Lab 4: Gathering Ground Control Points

Introduction:

The purpose of this lab was to learn how to establish ground control points (GCPs).  GCP's are used to georeference and geometrically correct images taken from unmanned aerial systems.  Therefore, the accuracy of the map is only as good as the accuracy and precision of the GPS used to establish the position of a given GCP.  In this lab, obtained the location of the GCP's using various GPS platforms so that a comparison in their accuracy could be assessed.  A minimum of three GCP's is necessary to adjust an image to a reference coordinate system but in general, the more topographically variable a location is, the more GCP's that will be required (Aber, J.S., Marzolff, I., and Ries, J.B., 2010, 124-128). Lastly, when placing GCP's, it is important to ensure that the GCP's are spaced out and not at the edge of the map area.  This is because the further you get from the center of the map area, the more distorted the map features become.

Map Area:

Photo 1: Map area outlined by yellow rectangle.  Map area is located between Fairfax Street and Hester Street, just south of South Middle School.  

Photo 2:  Ground view of map area taken from the cul de sac at the end of Hester Street. 

Conditions during the lab period were Sunny/Cloudless, and a temperature of about 60°F. Documentation of the weather conditions in addition to a hand drawn map may be found under the field notes section of this Blog.

Methods:

Before a mission could be flown using the Matrix Quad Copter, ground control points were established on the ground.  Based on the size of the map area and relatively flat topography, six ground control points were established (GCP locations visually represented on hand drawn map under Field Notes section of blog).  The GCP's consist of a black and white tarp approximately one meter wide and one meter long so that they can be seen from aerial photo (Photo 3).  As each point was established, the location was taken using the Dual Frequency Survey Grade GPS.  This device has millimeter accuracy, and served as the baseline for the other GPS devices to be compared to.  

Photo 3:  Using the Dual Frequency Survey Grade GPS to pinpoint Ground Control Point locations.  

Once the GCP locations had been pinpointed using the Dual Frequency Survey Grade GPS ($12,000), the latitude and longitude were taken again using the following devices: Bad Elf GNSS Surveyor GPS ($600 and claims sub-meter accuracy), Bad Elf GPS ($125), Garmin GPS ($100), and a Smart Phone GPS.

The final objective of this lab was to fly a mission over the map area, so that the images could later be georeferenced using data from the different GPS's listed above.  As shown in photo 4 and 5, a mission was planned using the mission planner software, and executed using the Matrix Quad Copter.  

Photo 4:  Showing the mission created
using the mission planner software

Photo 5: Showing the Matrix Mid Fligh

Discussion:  

The data obtained from this lab is not yet processed, so the discussion shall focus on the GCP's and different GPS's used.

In a commercial setting, it is easy to see how the Dual Frequency Survey GPS would be necessary for certain projects requiring the the most accuracy.  However, not all people an afford $12,000 equipment to do their mapping.  This is why technology is moving toward a more portable option such as the Bad Elf Surveyor.  Although the Dual Frequency Survey GPS provides high quality data, it is rather large, and may pose mobilization issues when collected data from areas with difficult terrain.  Therefore, in some circumstances it may be more practical to utilize more compact equipment such as the Bad Elf GNSS Surveyor if the results defend its claim to sub-meter accuracy.  Using the Bad Elf GPS (Photo 5) with the Tablet made data collection very quick and easy, and was by far more portable than the Dual Frequency Survey GPS.

Photo 5, Showing the compact shell of the Bad Elf GPS

Lastly, Ground control points could become less practical if the mapping area is covered by heavy foliage, and is extremely large.  As learned from the lab, establishing GCP's takes a substantial amount of time.  If the same process is scaled up to large area with tough terrain and foliage, the process would prove extraordinarily time consuming.  

Conclusion:

This lab taught me which factors to consider when placing Ground Control Points so that the aerial photos could be properly georeferenced when processed. Overall, we placed six GCP's around the map area, and made sure to pick locations equally spaced from each other, away from the edges, and on varying levels of topography.  In addition, it was interesting to explore the strengths and weakness of each GPS device, and I am interested in analyzing data obtained from the Matrix Quad Copter.