Datums

Introduction

Geographic Datums are important for processing any GIS-related data. A datum, simply put, is the reference point. For an aircraft, it would define where you locate components on the aircraft from. It's the 0,0,0 reference point. Geographic datums and referencing systems are a bit more complex because there is more than one point for the areas that one may be studying. In the lab, we learned how to correct a datum and reference point settings in ARC GIS Pro.  

Methods

To begin, we were given an excel spreadsheet of points referenced in X, Y, and Z. These points did not have any specific units of measure with them or coordinating reference data, which is where the reference systems and datums came into play. When the data points were put into the spreadsheet, the latitude and longitude were flipped, so the default results located the points on the opposite side of the globe. The corrections were made by swapping the latitude and longitude to move the points to their correct location.

These markers represent ground control points (GCP). They are at a construction site. One thing to note is that in figure 2, the GCPs are all biased towards the edge of the map, which distorts the map when processing the orthomosic. 

We checked the terrain elevations of the GCPs to see if they were within reason with each other. The height data was as follows:

Control 00, 280.1

Control 1, 246.7

Control 2, 243.69

Control 3, 241.37

Control 4, 247.98

Control 5, 271.96

Control 6, 280.58

Control 7, 270.51

Control 8, 266.98

Control 9, 249.04

Flag 3, 263.6

Then we checked the northernmost and southernmost points to compare them to the base map. 

Northernmost GCP 06, Z=280.58

Northernmost basemap 06, Z=310.14m

Southern Most GCP 02, Z=243.69

Southern Most basemap 02, Z=274 m

Figure 1. Image of GCP locations on a base map

There was a variance of roughly 30 meters between the basemap and the ground control points. This was a datum issue as it was uniform across all of the points. To correct this data issue, we entailed the National Geodetic Survey Data Explored to find out what the exact offset was and correct it. From the site, we gathered the marker information as follows:

Name: BLOOM S GPS
Elev Source: GPS OBS
Elev Order: None
Post Source: ADJUSTED
Post Order: None
Ortho Ht: 272.77
Ellip Ht: 238.926

For the GEOID12A, we got a geoid height of -33.852 meters. Applying this offset to our values to our GCP points, the points more closely resemble the basemap and are calibrated to the NAVD88 height. 

Conclusion and Reflection 

In conclusion, calibrating the datums for post-processing UAS data is important in ensuring the accuracy of the data. Also ensuring that each number coming out of an operation has a corresponding unit removes any guessing from the operations. 

I am a bit confused as to why we are digging so deep into the calibration of data because "Note: for data to be survey grade it must be certified by a licensed surveyor." and as far as I know the UAS major is not certifying the students as surveyors. If you could elaborate on this in the comments Dr.Hupy would be beneficial.