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Week 15: Datums Mystery
12/08/2025
This week was essentially a mystery to find how to correct incorrect GCP data. essentially, the geoid and ellipoid and orthomosaic height were mixed up (without metadata), and a known NGS point corrected the errors. To begin the assignment, the coordinates given for the GCPs were given in (x, y, z) format, and this dataset placed the points in antarctica. This is impossible, as the flight was conducted in the Midwest United States of America. This dataset was rejected, and the (y, x, z) format coordinates were used instead. These new coordinates placed the GCPs where they were thought to have been, which was in Wisconsin. Figure 1 below shows the topography of the area in which these points were found, and Figure 2 shows the coordinates of one of the points. Further below, Figure 3 shows the elevation value in meters for each of the control points, but something was off.
Figure 1: GCP Geography
Figure 2: GCP example coordinates
Figure 3: GCP elevation uncorrected (m)
Through some detective work, it was found that the vertical datum was set to the geoid height, but the ortho height was what we were after. The GCP height value was far different than that shown on the ortho imagery. Therefore, the National Geodetic Survey (NGS) Data Explorer was researched, and these heights would be changed. The closest recorded trustworthy NGS point was the “Bloom S GPS,” and this was found through a visual search in the menu seen below in Figure 4. For this known point, the difference between the geoid height and the ortho height was approximately 33.8 meters, and this difference was applied to the GCP heights to correct and calibrate them. Figure 5 below illustrates the new heights of the GCPs in meters.
Figure 4: Bloom S GPS search
Figure 5: GCP elevation corrected (m)
After completing that task, the mystery was solved and the heights of the GCPs were calibrated adequately with a trustworthy zero. In light of these findings, the GCPs in question and the data associated with them were used to make two maps. The first of these two maps includes the topography of the area’s elevation as well as the coordinates of each GCP and can be seen below in Figure 6. The second of the two shows ortho imagery which has the GCPs expressed as red dots on top, all overlaid with a coordinate reference grid. This map can be seen below in Figure 7, and the vertical datum is expressed in text for both maps. A full version of this lab assignment is available upon request.
Figure 6: Corrected GCP location layout with coordinates
Figure 7: GCP coordinate reference grid layout
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