Collecting Soil Data Using Survey Grade GPS

Introduction

  The purpose of this lab is to collect soil data which will combined with next week's UAS data to see the difference between pH levels, temperature, and soil moisture between different garden plots at the community garden located near the ponds by South Middle School. A grid like system will be set up, similar to the sandbox grid. However, instead of using a Cartesian coordinate system to record the survey points, a survey grade GPS unit will be used to garner the exact coordinates. The other materials and tools used include flags, a pH meter, a soil thermometer, distilled water, and a TDR 200 Field Scout device.
  Elevation data and aerial imagery will be collected using the M 600 UAS platform. This will cover the community garden and the ponds to the south. This day also consisted of collecting the X, Y, and Z coordinates of the GCPs.

Study Area

  Below in figure 10.0 is a map of the study area. The study area is the community garden located near the ponds by South Middle School in Eau Claire. Because it is only April, the garden mostly consists of dirt and mud. There were about 10 different plots which the grid system covered. The only known plant to have already been planted was some garlic. It looked like some other plots had been planted in, but the seeds were not sprouted yet so they couldn't be identified. The weather on the day data was collected was overcast, a cool 40°F, and fairly damp. It had rained the morning of, so much of the dirt in the garden was very soft or was mud.

Fig 10.0: Study Area

Methods

Field Day 1

Getting the Materials and Tools Ready
  First, the pH devices had to be calibrated. Because this was the first time using them, the directions were used to help calibrate them. Then, the survey grade GPS unit had to be set up. Professor Joe Hupy helped with this and showed students how to take survey points and how to enter in attribute information. Then, the flags, soil thermometers, distilled water, and the TDR device were handed out to some students to use and collect data with. Figure 10.1 , displayed below, shows Professor Joe Hupy demonstrating how to use the survey grade GPS unit to some students

Fig 10.1: Learning how to use survey grade GPS

Collecting the Data

Fig 10.2: pH meter used 

  There were four main groups of students. Two groups used the pH meters and soil thermometers to collect data. The first group was in charge of picking the spots to place the flags. The flags were aligned in a grid like system. Besides this, both of these groups did the same thing. When placing the flags, if a garden plot looked like it had already been planted in, the flag was placed on the edge of the plot so that no damage would be done to the plants.
  The pH meter used is shown on the right in figure 10.2. This device was used by first rinsing the sample container (located on the bottom) with distilled water. Then, a small clump of dirt located next to the flag was picked up and placed within the sample container. The pH meter would then give its pH reading. Unfortunately, the squirt bottle for the distilled water broke, so it was much more difficult and took much longer to rinse the sample container than expected.
  The soil thermometer was a bit simpler to use. To use it, one just had to clean off the thermometer rod and then stick it into the ground to get a reading. However, sometimes the soil thermometer would give an error message and would fail to record the soil temperature. When this was the case, the pH device was used to also record temperature as it also had this capability. The soil thermometer also took a while to acclimate itself to the soil temperature, but it was usually done before the pH value had been recorded. The soil thermometer is shown below in figure 10.3

Fig 10.3: Soil Thermometer

  Next, another group used the TDR device to measure the soil moisture content. This device can be seen below in figures 10.4. It measures the soil moisture level by sending a small electric shock between the two metal rods on the device. This measures the conductivity of the water content within the soil which is then used to calculate the soil moisture content. The value given with the TDR device is the  percentage of moisture content in the soil. Because there was only one TDR device for the class to use. This group took the soil moisture level three times for each flag / survey point.

Fig 10.4: TDR soil moisture measuring device

  The fourth group was responsible for getting the coordinates of all the flags using the survey grade GPS device. They also had to enter in the soil pH, moisture, and temperature values when taking the survey points.

  The data collected from the first three groups where written down in a notebook, and later entered into the survey grade GPS unit. The survey grade GPS group lagged quite a bit behind the other three groups as this was the most sophisticated tool being used in the lab. The screen used to put the data in the GPS can be seen below in figure 10.5.

Fig 10.5: Attribute screen on the survey grade GPS unit

   The person who recorded the values in a notebook were responsible for communicating with the person responsible for entering in the data into the GPS. This got to quite confusing at times because the first three groups each had their own communicator and there was only one person in charge of entering the data into the GPS. Also, the group using the survey grade GPS was behind the other three groups by usually 5 or 6 flags.

Field Day 2

FIg 10.6: Survey Grade GPS Unit

  First, seven GCPs were laid out throughout the area which would be flown over with the M 600. There were already 9 GCPs laid out by the UAS class the day before which had been left overnight so that they could be reused. The GCPs were laid out in a trail like pattern around the ponds in order of their numbers so that it would be easier to keep track of each GCP when using the survey grade GPS unit. The survey grade GPS unit can be seen on the right in figure 10.6. In the photo, three students are taking the X,Y, and Z coordinates of the 16th GCP. Before collecting this data though, the survey grade GPS unit had to be set up. This was done with the help of Professor Joe Hupy. Importantly, the coordinate system used was the UTM WGS 1984 Zone 15N, the same as the week before.

Fig 10.7: Portable MiFi

  Once the survey grade GPS unit was set up, it was fairly easy to record the X,Y and Z coordinates of the GCPs. All one had to do was press the button on the GPS screen which had the little surveyor man on it and then wait for the GPS unit to say that it was ready to collect data for the next point.

  The survey grade GPS unit needed WiFi in order to collect the data. This WiFi was provided by using a portable MiFi unit which can be seen on the left in figure 10.7.

Fig 10.8: Level Used for Accuracy

   While collecting the data, there were a couple things one needed to pay attention to before taking the survey point. First, one needed to be sure that the survey grade GPS unit was perpendicular to the earths surface. This was done by using a type of level included with the survey grade GPS. This can be seen on the right in figure 10.8. The bubble needed to be fully inside the circle for the GPS to be upright. Second, the data collector needed to make sure that the survey grade GPS unit was placed directly above the middle of the GCP so that the reading would be accurate. This can be seen below in figure 10.9. The placement didn't need to be perfect, but needed be within an inch or so.

Fig 10.9: Placement of the Survey Grade GPS Unit

This process of collecting data using the survey grade GPS unit was done unit all 9 of the locations of the GCPs had been recorded.
  Next, it was time to prepare the M 600 ready for flight. This included making sure the portable MiFi was nearby, updating some software for the controller, checking the batteries for both the controller and the M 600, and planning the mission. Figure 10.10 is a photo of the M 600. The M 600 costs around $13,000 when new. Figure 10.10 is a photo of the controller used to fly the M 600. An I-Pad is connected to this controller so it's easier to monitor the flight.

Fig 10.10: M 600 UAS platform

Fig 10.11: Controller and I-Pad used

Fig 10.12: RTK used for accuracy

  Then, the M 600 was ready for take off. Both a RedEdge camera and an X3 camera were used on the same flight. However, the overlap wasn't high enough to gather quality data using the RedEdge camera. The overlap was set to 80% and the altitude was set to 70 meters. an RTK was also used to help make the M 600 not encounter any interference issues and so that it had sub meter accuracy. This can be seen on the right in figure 10.12. After the flight was complete, the GCPs were picked up from the field.
  Next some maps were made. The imagery from the UAS flight was processed in Pix4D using the GCPs and the soil data was interpolated in ArcMap. The step by step directions for processing the data in Pix4D using the GCPs can be found by navigating to Contact → Unmanned Aerial Systems and then by looking for the Using GCPs to Process Data In Pix4D lab.

Results

  Two series of maps were created to display the data collected for this lab. The first series contains the aerial imagery and elevation data collected using the M 600. The second series contains interpolations of soil elevation, pH, moisture content, and temperature. Also, a map of where the soil data collection points was created. All of the soil data was interpolated using the Kriging Method.

Series 1

Fig 10.13: Orthomosaic of the M600 Flight

  This map above in figure 10.13 is a very high resolution image. The different plots in the garden can be identified, the location of cars can be found, and the placement of people can even bee seen. There are three main areas in the map: the north, the south, and the east. The north area is where the garden is. The trees here have leaves on them, but the grass is mostly bare. In the south region, this is where the two ponds are located. The vegetation here is mostly brown short and tall grass. The east region is where the parking lot and baseball field is located. Here the grass is a little greener, and this is where most people park their cars.

  This next map shown below in figure 11.9 displays the DSM overlaid with the hillshade of the M600 flight. The DSM was set to have 40% transparency so the hillshade can be seen beneath it. The hillshade scale is a grayscale so its legend isn't displayed because its values would be worthless. The hillshade is used in this map to help visualize the elevation differences.

Fig 10.14: DSM Overlaid With a Hillshade

  Looking at the north, south, and east regions in this map some trends can be found. For the most part, the south region has the lowest elevation. This makes sense as this is where the ponds are located. The north region has a higher elevation than the south region, but a lower elevation than the east region. The east region has the highest elevation of the three regions. These elevation trends make sense as they relate looking at the surroundings in the orthomosaic. The south region contains two water ponds. This makes sense as water is generally located at a lower elevation than its surroundings. The parking lot and the baseball field have the highest elevations probably so that water doesn't pool up in these area when it rains hard. 

  There are a couple of places in the map where the elevation values are not super accurate. This is because there were some trees which cause havoc when trying make the DSM look nice. The camera used on the M600 can only take the elevation of the surface, not the ground. Therefore, objects such as trees and buildings will cause there to be abrupt changes in the DSM values. The minimum value in the map is 264.2 meters which is located in one of the two ponds. The maximum value is 283.3 meters which is located on the tops of the trees either in the southwest corner of the map, or in the northern part. The average surface elevation is 273.8 meters which is displayed as the yellow color. This elevation covers most of the roadways and parking lot.

Series 2

  

  This first map in figure 10.15 is of the soil data points which were collected in part one of this lab. The points are arranged in a grid like pattern and are mostly located on the edges of plots. 

Fig 10.15: Soil Data Collection Points

  This next map below in figure 10.16 shows the interpolation of the elevation values. The highest elevation is 269.4 meters which is located somewhere in the southeast corner of the map. The lowest elevation is 268.9 meters which is found in the southwest corner of the map. There isn't really that much change in elevation as the range is only .5 meters. In general, the elevation slopes from east to west. The average elevation is 269.2 meters. The standard deviation is only .1 meters . The transition from high to low elevation is very gradual across the study area.

Fig 10.16: Elevation Interpolation

  Next is the pH interpolation map. This is displayed below in figure 10.17. The color scheme was set from red (low) to blue (hi) because this makes sense when relating to pH. Red is associated with acidic things, and blue is associated with basic things. The highest pH is 8.1 and is located in the southwest corner, and the lowest pH is 6.9 which is located in the southeast corner. The average pH is 7.5 which is slightly basic and the standard deviation of the pH values is .3. The pH values seem to change depending on the plot more than anything else. The pH value could be dependent on what type of plants were previously grown in the soil, but this is only an inference.

Fig 10.17: pH Interpolation

   The next map is an interpolation of the moisture content. The highest moisture percentage is 20.9%. There are a couple of possible locations for this. Both spots are located in the eastern part of the map. The lowest moisture percentage is 13.9% and is located in the northwest part of the map. The average moisture percentage is 16.86% and the standard deviation is 1.55%. The moisture content varies quite a bit between the plots. Overall though, the higher moisture percentages are located in the eastern part of the map and the lower moisture percentages are located in the western part.

Fig 10.18: Moisture Content Interpolation

  This last map shown below in figure 10.19 is the interpolation of temperature in Celsius. The highest soil temperature is 13.1 °C which is found in one of the darker shades of red in the eastern part of the map. The lowest soil temperature is 11.6 °C and is found in the darker shade of blue located in the southwest part of the map. The average soil temperature is 12.5 °C and the standard devation is .2 °C. The general trend with the temperature is that it increases as one moves from west to east. However, there are a couple spots that are out of place with this trend

Fig 10.19: Temperature Interpolation

  When comparing all the interpolations with each other a couple of general trends emerge. The first is that everything seems to change as one moves from east to west, not north and south. The second is that all the values tend to increase as one moves to the east. This is probably fluky more than anything else, but to find out for sure, a correlation between the rasters would have be calculated to see how strong the relationships are between them. The difference between the elevation interpolation and the DSM can't really be seen because there isn't enough variation in the DSM color scheme and the variation is significant enough.

Conclusion

  Survey grade GPS is a very accurate and precise way to collect coordinates for any location. The accuracy of this device is within one inch. The only downfall to survey grade GPS is that they are very expensive. The one used for this lab costed over $12,000. Besides its use in this lab, survey grade GPS could be used other applications such as for construction, for utility maintenance, or for surveying property lines.
  The process of setting up and collecting the soil data for this lab took about 2.5 hours. It could have gone much faster, but the lab wasn't very organized. This was because the pH measuring devices weren't available until the day of the lab, so the plan for the whole lab changed the day of the lab.
  If this lab were to be done over again. Everyone should have to use each tool at least once so that each person know how to use them. Also, the grid system could be set up better so that there are two survey points in each garden plot. When the lab first started, it wasn't known that the survey points were supposed to be inside the garden plots. This caused some of the first survey points to be located on the pathway instead of inside a garden plot. This issue could easily be fixed with just a little clarification when explaining the lab.

  The results of this lab indicate that there is a potential correlation between soil elevation, temperature, pH, and moisture content. However, much more analysis would need to be done to prove this because this is such a small sample. It is more likely that these attributes change depending on the garden plot. It is likely that people place and grow similar garden plots and plants next to each other therefore causing the relationship between the attributes.