Field Navigation: GPS

Introduction:

In continuation with the past two week’s activities (creating a map of the navigation area and navigating using a map and compass), our task this week was to navigate the Priory in Eau Claire using only a GPS unit. The main objective of this activity was to analyze the efficiency of using a GPS unit to find points. We were also interested in comparing the efficiency of the GPS unit to the efficiency of a map and compass (distance and azimuth) to find points as we did last week. Each technique has its benefits and drawbacks. The GPS is easier to understand, but our navigation was less direct than with the map and compass. The GPS functions were not entirely accurate either—the compass in particular was unreliable.

For this activity, we were working with the same groups of three students and within the same area in the Priory as last week. However, we were given a different course than we had last week with six different navigation points to find. There was a total of six groups of three students. Two groups were assigned to each course of navigation points working in opposite directions.

Each person was provided with their own GPS unit, an etrex Legend H by Garmin (Figure 1), and coordinates of the points in UTM and latitude/longitude. We were able to find the points by comparing the given coordinates and the coordinates on our GPS units. Each group of three worked together to find their assigned points despite the fact that every individual had their own materials. We used the track log function of the GPS units to record our positions in the field (further details in the Methods section) to see how directly we traversed between points as well.

Figure 1: This is the Etrex Legend H GPS unit by Garmin that we used to do our GPS navigation for this week.

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Methods:

Before we were able to begin finding our points, we had to set up the track log function on our GPS units. The track log records the location of the GPS at a fixed time interval. For this activity, the units were recording our locations every few seconds. The time interval varied slightly between units, however, because we did not establish a decided interval before navigating. The track log was very simple to set up—we turned it on when we wanted the unit to begin recording and turned it off when we were done. It was important to make sure we turned the track log off after we finished our navigation because we would have far too many irrelevant points otherwise. We also had to make sure we had enough battery life for our GPS units before setting off.

Once we started the tracklog, we began searching for our points. With only the GPS to guide us, we were very limited. The GPS could provide the coordinates of our current location and a compass. However, the compass was extremely inaccurate and could not be relied upon.  We had our coordinates set in UTM and could orient ourselves by comparing the coordinates of the points to the coordinates on the unit (Figure 2). Knowing that our origin in the UTM coordinate system was to the southwest of our study area, we determined that the higher the X coordinates went, the further east we were traveling and the higher the Y coordinates went, the further north we were traveling. We had to test which direction we were traveling simply by walking in a certain direction and seeing if the coordinates were getting higher or lower. We would navigate to each point by comparing the coordinate values from one point to the next. For example, if Point 2 had a higher X coordinate value and lower Y coordinate value than Point 1, we would walk southwest by going in the direction that would give us higher X values and lower Y values according to the GPS. This was simple conceptually, but it proved to be tricky in the field. We eventually decided to get as close to the Y coordinate value of the point as possible first, and then try to match the X coordinates.

Figure 2: This is the data, given to us by our professor, that we used to find the  points for Course 3. We matched the GPS coordinates to the coordinates in this table to locate the points.

The next step was to load the track log points and save them as a shapefile. I connected my GPS unit to a computer using a USB cord and loaded the data into a program called DNR GPS. The program was relatively intuitive. It was important to remember to save the data in the correct coordinate system and as a point shapefile rather than a line which was the default. After saving the data as a shapefile, I created three maps of the track log data using ArcMap—one for my track log points, one for my group, and one for the entire class.

Figure 3: This is the map of my track log data. As can be seen, I was not traveling directly from point to point.

Figure 4: This is a map that includes the three track logs from my group. The track logs follow a similar route but are not exactly the same.

Figure 5: This is a map of the class track log data. It is sorted by the different courses and shows that there were many different paths taken to get to the same points.

Results:

In comparison with last week’s navigation exercise, using the GPS seemed to be much more difficult than using the map and compass. Without a map and properly working compass, it was hard to orient ourselves, and we found ourselves looping around our own tracks—those woods began to feel like the twilight zone after a while (Figure 6). As seen in Figure 5, there was a lot of variation between groups that were navigating the same route because it was hard to keep track of the direction we were traveling. There was variation among the track logs within our group as well, which may be due to problems with the GPS units or slight variation in the paths we were taking (Figure 7). Without the luxury of a map we also were unable to anticipate the type of elevation changes and terrain we would be going through. We found ourselves crossing a rather steep gorge at one point. The upside to the GPS unit is that it can be quick to set up and easy to understand. The track log function is another added benefit to a GPS because it can record exactly where the user is traveling, and the data can be used to create maps.

Figure 6: This is the area that we looped around to get to Point 4. We went around this loop  clockwise meaning that we went way out of our way to get to the point and crossed our own tracks on the way to Point 5.

Figure 7: This is one example of an area that the track logs varied among my group. It is also showing the difference in the time intervals that were used. The yellow track log (mine) had a time interval that was larger than the other two so the points are more spread out.

There we similar challenges between this week and last week in terms of the nature of the field. There was quite a bit of snow cover (Figure 8), though somehow there was even more snow than last week (I remembered my snow pants this week though—score!). The brush was also bothersome. Unfortunately, keeping an eye on the GPS meant that I was not looking out for tree branches and walked into a couple solid branches along the way. We had to navigate around certain features in the field as well, which threw off our navigation as we lost track of the direction we were going.

Figure 8: This is a photo of one of my group members walking through a clear area in the Priory. It  gives a slight idea as to the depth of snow we were walking through.

Conclusion:

This week’s activity consisted of using only a GPS unit to navigate in the field. We learned how to use the track log function on the GPS to record our locations and save the data as a shapefile to be used in ArcMap. Working in groups of three, we navigated to six points in a large wooded area by matching the coordinates on our GPS units to the given coordinates of the points. We found that using a GPS has both benefits and drawbacks while navigating. I thought that the GPS was more difficult to use than the map and compass for navigation because we could not orient ourselves properly and were less aware of our surroundings. We were definitely not travelling directly from point to point as we were with the distance and azimuth technique because of this.