Activity 4 - Gathering Ground Control Points

Introduction:
The primary focus on this activity was to get the class accustomed to using Ground Control Points or GCPs as a way to georeference and be able to geometrically correct aerial images taken by a UAS. To get the utmost accuracy one has to be very careful with the methods and equipment used in collecting GCP data. To prove this point we used a plethora of devices with GPS recording capability and compared them. When using GCPs in an area a bare minimum of three points has to be used. The more points you use, the better your end data will be. If you area has a lot of topographic variance more points will be needed to accurately gather data for that area. When focusing on an area GCPs should be well spread out and one should avoid placing them on the edge of the desired area because the further you get away from the center the more an image is distorted.

Study Area:
For this activity we moved to a new area (Figure 1) not as cut and dry as the soccer fields we are used to.

Figure 1 - Highlighted area where we placed our GCPs, located south of South Middle School

Weather conditions that day were adequate for the task at the hand, it was sunny and remained in the low 60s.

There was some varying terrain but nothing more than some tall grass and some cut trails. We focused on covering the area around the small pond. If there was more variance in elevation we would have had to of used more GCPs. 

Methods: 
The first step was laying down our GCPs in different locations, for this area we placed 6 around the pond in different spots in the study area shown in (Figure 1). You can use a number of different items as markers but for this exercise we were lucky enough to survey marker mats (Figure 2). 

Figure 2 - Marker we used as a GCP

These markers were nice because they were all uniform is size and gave us an exact cross to focus on in the middle. Consistency is key with GCPs to keep a high level of data integrity.

For data collection we used a wide variety of methods each with varying levels of accuracy. 

The first tool we used was a Dual Frequency Survey Grade GPS, the level of accuracy with this is unmatched but it comes with a price, 12k for educational use and 18k for commercial. (Figure 3)

Figure 3 - Dual Frequency Survey Grade GPS 

With all the devices we used it was very important we took the reading from the same spot on the GCP as you can see above (Figure 3) Ethan took his time making sure the device was level and in the center of the GCP.

The next two devices we used were similar to one another just had varying levels of preciseness and also different costs respectively. Both were Bad Elf GPS just one was of survey grade (Figure 4) at $600 while the other (Figure 5) was at the enthusiast level at $125. We paired both of these with a tablet to record the data.

Figure 4 - Survey Grade Bad Elf

Figure 5 - Bad Elf GPS Pro

The next device we used was a basic Garmin GPS unit, these can be purchased for less than $100. They do not claim to be pinpoint accurate but are more so aimed at giving you a general idea of where you are. Nonetheless we used this device for the sake of comparison. 

Also for the sake of comparison, we used a smartphone in two different ways to show how truly inaccurate cell phone GPS is and how it should not be used in surveying or with CGPs. We used ArcCollector and the GPS data associated with images taken on an iPhone. 

Lastly, we drew up a mission on Mission Planner (Figure 6)  and used the Matrix Quad Copter to do aerial imaging. This data will be processed and I will post the results after that is completed.

Figure 6 - Mission Plan we used for aerial imaging with our GCPs below.

Discussion/Results:

The devices we used all had their pros and cons. Generally the more expensive and complicated the better results you will get. The Dual Frequency Survey Grade GPS was cumbersome and expensive but highly accurate. The Bad Elf GPS units were easy to use and cheaper but you sacrifice some accuracy with those trade offs. The last devices really have no place in the GCP world but were interesting to use and compare to the more specified devices. Our results of the comparison can be found below (Figure 7). 

Figure 7 - GPS Unit Comparison

You may think by glancing at this that they were all pretty close but when it comes to surveying and GCPs we want pinpoint accuracy and that is where the expensive, harder to use equipment really shines through. 

Surveying technology, like any other field is rapidly growing and more portable options with precise accuracy will only become more and more available. Although the Dual Frequency Survey Grade GPS is amazingly accurate it is bulky, takes an extra amount of time to use and could be difficult to use with different terrains. The Bad Elf on the other hand is small, portable, much cheaper and still fairly accurate. 

Gathering GCPs is time consuming because one must be very precise with GCP location selection and collecting the data. Depending on the area being analyzed many factors such as topography, weather conditions, foot traffic, etc all have to be considered. 

This activity relates to commercial practices in a few ways but mostly in the sense that these are the tools professionals are using today and to be successful in this industry you not only have to know how to accurately collect the data but processing it correctly is equally as important. 

GCPs are important whenever accuracy is paramount. As our readings say, the more GCPs you have the better the quality of data but you must have over 3. 

GCPs relate to UAS missions because they are used as a second form of measuring an area. When processing the aerial images, GCPs will come in very handy for verifying the calculations are correct and the data is valid. 

Conclusion:

Although sometimes time consuming and not nearly as thrilling as flying a UAS to collect data GCPs play a vital role in GPS data collection and verifying the information you collect is correct and as accurate as possible. Depending on the device you use the level of accuracy is directly related. When using GCPs you must have at least three markers and if your area has varying levels of elevation that all has to be considered. As technology improves collecting GCP data will get quicker and easier while remaining very accurate.