Constructing GCPs

Introduction

For this week's activity, the instructor guided students through the process of constructing homemade ground control points (GCPs). As discussed previously, GCPs assist in georeferencing aerial imagery by attaching a coordinate to the GCPs themselves which is critical for the orthorectification of remotely sensed imagery. Creating GCPs by hand can be cost effective and will last a very long time depending on the quality of the material used.

Methods and Results

Figure 1: GCP construction material

The material used for the GCPs was a black, high density polyethylene sheet (8' x 4' x 1/4")(figure 1).

Figure 2: GCP cut down size

Using table saws, the polyethylene sheets were cut down into squares as seen in figure 2. The pieces were then laid down across bed sheets in preparation to be spray painted.

Figure 3: Spray painting triangle side 1

Using a wooden cutout in the shape of a triangle, the cutout was placed on top of the black square. Using neon pink spray paint and firmly pressing the cutout to the black square, the triangle was painted onto one side of the square (figure 3).

Figure 4: Spray painting numbers

While allowing the first triangle to dry, using neon green spray paint, numbers were painted onto what would be the adjacent black triangles on both sides in numerical order (figure 4). Each GCP was numbered "1, 2, 3,..." and so on.

Figure 5: Spray painting triangle side 2

Once the first triangle was dry enough, using the wooden cutout once again, the second pink triangle was painted opposite of the first triangle (figure 5). Once all GCPs were spray painted with triangles and numbers, they were then allowed to completely dry (figure 6).

Figure 6: Final result of GCPs

Conclusion

The GCP construction went smoothly without incident. Approximately 16 GCPs were made within about a 45 minutes time span which was pretty efficient. Initially the triangles were going to be green and the numbers pink, but these were switched at the last minute in hopes that the pink will stand out better in the imagery compared to the green color. These GCPs will be used in a future activity.