Correcting Datums

12/05/2024

    Not all UAS data is going to be properly organized or labeled, and being able to identify datum errors caused by lack of properly organized information is a valuable skill in the UAS industry. 

    

    In this assignment, we were presented with an Excel file containing GCP information for a flight. However, there is no metadata explaining what any of the values mean and the labels of the columns are not aligned. There is a file labeled GCP_groundvalue_xyz.txt, however, when we go to open the .txt file for the GCPs in ArcPro, the coordinates appear over Antarctica, indicating that the x and y fields have been mixed up. Fortunately there is a file with the correct datums, which when imported as shown below places the GCPs in the correct x and y position. 

    While the x and y values of the GCPs are now placed, we still need to check the Z values. To do so we can simply import the GCPs into ArcGIS Earth and hover the mouse over the points to check the ground elevation and compare this to the Z value of the GCPs. As suspected, the GCPs all appear to be about 30m less than the real elevation values. 

    To figure out what exactly is the issue, we can go to the National Geodetic Survey website. If we go to the marker nearest to the survey area, it becomes evident that the difference between the Ellipsoid height and the orthometric height of the datum also appears to be about 30m, same as before. We can then go into the datasheet for the marker to determine which Geoid Model was used to calculate the NAVD88 orthometric height (GEOID12A), and use this value to correct our Z values.

After applying the 33.852m correction to each of the Z values, the x, y, and z values of the GCPs are now accurate. Below is a map created in ArcPro displaying each GCP and the new, corrected x, y, and z values.