PE&RS October 2003

Direct Georeferencing

This article describes the quality assurance and quality control (QA/QC) tools currently under implementation in the Z/I ImageStation Automatic Triangulation (ISAT) software for imagery acquired by an aerial camera and direct exterior orientation data derived by the Applanix POS AV system.

Detection, Location, and Correction (DLC^TM) is the concept behind the direct Exterior Orientation (EO) parameters QA/QC process in ISAT. POS AV data, project aerial imagery, and available GCP (control/check points) are simultaneously used to efficiently perform DLCTM. By “Detection” we mean, to detect whether or not there is a perfect fit (according to some predefined threshold) between the directly derived EO parameters, the images, and the available GCP. If there is no perfect fit then “Location” is performed, where ISAT tries to identify the location and possibly the reason for erroneous EO parameters.“ Correction” is where the erroneous (inaccurate for some reason) EO parameters are corrected. The DLCTM is performed partially on mapping projects of any size. It is recommended that the DLCTM be used in one or more parts of the mapping project for quality control purposes.

Figure 1 shows the DLC workflow in ISAT. Typically, the raw GPS and IMU data are processed in POSGPS, POSProc, and POSEO, where the derived trajectory parameters are translated into camera exposure station coordinates and image orientation angles with respect to a local mapping frame. ISAT then reads the information where it first performs the automatic interior orientation (IO) then checks in the POSEO output file whether or not the EO parameters have high standard deviations. If the standard deviations are higher than what is suitable for the project at hand, then it will issue a warning to the operator that the EO data should be improved by reprocessing the GPS data.

The operator then becomes responsible for improving the GPS data processing quality, runs the data through POSProc and POSEO, respectively and imports the improved X, Y, and Z and w, j, and k, data into ISAT. If the GPS data cannot be improved, ISAT will check whether the standard deviations are consistently too high for the project at hand. If the standard deviations of the EO data are consistently too high because of equipment or human errors, then ISAT will issue a warning that it might be possible to use ISAT’s automatic aerotriangulation engine to its fullest. Once the problematic area in the project has been located, ISAT will run the following checks on those areas of the project that have high EO standard deviation.

1. ISAT will manually or semi-automatically measure image coordinates of the ground points (check points). The user can then revisit the ground point locations manually to make sure that they are precisely located on the imagery.

2. ISAT will then perform the space intersection using the given EO data and the image coordinates of all available checkpoints.

3. ISAT will compare the computed checkpoint coordinates with the given values. If the checkpoint residual test passes, ISAT will issue a statement that all checkpoint residuals are within the required accuracy.

4. If checkpoint residual testing did not pass, ISAT will issue a warning and let the user decide on running a local automatic aerial triangulation using GPS/INS assisted triangulation concept applying the shift and drift parameters.

5. If the checkpoint residual test passed, ISAT will automatically generate model pass points and performs space intersection to check analytically if model pass points have any parallax. If parallax is evident, the user is given an option to eliminate the parallax by using the localized GPS/INS-assisted AAT option.

6. If the pass point test passed, ISAT will move on to automatically generate tie points between strips. These points will be used again in the space intersection mode to determine whether or not there is remaining parallax between the image strips. If no parallax is discovered, then the software will issue a statement that no errors are found in this project and the EO data can then be used directly in the map compilation mode.

7. If parallax is evident, the user is given an option to eliminate the parallax by using the localized GPS/INS-assisted AAT option.

8. Each time there is any need to run a localized GPS/INS-assisted AAT, all the check listed in steps 1 to 7 will be checked again. If all those checks are passed, ISAT will issue a statement that all errors found in this project are fixed and the EO data can then be used directly in the map compilation mode.

If any of the checks fail, the user will be given the option to run the entire project using ISAT’s GPS/INS-assisted automatic aerotriangulation engine, to fix the remaining problems.

Dr. Mostafa Madani, Chief Photogrammetrist and Product Manager,Z/I Imaging Corporation.

Dr. Mohamed M.R. Mostafa, Applanix Corporation.

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