Activity 4: Gathering Ground Control Points (GCPs) using various Global Positioning System (GPS) Devices

Introduction

This weeks activity was all about collection of Ground Control Points (GCP's). GCP's are vital to collection of high quality aerial data. They are used to tie down the images collected to the surface of the earth and can increase the spatial accuracy of those photos to as close as millimeters from the actual location on the Earth's surface. The opposite is also true that if the GCP's you collect are not accurate they can distort the data and make it useless. Use of GCP's is important especially when working with temporal analysis such as vegetation change. The pixels in all of those images need to line exactly over each other from each weeks image in order to be able to display the vegetation change accurately. After we collected the GCP's we then conducted a flight with the Matrix platform so that we had some data to use these the GCP's later in the semester.

Study Area

This week we met at the community garden and swamps in Fairfax Park (Figure 1) behind South Middle School which is about 5 minutes from campus. During summer in my work as the GEI technician and the my research this summer with Dr. Hupy we flew this area every week so it is a familiar area to me. The weather was pretty decent it was about 63 degrees with winds 3 to 5 MPH. It was a mix of sun and clouds but being late in the day and approaching the winter months the sun was quite low in the sky.

Figure 1 This was our study are for activity 4. The yellow are is where the GCP's are placed inside of and where the imagery collection flight was flown with the Matrix platform and an SX 260 camera.

Methods

The first thing we did when we got to the study area is we set out GCP markers (Figure 2). These markers need to be an object easily seen in the imagery that the UAS collects. If they are too small or hidden by ground cover they do you no good. This gets into the considerations when placing GCP targets. First is what I mentioned above. The targets have to be visible. Avoiding trees or other ground plants that would cover the target as well as making sure the targets are large enough that they can easily be seen in the imagery. Next you want to think about the spacing of the markers. It is mandatory to have at least 3 GCPs in an area that you are collecting data in. In places that there is dramatic elevation changes more should be used. The GCP's are what gives the computer the elevation data for an area so if you want to accurately model the terrain in an area more GCP's are essential. They need to be spread out as well. Putting them right next to each other or in a line does not do any good. You want them to be spread out and many times they will end up being placed in a triangle pattern to ensure proper spacing. Aerial images get more distorted as you get further away from the center of the image we can put a couple GCP's towards the edge of the study area to help correct this however you want to make sure the majority of the GCP's are spread out equally throughout the study area. Taking notes while placing the markers is good practice. If for some reason you lose a marker or your lat/long positions of the markers gets messed up having a drawing of the approximate location of the GCP's is important.Field notes are always important to take while in the field. Even if you don't think something is important to record, you never know it could really be helpful later on during image processing or years later if you want to remember how you did a specific field activity. I made a drawing of the area we were working in (Figure 3). It may not be super accurate but it gives me a general idea of where all the markers should be.

Figure 2 This is one of the ground targets we used that will show up in the aerial imagery. We collected the GPS location for each of the 6 markers like this one we placed out.

Figure 3 This is the field drawing I made while we were collecting the GCP points. You can see it gives the general arrangement of the area and has the approximate location of the GCP markers. Sketches like this should be made whenever you are out in the field.

Taking into consideration what I discussed above the class set out 6 GCP's around the study area. Then we collected the GPS positions for each of them with 4 different units. This was done by placing the GPS unit directly over the center of the ground target and collecting the point (Figure 4). The first is the Topcon survey grade GPS unit (Figure 5). This was our gold standard for GPS accuracy. This unit collects points to with in a few millimeters of the actual location. It is a very expensive unit, around 5,000 dollars, but when precision matters this the unit to use. The second unit was the BadElf Survey Grade (Figure 6) unit which runs about 500 dollars. This is supposed to be a pretty good unit with an error variance of about 1 meter. We were expecting this to be the second best accuracy wise but when we ran the data and compared it with the Topcon points and the other units it wasn't extremely accurate. The Garmin GPS (Figure 7) unit we used was the second most accurate. This unit is around 200 dollars to purchase and looking at the results map (Figure 9) comparing the units it is much more accurate then the BadElf and not bad compared to the Topcon unit. Finally we used Dr, Hupy's iPhone (Figure 8) to collect he GCP location. Cell phones should never be used to collect GCP coordinates. They are terrible. The only reson Dr, Hupy used this method is to show the class how inaccurate they are compared to the other units we have available.

Figure 4 Dr.Hupy demonstrating how to collect points with the Topcon GPS unit. Notice how the unit is directly over the center of the target.

Figure 5 This the Tesla ground station component of the survey grade GPS unit we have on campus. You can collect GPS with just this device but they are not super accurate. This gives you the easy user interface.

Figure 5 This is the Topcon survey GPS unit itself. This connects to the ground station via Bluetooth. With this unit you get accuracy within millimeters of the actually location on the Earth's surface. 

Figure 6 This is the Bad Elf survey GPS unit. It is nice and compact which makes it convenient for feild work and the accuracy is supposed to be pretty good with it. In this activity we found the accuracy to not be as good as it is supposed to be.

Figure 7 These are Garmin GPS units similar to the one we used for this activity. Compact size makes them good field tools and the accuracy is pretty good for the price of the unit.

Figure 8 This is your average iPhone. NEVER USE THIS TO COLLECT GCP POINTS WITH. The accuracy is terrible.

Figure 9 This is the results of the 4 different GPS units we used. You can see the Garmin unit was second best to the Topcon unit and the Bad Elf we after the Garmin. The iPhone points are terrible as expected. These location are where the GCP markers were placed and when the imagery is processed we will tie the images to these GPS points.

The final portion of the activity was to fly a mission over the swamps where we placed the GCP's. As always we went through all the preflight checks and took all the same safety precautions as any other time we fly. Having a PIC, PAC and spotter. Please see Activity 2 and Activity 3 for the pre flight procedures and image collection details. It was gettting close to dark when this mission was flown so it will be interesting to see how the images turn out. I am anticipating there will be alot of shadows and dark areas that the processing software will have a hard time with so we may lose chunks of the data. That will be determined when the data is processed in a couple of weeks.

Discussion

This was a very hands on activity for the class. It was good to have an activity that the whole class could do at the same time instead of being split into groups like some past activities. The biggest thing I took away from this activity is that GCP placement and the quality of your GPS unit are what makes or breaks your GCP data and eventually the accuracy of your imagery data. We have access to some of the best GPS units money can buy. The Topcon unit is top of the line and it shows with the accuracy of the points it collects. This isn't always necessary.For our purposes in this excercise using the Garmin or BadElf GCP points would give a good accuracy in our aerial imagery and the Topcon is overkill.In a situation like the vegetation change I talked about at the beginning the Topcon would have to be used because the other units aren't accurate enough. Another instance where you would want the Topcon is when creating a terrain model of a mine or other area with large elevation changes. Just like choosing a UAV platform and sensor is mission specific so is the GPS unit used for GCP's. Knowing the level of accuracy you need and what unit will give you that accuracy is vital to good field work and data collection. Location and number of GCP's is the other piece that is very important. A lot of the placement is common sense but just being aware of your surrounding and the lay of the study area, as always in field work, will greatly increase the quality of work you conduct. Being able to tie all this information into the data processing step is another valuable skill that we will learn in the next couple of weeks.