Survey of Point Features

INTRODUCTION

This lab explored surveying points and finding attribute data for each point. The data measured included location, pH, temperature, and volumetric water content. A variety of tools was necessary to measure this information. This lab ties into the subsequent lab, involving UAS, to combine aerial imagery with ground data.


METHODS

Ground Data

The survey was taken at a community garden in Eau Claire, pictured below in figure 1. To determine locations of data points that would be measure, flag were inserted into the ground every ~3 meters across a section of the garden. First, measurements of the point were taken. This data was then entered into the GPS unit, which would store the data with the coordinate.

Figure 1: Garden that was surveyed

The pH, temperature, and water content were measured with handheld tools. The pH measurement was performed by scooping a small amount of soil near the flag into a plastic cap, where water was added to create a slurry. Then, the instrument was inserted into the cap, as shown below in figure 2, and the pH readout was shown on the LED screen. Note that the instrument had to be calibrated using samples of known pH before use. The temperature was recorded by inserting an electronic thermometer a few centimeters into the soil next to the marker flag. The volumetric water content was measured using a TDR probe. This probe used time delays in electrical signals to measure the percent water per volume in the soil. Similar to the thermometer, this was inserted into the soil and a measurement was recorded.

Figure 2: Measuring the soil pH

To measure the location, a TopCon Dual Frequency Survey Grade GPS with sub-centimeter accuracy was used. This was placed over the flag, as shown below in figure 3. After the GPS was centered, the previously recorded measurements for the point were entered into the GPS unit using the touch-screen on the device. After the data was entered, the GPS took 30 readings of the coordinates for the point and averaged them to find the location within centimeters. The attribute data was stored with each point of the device.

Figure 3: Using the GPS for a data point

UAV Data

The next step was taking aerial imagery through the use of a UAS, or unmanned aerial system. The UAS used, pictured below in figure 4, was a DJI Phantom. The Phantom was fitted with 6 propellers and a bottom-mounted camera. It could be flown manually or follow a flight plan.

Figure 4: DJI Phantom UAS before flight

Before the flight, however, GCPs, or Ground Control Points, had to be set up. This was done by placing a clearly visible numbered plate on the ground, as shown below in figure 5. The precise location was taken through the use of a survey GPS unit, used previously. This unit was done by centering the GPS tripod directly over the center of the plate, as shown below. A built-in bubble-level was used to ensure the GPS sensor was directly over the center. Note the sensor is not the same unit as the unit with the screen. The sensor itself is at the top of the center pole of the tripod.

These GCPs were placed across the study area to provide data necessary to properly align the aerial imagery. The plates were left on the ground for the flight, as their purpose is to be seen in the aerial imagery to assign coordinates to.

Figure 5: GCP preparation

With all the GCPs set up, the flight could then begin. After numerous software updates, professor Hupy began the flight manually and flew the Phantom to altitude. He then began the flight program designed for the exercise. The flight was designed to have enough overlap for multi-band imagery. Once the flight program was finished and the data collection had concluded, the Phantom was manually landed as shown below in figure 6.

Figure 6: Landing the Phantom UAS

RESULTS

Shown below in figure 7 is the orthomosaic map created with Pix4D. An orthomosaic is a number of smaller tiles mosaicked together. Notice there are no seems within the image.

Figure 7: Orthomosaic map

Shown below in figure 8 is the DSM of the area. The taller objects in the center are trees, and the rest of the area is relatively flat. You can see a road on the western side and curving south of the treeline. Pix4D uses overlapping images to generate heights throughout the area. Note there is a hillshade effect on the DSM.

Figure 8: DSM of study area

Finally from the imagery there is a collection of oblique views of the area shown below in figure 9. The views were taken in ArcScene by setting the elevation of the orthomosaic to the DSM image. The numbers represent where the viewer is.

Figure 9: Oblique views of study area

Below are surface maps generated from the ground data taken. First, in figure 10 are pH values taken in the garden. The interpolation method used was IDW.

Figure 10: pH values of garden

 Next, in figure 11 is the temperature data taken by inserting a probe into the soil.

Figure 11: Temperature data

 Finally, figure 12 shows water content data taken with the moisture probe.

Figure 12: Water content data

DISCUSSION

This lab explored several methods of field data acquisition. Ground data was used in conjunction with aerial imagery. These methods are commonly used in agricultural studies, among other things.