Geographic Referencing and Datums

    We are going to look at a variety of Geographic Datums and we were provided with several examples of how improper use of datums can ruin UAS datasets. We are going to deliberately work with a Ground Control Point file that is bad on several levels. We are going to work with a less-than-ideal file since sometimes the best way to see what not to do is to experience the problem. That is, rather than give you a nice and clean, tidied up data set, we are going to work with something more along the lines you can expect to experience at some point or, at the very least, to prevent from occurring ahead of time by informing those doing the flight and ground control work what information you need.

    The file we were provided is called GCP's_Ellipsoid. The concept of metadata is important because then we don’t have to speculate on what the information and units are. We don’t have to guess and it’ll give us what we need to know. We will revisit the topic of projections, but it's important to never guess on the projection of a dataset. In this case, we were able to confirm it was WGS84 from the numbers that this is Lat/Long

Figure 48: Shows The GCP Data File We Will Be Working With

After reviewing the metadata, it's time to open ArcPro and then open the GCP project. When you first load in the project, you should see a basemap with nothing but a basemap in the table of contents. This is because we are going to import XYZ data the correct way, and the incorrect way. It' important that we move away from the general understanding of Lat/Long. If you are familiar with navigation the conventional way to list the coordinates of a location is Latitude and Longitude. In the GIS world, we don't have Lat/Long as we use X, Y, Z.

• X is always longitude, or the lines running North and South
• Y is always Latitude, or the lines running East and West
• Z is the vertical value

We are going to import our data into ArcPro as a point feature class file. Click on the map banner tab, then click on the down arrow of the data icon and select XY data.

Figure 49: Shows XY Point Data On The Add Data Icon

For the first test, we are going to add the data by setting the remaining XY table to point processing parameters to what is shown below (Figure 50).

Figure 50: Shows First XY Parameters

After running the processes, we get a series of points that show up on the map. We can see that the coordinates are 43.4670048E, 89.7383326S. The points are located in Antarctica because the coordinate system or the datums were not set correctly. We are going to try it again, but the right way. Using the same data tool as before (Figure 48), but this time using the correct parameters shown below (Figure 51).

Figure 51: Shows The Correct Parameter Settings

Our data is now in the right location, but there are still some problems. This data is meant to represent ground control point markers and they are important when accuracy and precision are needed in UAS work; in this case, it was for a construction company. It's important to always look at the GCP layout before doing any processing and analysis to get an idea of the placement and to see if there are any issues. 

Figure 52: Shows GCP Points On The Basemap

I want to look at the Z values of these markers to make sure they correspond with the terrain. You can do this by clicking on each marker, the Z values are recorded below:

• 266.98
• 280.1
• 280.58
• 271.96
• 270.51
• 263.61
• 249.04
• 247.98
• 246.7
• 243.69
• 241.37

Now we want to compare these to what the elevation values are for this area. We can do this in ArcPro by adding the terrain layer from the living atlas portal, but we are going to do this with ArcGIS Earth because it's more likely you'll have access to it in the field. To add your data into ArcGIS Earth, click on the add data icon, then click add files option and select your files (Figure 53).

Figure 53: Shows Where To Add Data In ArcGIS Pro

We will start with a table import because this is what you commonly get from a survey crew. It's important to note that the data must be labeled correctly or else it won't import. Now, the points will appear on the map (Figure 54) but now you can also see the terrain. Notice how the land has a good amount of relied and the North end is higher in elevation and the South.

Figure 54: Shows The Points And Terrain Features

Now if we click on the Northernmost point on the hillslope, and the Southernmost point by the creek, we can record their elevation values.

• Northern Point: 280.58
• Southern Point: 243.69

Now if you move your mouse over near those points, we can record the approximate elevation.

• Northern Approximation: 310.93
• Southern Approximation: 274.91

The difference for the Northern point is roughly 30m and the difference for the Southern point is roughly 32m. This can be attributed to a datum error. We have the classic issue of the survey data set to the geoid, but we want to get the ortho surface measure. Ortho surface is a way of surveying types referring to elevation about mean sea level. We are going to use the internet to figure out how to calibrate these numbers. Since we know it's a datum issue, we can find out what the offset is and apply it to fix them. We are going to use the National Geodetic Survey Data Explorer to do this. This U.S government service provides free information that is related to billions of revenue in multiple industries. 

Use the go-to location on the menu to the left and zoom into the region and plot marks. Right-click on the map in your area of interest and select "Place X". You can change the radius used to plot marks under the map layers. It helps if you use the search bar to find your location; for this case, it's in Bloom City, Wi (Figure 55).

Figure 55: Shows The Search Bar

Now using either ArcPro or ArcGIS Earth to locate where the markers are on the map and click on an icon near where the survey is located, the following information is shown:

• Name: BLOOM S GPS
• Elevation Source: GPS OBS
• Elevation Order: None
• Position Source: ADJUSTED
• Position Order: None
• Ortho Height: 272.77
• Ellipsoid Height: 238.926

Orthometric height is the vertical distance from a location on the earth's surface distance to the geoid. The ellipsoid height of that same point on the earth's surface is the vertical distance from that point to the ellipsoid. This relates to the differences we noticed because that’s why we were getting two different values.

Now if you click on the datasheet link, this is the information that is needed when doing very precise work with UAS to make it survey grade.

• NAVD 88 Ortho Height: 272.77m
• NAVD 88 Ortho Height In Feet: 894.9ft
• Which Geoid Model Was Used: GEOID12A
• Geoid Height Under The Geoid12A: -33.852m
• Geoid Height Under The Groid18: -33.853m

Now apply the Geoid12A height difference to each of the Z values we collected earlier from the survey points. The corrections can be seen below:

• 233.128
• 246.248
• 246.728
• 238.108
• 236.658
• 229.758
• 215.188
• 214.128
• 212.848
• 209.838
• 207.518

These points match up a lot closer to the elevation points; they are not exactly the same, but they are much closer than they were before. We did it, we solved the XYZ and datum mystery!