Agisoft Metashape Professional supports the processing using control and check points. We recommend reading the Image Capture Tips section which explains general principles of taking and selecting pictures that provide the most appropriate data for photogrammetric processing. Current instruction includes the following steps:
- Add photos
- Align photos
- Optimize based on measured camera locations
- Add markers
- Ground control and check points
The dataset used to benchmark aerial data processing consists of 920 images, 40 MPix resolution each. The data has been acquired using Wingtra platform (WingtraOne VTOL mapping drone) with Sony RX1RII camera, coordinates measured with a precise PPK method. No GCPs data was used for processing. For evaluation purposes the dataset can be downloaded from the Wingtra website ("High-resolution quarry mapping" dataset):
1. To add photos select Add photos... or Add folder... command from the Workflow menu.
2. Then browse the source folder and select files to be processed. By default the coordinate information for the camera centers will be loaded from the image EXIF:
At this stage, Metashape tries to find the camera positions and orientation for each image and generates a tie point cloud consisting of the tie points between the images.
1. Select Align Photos... command from the Workflow menu.
2. In the Align Photos... dialog box select the desired alignment options.
In most cases, Generic preselection will help you to process the images faster.
3. Click the OK button. The progress dialog box will appear displaying the current processing status. To cancel the processing click the Cancel button.
4. Alignment having been completed, computed camera positions, and a sparse point cloud will be displayed in the Model view.
Optimize based on measured camera locations
1. Click Optimize toolbar button on the Reference pane.
2. In Optimize Camera Alignment dialog box check additional camera parameters to be optimized.
Fit additional corrections option is reasonable if the current camera model cannot be accurately described with Brown's Model used in Metashape. Enabling this option may help to achieve better accuracy for RTK/PPK datasets without ground control points (GCPs) utilization. The optimization process usually improves the alignment results and reduces the errors, so it should be included in the common aerial data processing workflow.
3. Click the OK button to start optimization.
The results of the optimization procedure can be evaluated with the help of error information on the Reference pane. In case optimization results do not seem to be satisfactory, you can try recalculating with lower values of accuracy parameters, i.e. assuming ground control measurements to be more accurate. View error models respectively using View Estimated and View Errors toolbar buttons.
Generally, it is reasonable to run an optimization procedure based on markers data only. It is due to the fact that GCPs coordinates are measured with significantly higher accuracy compared to GPS data that indicates camera positions. Thus, markers data are sure to give more precise optimization results. Moreover, quite often GCP and camera coordinates are measured in different coordinate systems, which also prevents using both cameras and markers data in optimization simultaneously.
If markers will not participate in optimization, we recommend that they are added after the optimization process has been completed. If you want markers to participate in the optimization process, you need to add them to the project and place them on all images, then start the optimization process (for markers to participate, you need to have checkboxes enabled in the Reference pane).
1. Click Import toolbar button on the Reference pane. Browse to the file with Ground Control Points (GCPs) and coordinates, click the Open button.
2. In the Import CSV dialog set the coordinate system if the data present geographical coordinates.
3. Select Delimiter and indicate the number of the data column for each coordinate.
4. Click OK button.
To enable the display of markers, click Show Markers button on the Toolbar button:
5. Open the photo where the marker is visible by double-clicking on the photo's name. To filter photos by marker use Filter by Markers command from the 3D view context menu.
6. Switch to the marker editing mode using the Edit Markers toolbar button. Move the marker projection to the desired location by dragging it using the left mouse button.
If you indicated the position of the marker on the images - then the flag is green, automatically placed markers will be marked with a blue flag. To determine the position markers should indicate their position in at least 3 photos. The larger the number of photos used to indicate projections markers, the higher positioning accuracy.
Information for reference:
- Pinned (green flag) - this means that the user checked the projection and manually specified it (moved the photos accordingly, made sure that the marker was marked in it precisely), it is taken into account in bundle adjustment.
- Block marker - blocks marker on this camera, automatically the projection of the marker in this camera will not be displayed, in fact, a "ban".
- White markers - not used in bundle adjustment.
Ground control and check points
In Metashape you can use 2 types of markers. They are control and check points. Control points are used to reference the model. While check points are used to validate the accuracy of the camera alignment and optimization procedures results.
1. Select marker on the Reference pane.
2. Ensure that the selected marker has at least 2 projections placed on the aligned images. You should check point box. If the checkbox is enabled, the point is the control point. When the checkbox is disabled, the point is the check point.
3. Control and check point errors can be inspected on the Errors Values tab.
After you change the point type click Update toolbar button from the Reference pane.
Starting from Metashape Pro version 1.5.4 estimated control point coordinates correspond to the adjusted values, that is, they are calculated taking into account both reprojection error and measured object space coordinates. Similar behavior is common for other software packages and also allows for better detection of measurement errors. If you are interested in the difference between measured and triangulated control point coordinates, you can temporarily uncheck the corresponding entry in the Reference pane (thus making it a check point).