Field Activity #4- Gathering Ground Control Points using various GPS Devices


     In our fifth field activity we were introduced to Ground Control Points (GCP's). GCP's are used to improve the quality of your aerial imagery acquisitions. When gathered properly they can produce data with sub-meter, and even millimeter accuracy. Likewise, when gathering your GCP's, if you do not gather them properly they can then diminish the quality of your accuracy. Another reason we want to use GCP's is that we are then able to tie down our imagery to a given coordinate system if the digital sensor doesn't have GPS on it (GoPro, Canon S110). It is important to have a coordinate system when displaying our imagery, especially for survey quality data.
     When looking at our survey area it is important to place our GCP's in a visible spot for our UAS to take images of. You don't want them placed under trees or other debris obstructing our field of vision. When placing our GCP's in our survey area it is important to have them spread out over the entire field. The closer you are to the edges of the survey field the more distorted our GCP's become. So, we do want one or two near the edges but not right on the edge while placing the other GCP's randomly throughout the rest of the survey field. Another point to bring up is when placing your GCP's with different elevation. If there is a hill in your survey area it is important to place more GCP's around and on that area. This will help with displaying the elevation of that area. It is a rule of thumb to have a minimum of three GCP's in your survey field, while it is recommended for better quality to have more. GCP's are very time consuming as you have to chart and mark where they all are. It is vital to have good field notes that you can look back on when you sketching your survey area with your GCP's. This will allow you to look back if there is any error when recording your GCP's. Pictured below is a sketch of our survey area that I took in the field while also labeling my GCP's.

    We placed six GCP's over the survey area in field activity five. We spaced them out relatively evenly throughout the survey field while making sure they weren't to close to the boundaries for distortion. Upon placing our GCP's we also recorded the GCP's with a Dual Frequency Survey Grade GPS. This GPS for us was our gold standard as it will get accuracy down to millimeters. This was the first time that we were introduced to this method of recording our GCP's so we were all relatively new to it. Some important things when dealing with this GPS was to make sure it was in the exact center of our GCP along with being level. This would allow for us to get the most accurate reading for our GCP's. Now when we recorded our GCP in the GPS it would tell us the horizontal and vertical extent along with the exact coordinates for the GCP. This would then allow us to tie down our GCP's to a coordinate system upon uploading the images. Pictured below is the gold standard GPS that we used to record our GCP's the first time. Also pictured is the first GCP that we placed in the survey field.

    The second method we used when collecting our GCP's was the Bad Elf GNSS Surveyor GPS. The GPS can produce sub-meter accuracy. This is a relatively small GPS that we lay in the center of the GCP on the ground then connect to a tablet app that will in turn allow us to record our GCP. We can also put field notes in the tablet allowing us to know the exact area for this GCP along with any other field notes that are worthy. Pictured below is the Bad Elf GNSS Surveyor that we used in the field. You can see that it is about the size of a stop watch, but is a very good device when collecting GCP data location when teamed with a tablet app.
 

    The final way we collected our GCP's was with a mobile phone. The reason we used a mobile phone GPS, is that after we analyze the data later this semester that we can then see the difference between the accuracy between the three different GPS's. We know the survey grade GPS will be the most accurate, but we want to see just how inaccurate your mobile phone GPS actually is. In today's society people rely on their mobile phones everyday when traveling and it would be good information to show and share just how inaccurate these devices are.
    The final part of our field activity was to carry out a flying mission over our GCP's and survey area. We used mission planner to plan a mission and was able to fly over our survey area. Although it was getting relatively dark when carrying out our mission, approximately 6:30 pm, we still were able to capture the GCP's over our survey field. Now that we have our mission carried out, we can now analyze the data and tie down our GCP's to a coordinate system with the three different means that we collected. We have not done this yet but I look forward to analyzing this data to see how accurate or inaccurate each means of collection was. The picture below is of our first GCP in our flight mission. This picture was taken on the first pass in our flight mission.

     When collecting our GCP's the easiest and fastest way in my opinion was with the Bad Elf GPS. All we needed was the tablet and the Bad Elf and took 10 seconds for the GCP to be collected. I also liked that we were able to put field notes right into the tablet, this could prove very helpful when analyzing the data. The second fastest method would be with your mobile phone. Just by placing your mobile phone in the middle and snapping a picture you are able to capture the GCP. Only down fall is that it is least accurate. The slowest but most effective way to capture them was with the survey grade GPS. Although it was slow and you have to be precise, you also get the most accurate GCP's. The reason GCP's are so time consuming is that you have to scan the entire survey field and determine where the proper place is to collect your GCP's. Also depending on how large or how much elevation change there is in the field. Another reason it can be time consuming is by the means you are collecting your GCP's. Are you using just your mobile phone or are you using a survey grade GPS, these will play a factor in how much time you put into collecting your GCP's. When talking about commercial and survey grade GCP's many different types come into mind. These deal a lot with permanent GCP's that need to be collected weekly, and sometimes even daily. These could deal with mines or areas that are being excavated or even areas of washout. With having permanent GCP's you know exactly where your last recordings came from and allow you to go back to that exact spot. Now when using permanent GCP's you don't need to necessarily put something in the ground. You could use a sprinkler head or another object that you know that will not change by the next time you come collect your points.
     Upon completing this field activity I was fairly educated on how to collect GCP's and the importance of them when dealing with survey grade quality mapping. Without GCP's your imagery could be well off your intended survey zone leaving you with poor quality images and mapping.