AT 309 Week 9: GCPs and Datums
Introduction
This week, the lab utilized ArcGIS Earth and ArcGIS Pro to demonstrate various problems users may run into if they do not know how to handle the data. The problems revolved around GCPs and datums. If data is not handled properly, the project can be skewed or even unusable in some cases.
Metadata
One of the lessons learned earlier in the semester was the importance of metadata. Metadata provides context about the mission that is necessary for analysis. The data used in this lab did not contain metadata. The only data provided on the flight was a file of the GCP ellipsoid information.
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| GCP Ellipsoid File |
The GCP ellipsoid file contained the information in the photo above. The GCP points are labeled and the XYZ fields represent latitude, longitude, and altitude for the GCPs. The XYZ fields are not labeled and this table is difficult to interpret. Because of the lack of metadata, the class had to input the data into ArcGIS Pro and fix any problems that came up.
XYZ Order
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| GCPs imported from file |
The mission in this lab took place in Wisconsin. When importing the GCP data without changing it, the GCP points were not located in Wisconsin. The order of the imported information was X, Y, Z.
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| GCP (XYZ) map zoomed out |
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| GCP corrected to YXZ order |
After making this change, the new GCP location was in Wisconsin where the mission took place. This is an important lesson because the data cannot be analyzed if the GCPs are on the other side of the planet. GCPs are used to provide accurate geolocation data in missions. It is vital that these ground control points show up correct location. Now that this issue was solved, the data was loaded into ArcGIS Earth to demonstrate another problem.
Datums
Putting the GCP data into ArcGIS Earth provided a 3D map of the mission area.
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| GCPs in ArcGIS Earth |
Initially, the data looks fine because the GCPs were placed in the correct area. When looking at the altitude values of the GCPs and comparing it to the projected height in ArcGIS Earth, the next problem showed up. The Z values for the GCPs were about 30 meters lower than the projected height in ArcGIS Earth. This is a datum issue. The issue is that the GCPs are in orthometric height and ArcGIS Earth is showing ellipsoidal height. To solve this problem, there is an online service called National Geodetic Survey Data Explorer provided for free by the U.S. Government.
NGS Data Explorer allows users to input the coordinates of a desired area and see information that is necessary for survey grade UAS work.
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| NGS Data Explorer for the mission area |
Clicking on one of the icons in the mission area pulled up a box of information that included orthometric height and ellipsoidal height. The height difference in these two numbers is roughly the same as the difference noticed in ArcGIS Earth.
The datasheet link in this box takes users to more in depth geo information about the area.
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| NGS Data Explore data sheet for Bloom City, WI |
The data sheet provides information like the geoid model that was used to determine orthometric height. In this case, the geoid model used was GEOID12A. Geoid height is the difference between orthometric height and ellipsoidal height. The geoid height for GEOID12A is -33.852 meters. This matches the height difference from earlier and solves the problem.
Conclusion
In the week 6 lab, we learned how much information can be interpreted about a mission through the metadata. Normally metadata would include information like the geoid model used. In this lab, we got to experience what happens when you have to process information without metadata. This shows the importance of metadata in a mission. Without the metadata it can take a while to figure out how the data was formatted. This is a waste of time that could be spent analyzing data. The problems dealt with in this lab can be easily avoided if time is taken to create a proper metadata file during the mission.
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