Agisoft Metashape Professional supports the processing using control and check points. Ground control points are used for georefrerencing, and check points are used to evaluate accuracy. 


We recommend have at least 3-5 points in the project from which an accuracy assessment can be performed.


The shooting scenario and the source data are very important. Please check 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:


Import markers from CSV file

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. Select Delimiter and indicate the number of the data column for each coordinate.

If the file contains the coordinate accuracy value, then enable the Accuracy box for the coordinates and also specify the corresponding columns. In our example, this information is not stored in the file, we will specify the accuracy value manually.


4. Click OK button. And to create a marker you need to click Yes to All button:



The coordinates are imported into the project and will be displayed in the Reference pane. To enable the display of markers in Model view, click Show Markers button on the Toolbar button: 



Markers are shown as blue flags in Model view.

 


Set accuracy

If the coordinates for the markers were obtained with the same accuracy, then you can specify this value in the Reference Setting dialog window. Click Settings button on the Reference pane:


And specify the Marker accuracy value:

If the XY and Z coordinates were measured with different accuracy, then use the - "/" to indicate different values. For example, the XY coordinates were measured with one accuracy, and the Z coordinate was measured with a different precision, then the values will be shown as in the screenshot below:


If you need to change the accuracy for a specific single point, then you need to select Modify... command from the markers context menu on the Reference pane. After that select Column - Accuracy and Value in the Modify Reference dialog window:






How to set the marker type

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.
NOTE: 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).



Starting from version 2.2.1, control and check points have different icons and will be displayed differently:




You need to enable or disable checkbox for markers on the Reference pane to change the type: 

  • If the checkbox is enabled, the point is the control point. 
  • When the checkbox is disabled, the point is the check point. 




After you change the point type click Update toolbar button from the Reference pane.




Place markers on the images

Open the photo where the marker is visible by double-clicking on the photo's name. To filter photos by marker use Filter Photo by Markers command from the 3D view context menu.  


If the markers are not located on the surface of the tie points, then filtering will not be applied. In this case, we recommend checking that the altitude coordinates have been loaded correctly for the images and that the coordinate system is set correctly.


The filtered images will be visible on the Photos pane (select View > Panes > Photos):


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. 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.



If you indicated the position of the marker on the images - then the flag is green.


In Metashape, it is possible to automatically refine the position of markers in images. The Refine Markers tool is used for this (Tools > Markers > Refine Markers). Automatically placed markers will be marked with a blue flag.  You need to manually place marker on at least 3 images. After that, the position of the markers in other images will be automatically adjusted:




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. 


NOTE: Blue flags are also used  in bundle adjustment (as green). If suddenly their position was determined incorrectly, then they will contribute. We recommend double-checking whether their position was correctly determined on the photos, if not correctly, then manually clarify their position.



Optimization

Optimize Cameras command performs a full bundle adjustment procedure on the aligned photogrammetric block, simultaneously refining exterior and interior camera orientation parameters and triangulated tie point coordinates. The adjustment is performed based on all available measurements and corresponding accuracies, i.e. coordinates of projections of tie points and markers on images in image coordinate system; GPS coordinates of camera positions; GCP coordinates; scale bar distances.


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.


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).


To run optimization you need to click Optimize toolbar button on the Reference pane:



In Optimize Camera Alignment dialog box check additional camera parameters to be optimized. For most projects, it is sufficient to use the following parameters:




Adaptive camera model fitting - this option enables automatic selection of camera interior orientation parameters to be adjusted. 

Estimate tie point covariance - the function allows to estimate the covariance values of the tie points.

Fit additional corrections - with this option enabled Metashape estimates additional coefficients that are necessary to achieve better accuracy. There are different corrections that allow to compensate the distortions that cannot be described by the Brown's model.


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: