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

Introduction:

In this field activity, our class experimented with various methods of collection ground control from the very accurate to the very approximate. Ground control points, also known as GCPs, are used to improve the quality of aerial imagery and data collection if gathered correctly. At their most accurate, you can measure out the altitude, latitude and longitude within millimeters. Accuracy is important when it comes down to collecting survey grade data which some companies may need when require geospatial data to be collected at high temporal frequency to monitor a development plot in the mining or agriculture industry for example over time.

Study Area:

This weeks field activity took place near the South Middle School/Southside Community Gardens of Eau Claire, Wisconsin where just south is a large open area with walking paths and a few small bodies of water. This location was chosen due to it's varying landscape and to provide a different location other than a soccer field complex. The study area fell inside and included the gravel walking path that surrounded the northern most body of water, which wasn't visible from ground level due to it being surrounded in tall grasses and weeds. Ground control points were placed along the path around this body of water and with three of the six being placed inside the path in the grass. The weather was clear with a considerable amount of wind to the West.

Figure 1: Image of study area from google images. Area studied is
 outlined in red and North direction indicated with red arrow 
in bottom right corner. 

Methods:

Six ground control points were placed around the study area. Each of the GCPs were made from tarp-like material with holes at each corner for nailing the GCP into the ground. Each side of the square GCPs was the base of a white or black triangle, whose points met at the center to form what could be described as intersecting black and white hourglasses. When placing the GCPs it is key to place them within the borders of the study area where they are visible from above, that way they can be seen and will not be warped by being near the edge of a image. The first GCP was placed on the gravel path just south east of the end of Hester St.

Figure 2: Nadir image taken with Matrix UAS above
the first ground control point.

The second GCP was placed South of the first in the tall grass approximately half way down the path before the first turn. The third GCP was placed South of the second at the first Eastward turn of the path. Approximately halfway up the Northeastern pointing path there was a thin gravel path that extended towards the small body of water, at the edge of this path the fourth GCP was placed. At the end of the Northeastern path where the direction changes to go Northwest, the fifth GCP was placed and finally halfway up that path to the Northwest, the sixth GCP was placed in the tall grass on the inside of the path. While placing the GCPs the altitude, latitude and longitude were taken with the Dual Frequency Survey Grade GPS which is a high precision unit that would act as a comparison the other GPS units used.

Figure 3: Dual Frequency Survey Grade GPS
on mounted system 2m above the ground at GCP 5

Figure 4: Topcon display connected to
the Dual Frequency Survey Grade GPS
showing the reading at GCP 5

Figure 5: Bad Elf GNSS Surveyor
(yellow) used in data collection

The class was then broken up into groups and each group collected data at the GCP for a different type of GPS unit. Each device was placed at the center of the GCP (Where the triangle points met) and a data point was collected. This was done at all six GCPs. My group collected data using the Bad Elf GNSS Surveyor GPS, a unit that should have accuracy to within one meter and tens of thousands of dollars cheaper than the first unit used. A tablet app was used to gather data with this device. The next group collected data on a tablet device with the even cheaper Bad Elf GPS which was not survey grade. Other groups collected data with the Garmin GPS and collected geotagged images with a smart phone. After these data points were collected, a mission was flown using the Matrix quad-rotor UAS to determine how accurate the sensors were and how they related to the GCP markers. After all experimentation was done, the temporary GCPs were collected.

Results and Discussions:


Below are some of the data sets collected by the the Topcon Survey GPS and Bad Elf GNSS GPS. Notice the precision in latitude and longitude of the 12 digit Topcon compared to the 8 digit Bad Elf.

Topcon Survey GPS

GCP:  Latitude,            Longitude,       Altitude

1:  44.7773757474,  -91.4729374398,  267.839
2:  44.7767401655,  -91.4729385628,  267.670
3:  44.7758957110,  -91.4728733066,  267.487
4:  44.7763877743,  -91.4721470321,  267.580
5:  44.7769016214,  -91.4711962560,  269.177
6:  44.7773686301,  -91.4723883271,  267.341

Bad Elf GNSS Surveyor GPS

GCP:  Latitude,  Longitude,  Altitude

1:  44.777397,  -91.472922,  267.839

2:  44.776792,  -91.472942,  267.670

3:  44.775911,  -91.472892,  267.487

4:  44.776406,  -91.472128,  267.580

5:  44.776922,  -91.471192,  269.177

6:  44.777378,  -91.472386,  267.341

Figure 6: Study area with overlayed GPS location as recorded by the various units described on the key in the bottom right-hand corner for purposes of analysis of accuracy the XYZtopconsurveygps should be used as the most precise coordinates.

One of the main goals of this activity was to experiment with collecting GCPs with various GPS devices and determine their accuracy. If you refer to figure 6 above, you can see that the different GPS devices were indeed varying in their capability to provide accurate coordinates. It was expected that the most expensive and precise Topcon would outperform the rest in terms of accuracy and that appears to hold true. It was also expected that as you downsize in price range down to the collector app of an iPhone that the quality of data would decrease. Based on figure 6 we can see this theory is only somewhat true. Based on the image we can see that the iPhone does produce some obscure points around GCPs 5 and 6 on the right of the image, but the one that produces the most obscure results is the Bad Elf pro collector which is a surprising result.

As you can imagine the cheaper alternatives to the Topcon were easy to used and to collect data from with minimal set up at each GCP. All the user had to do is place the device approximately near the center of the GCP and click a button to record the point, however this simplicity shows in the accuracy. If you want to get precise data for high frequency collections to observe changes it is necessary to use precise equipment. Though it was time consuming to stabilize and re-stabilize the Topcon at each and every GCP it did provide accurate measurements that can be analysed over many surveys to be used in a commercial regard.

If you want to gain accurate and useful data it is important to have enough GCPs. It is key that you have no less than 3 GCPs for you study area. The number of GCPs also depends on the terrain. The more variance in elevation, the more GCPs you will need, likewise, the more monotone the study area, the more GCPs you will need to help stitch the images together. Obviously the more GCPs you have the more time consuming the process of collecting accurate data will be, however, your data will be more useful if you do this.   

Conclusion:

In this field activity, various types of GPS units were used to collect data at six different GCP locations. It was shown via the Topcon that the more time and expense it took to gather a point, the more accurate the data you will receive. However, cheaper survey grade devices had some accuracy, and could be used to collect data over a single study. The GCPs we used were only temporary, and therefore useful for a short term study like ours, but for in a commercial study our methods would not have been accurate and we would want to elect to use more permanent GCPs.