When an ortho mapping workspace is created, the next step is to perform block adjustment using the tools in the Adjust and Refine groups. The adjustment tools encapsulate operations of computing match points (tie points) from overlapped images and performing triangulation calculations. The adjustment tools use an algorithm and processing procedure that are suitable for each ortho mapping workspace type. For instance, the algorithm performs RPC triangulation for satellite images and frame triangulation for aerial images. The parameters used in the adjustment computation can be defined on the Adjustment Options dialog box. The adjustment quality report can be viewed using the Quality Report tool.
The Adjust tool is used to perform block adjustment of the image collection in the current ortho mapping workspace. The processing is always done at image source resolution—computing tie points from source images, and then performing triangulation using the computed tie points. Drone and scanned aerial images normally come with poor-quality or no exterior orientations and coarse GPS information. In these cases, an initial adjustment will be performed at a coarse resolution—a user-defined pyramids level is used to estimate the image orientations, and then an adjustment will be performed at the source resolution. The tie points generated at the initial adjustment will be used to seed the tie point computation and triangulation calculation at the source resolution. Having pyramids built for the image collection ahead of time or during ortho mapping workspace creation will reduce the time of initial adjustment process.
The Quick Adjust tool is applicable for processing drone images. It performs adjustment at a coarse resolution to estimate exterior image orientations and improve GPS location. This initial adjustment is done quickly and allows you to get an understanding of the data coverage of your study area and the choice of the processing parameters for this collection before taking the time to run an adjustment at source resolution. For example, when you collect data in the field, you can run this tool and run Adjust again to compute the full adjustment. This can also be used as a step to establish the camera model before adding GCPs.
To further improve the adjustment accuracy with ground reference, you can add ground control points (GCP) using the Manage GCP tools. The adjustment quality can be viewed from an adjustment report using the Adjustment Report tool. If the adjustment errors are larger than expected, there are several ways you can check your tie points:
- Check the solution point layer in the Content pane, review the residual map or analyze the residuals in the Residual field in the attribute table, identify point IDs that have large residuals, and delete tie points that have the same point IDs in the control point table.
- Run the Analyze Tie Point tool to generate a coverage feature class and an overlap polygon feature class. The two feature classes help you understand whether you have enough tie points and where you need to add more tie points.
- Use the Recompute Tie Points tool to compute tie points and to also append more tie points.
These tools will update the control points in the ortho mapping workspace. You can run the Adjust tool again to perform triangulation and update the image collection.