method of reading the file and averaging the position will always obtain an elevation close to ground level; this, in turn, will lead to estimating the wrong combined scale factor for the import scaling. Because photogrammetry projects are typically larger than terrestrial projects, it’s even more important to apply the correct scaling. If you determine that the scaling is wrong, you should check the More options check box, select the Grid-scaled in project's coordinate system option, check the Advance check box, and either enter an Elevation value that is larger than what is logged in the file or slightly change the Combined Scale Factor. | | a. Check the More options check box. b. Select the Georeferenced point cloud option. c. Select the Grid-scaled in the project's coordinate system option. |
Grid-scaled in a different coordinate system than the project More... This option allows you to specify the coordinate system in which the file was created. When TBC estimates the combined scale factor for the file, it will use the selected coordinate system instead of the project’s coordinate system. TBC will transform the position of the point cloud from that coordinate system to that of the project in the standard way. In addition, TBC will compute the change of convergence angle between the two coordinate systems and rotate the point cloud by that amount. The goal is to preserve geo-referencing and registration when importing multiple LAS/LAZ files. | | a. Check the More options check box. b. Select the Georeferenced point cloud option. c. Select the Grid-scaled in the selected coordinate system option. d. Click the Select button and select the appropriate coordinate system. |
Of unknown origin More... This option is applicable if any of the following are true: - You do not know whether or not the point cloud you are importing was georeferenced prior to import.
- You do not know if the point cloud is ground-scaled or grid-scaled.
- The point cloud you are importing is scaled to grid and is being imported into a project in which there will never be station-based point clouds. In this case, the file will not be scaled from grid to ground and from ground back to grid for display purposes. In addition, there will be no need to determine the point cloud center or the combined scale factor, and there will be no loss of fidelity.
Because the point cloud is imported without a scan station, it does not influence the scale factor used in the project. Note: The process of scaling a point cloud from grid to ground on import and from ground back to grid when the point cloud is displayed in TBC may lead to a small loss of fidelity. For the UAS-derived photogrammetry, there is also the complexity of estimating the best combined scale factor. All of these issues can be avoided if there is no need to intermix ground-scaled and grid-scaled data in the same project. In this case, you should select the Unknown/Generic option to disable all automation related to point clouds and ensure no scale factor will be applied to the point cloud on import or for display purposes. | | a. Check the More options check box. b. Select the Point cloud from unknown source option. Note: This option disables automatic scaling. |
The next is optional and is useful if you know the specific coordinates and elevation you want to use to center the point cloud (perhaps closer to an area of interest). Otherwise, you can skip this step and allow TBC to automatically compute the coordinates and elevation.
- Optional step: if you know the specific coordinates and elevation you want to use to center the point cloud...
- Check the More options check box.
- Select the Georeferenced point cloud option.
- Select the appropriate Georeferenced point cloud sub-option.
- Check the Advanced check box.
- In the three Point Cloud Center (optional) fields, enter the coordinates and elevation to use to center the point cloud.
Optionally, you can click the Estimate from File button to allow TBC to compute the centroid and populate the fields for you. (This will provide the same centering values as if you had not used the Point Cloud Center fields.) Then you can change any values as necessary.
- Click OK to accept your option selections and continue the import process.
Related topics
Import Point Cloud Files
Register, Refine, and Georeference Point Cloud Scans
