GPS Topographic Survey

Introduction

For this activity, as a class, students went outside on campus and conducted a topographic survey using a survey grade GPS unit. With the collected data, students were to then import the data into ArcMap and run interpolations and create continuous maps showing the elevation changes. 

The objectives included:

• Surveying the study area using a survey grade GPS unit
• Importing the collected data into ArcMap and displaying the X, Y, and Z data
• Run interpolations using IDW, Natural Neighbor, Kriging, Spline, and TIN interpolation methods and creating continuous maps

Study Area/Methods

The study area was a grassy knoll that featured a small hill on the lower campus of UWEC located between Centennial Hall and Schofield Hall. Due to inclement weather, the survey was brief and only 20 points were collected. The sampling method used was a random stratified method where random points were selected on the grassy knoll and the elevation changes of the hill were attempted to be captured well enough for interpolations that would represent the hill adequately. This was unlike the method used for the sandbox survey conducted previously in the semester where a systematic point sampling method was used where measurements were taken on a grid that was set up with 5 cm increments. 

The first step was to conduct the survey on the grassy knoll using the survey grade GPS unit which can be accurate to sub-centimeter accuracy. To collect points, the GPS receiver was set onto the ground, the two legs were extended to provide stability, then the legs were adjusted until it was as level as possible, which would provide the best accuracy (by lining up the bubble in the level). Once the receiver was lined up properly, students used the handheld device to collect the point. These steps were then repeated until 20 points were collected.

The next step then was to upload the data into a text file format that students could then import into a geodatabase in ArcMap. First a folder was created (GrassyKnoll), then a file geodatabase which the data would be uploaded into and subsequent interpolations would saved in was made (GrassyKnoll.gdb), the text file was copied from the TEMP folder and pasted into the GrassyKnoll folder, and finally the text file was imported into the geodatabase by right clicking on the file in ArcMap and selecting 'Import' and 'Into Geodatabase'.


Results/Discussion

Looking at the results of the interpolations, it's clear to see that the grassy knoll was not represent very well. The grassy knoll featured one, elongated hill the sloped down on each side until it met the edges of the sidewalk (a picture of the knoll may be posted at a later time to show what it actually looks like). An attempt was made to find a basemap image in ArcMap that would show the extent of the knoll where the extent could have been traced and then used as an extent when running the interpolations. However, no basemap image was found to be up to date on what the knoll looks like present day (since it was just recently constructed) and therefore a default, rectangular extent was used when running the interpolations except for the Natural Neighbor interpolation which captures the extent of the knoll best by only extending the interpolation out to the edges of the most outside data points. 

The maps represent elevation where dark green is the lowest elevation and light pink is the highest elevation. The legends were placed in the lower left hand corner of each map so as to not cover up any data points (represented by the dark blue circular dots). The resulting interpolation maps are as follows:

1. IDW

The IDW map appears to be one of the least accurate interpolations where isolated, circular elevations are captured, and elevation is highest towards the left side and lowest towards the right side (figure 1). 

Figure 1: IDW interpolation map

2. Kriging

The Kriging map appears to be one of the most accurate maps where a circular, light pink shape which represents the highest elevation is towards the bottom left side and strands of lowering elevation form around it (figure 2).

Figure 2: Kriging interpolation map

3. Natural Neighbor

The Natural Neighbor map may be the most accurate of the maps elevation-wise and extent-wise where the knoll and its elevation changes are best captured. Elevation is highest towards the lower left hand corner where circular-like shapes of lowering elevation surround it (figure 3).

Figure 3: Natural Neighbor interpolation map

4. Spline

The Spline map appears to be one of the least accurate maps where the highest elevation juts upward from the lower left hand side and uneven, lowering elevation changes surround it (figure 4).

Figure 4: Spline interpolation map

The fact that the results do not very accurately represent the grassy knoll could be due to not enough data points being collected, the knoll not being sampled well enough, equipment error, or a combination of some or all of these reasons.


Conclusions

This activity goes to show how important it is to properly sample a chosen study area. The grassy knoll was not represented well enough as is shown in the interpolations made using the collected data. In future surveys, it will be important to more carefully sample a study area well enough for it to be represented more accurately.