Geographic Datums

 Introduction:

A datum is a coordinate system with a reference surface that provides already known locations in order to create maps accurately. Currently, for the United States, there are two datums. The North American Datum of 1983 and the North American Vertical Datum of 1988. These serve has a reference point to ensure that the coordinates and points on our maps are accurate. In this lab, we used a set of ground control points that were bad in order to gain a better understanding of how to work around these errors. We used ArcGIS Pro in order to analyze the data.

Objectives:

#1: Discover, identify, and apply basic concepts behind geographic datums

#2: Recognize, relate, and compare different types of datums in ArcPro

#3: Demonstrate proficiency and knowledge on how to effectively bring XYZ coordinate data into both ArcGIS Pro and ArcGIS Earth

#4: Demonstrate proficiency and knowledge on how to effectively determine the quality of a dataset and identify issues with the vertical datum

Method/Assignment: 

To start with this lab, we began by looking at the GCP data in an excel document. The data in this document was leaving a lot to be desired. There was not much information in terms of explaining what the various things were. Most importantly it did not display a projection for data analysis. It is important to never guess what the projection of a dataset is. In this case it was able to be figured out by looking at the numbers. (Figure.1) 

(Figure.1 GCP Data)

After analyzing the GCP data we moved into ArcGIS Pro to dive deeper into our analysis. The map that we started with was a simple base map with only the GCPs placed. (Figure.2) From here we imported a .txt file that contained the XYZ coordinates for the GCPs. (Figure.3) 

(Figure.2 Base Map)

(Figure.3 .txt File)

From here we then wanted to import the point data into ArcPro. By clicking on the 'Add Data' dropdown we were given options in which this case we chose 'XY Point Data'. A window appears on the right hand side of the screen with fields that we must fill in. For starters, importing the table of point data into the 'Input Table'. The 'Output Feature Class' will fill itself out automatically in this case. Then we verified that the XYZ fields were correct and that the coordinate system was set to WGS84. (Figure.4) 

(Figure.4 XY Point Data Window)

After running the process it is clear that there is an error. While the GCP points did load in with the correct spacing, they are in the completely wrong location.(Figure.5) By zooming out we can see that the points are all the way down in Antarctica. (Figure.6) There is a very large issue here that the people who collected the data did not account for. The coordinates are swapped. The reason that the points appeared in Antarctica is because the X and Y values are swapped. 

(Figure.5 Point Data Loaded in)

(Figure.6 Points in Antarctica)

This is a very large error but it is easy to fix. We deleted the XYZ data set since it is no good and instead uploaded the file YXZ which is the same coordinates but the X and the Y are swapped. After running it again, we can see that the GCP data is now in the correct location. (Figure.7)

(Figure.7 Correct GCP locations)

From here we want to see the elevation values for the GCP points. To do this we opened up ArcGIS Earth and loaded the table of points in a similar way we did it in ArcGIS Pro. (Figure.8) By looking at the Northern and Southern most points, the elevation values are off and that is because of a datum issue. The survey was conducted according to the geoid but we want the ortho surface measure. 

(Figure.8 ArcGIS Earth Data Points)

In order to get the correct measurement for elevation, we used the National Geodetic Survey Data Explorer website. From here we located the area that the operation was conducted and found the marker that was closest to our data. The information showed that the Ortho height was 381.550 and the Ellip height was 347.830. The difference between those numbers is roughly 34.(Figure.9)

(Figure.9 National Geodetic Survey Data Explorer)

From here we can do some math to calculate the correct elevation  heights that we need. The difference is 34 adding that to the values will give us the correct elevation. 

Discussion:

Understanding datums and how they can effect the quality of UAS data is important to ensure accurate results. If the point data is in the wrong order, the outcome can be vastly different than what is intended. In the future, I can expect to get data that is not correct and it will be up to me to figure out how to fix it. I feel that this lab gave me a good understanding of how I can correct common issues with XYZ coordinates and datums.