Lab 4: Distance Azimuth Survey

Introduction:

Surveying a plot of land can be a complicated process, full of many possible methods of completion. If the area is small enough, or if a high degree of detail is required, grid plots can be constructed in the area of interest, and points can be taken in each grid, providing a large number of data points that can be entered as X and Y coordinates in a Cartesian system. In large areas, however, this method does take a significant amount of time, and may not be appropriate in situations where an accurate survey needs to be done quickly and efficiently. One such method for quickly finding direction and bearing is using a laser rangefinder, like the TruPulse 360, as shown in Figure 1.


Figure 1: Me using the TruPulse to find the azimuth of a tree in Owen Park.

According to the product’s information website, the rangefinder provides a very accurate compass technology to get the best azimuth reading. The TruPulse can provide Distance, Inclination, Height, Azimuth, and Missing Line ratings using a laser, receiving a reading when the laser hits the object to be surveyed.

Methods:

When getting acquainted with the rangefinder, the first area to be surveyed was the parking lot outside of Phillips lecture hall on the campus of the University of Wisconsin-Eau Claire. A few points were taken, including a statue, an emergency post, and a few trees. The distance and azimuth for each object was recorded, and put into a table format, as well as the latitude and longitude of the start point in decimal degrees. Exact coordinates were needed to get a perfect location for the start, as we were quick to find out, because if it was not accurate, the lines would not be in the proper position on the map, as demonstrated in the map in Figure 2. For the actual laboratory exercise itself, the class was split into groups of two and told to find a survey area where at least fifty data points could be found and recorded. We decided to survey tree locations in Owen Park, a small area near the Chippewa River in Eau Claire, near the University. A satellite view of the park can be seen in Figure 3, and additional pictures of the scenery can be seen in Figures 4 and 5.

Figure 2: Initial Points taken during familiarization with the TruPulse. As visible by the image, if the full coordinates were not input, the position of the lines would be incorrect in relation to the starting point.

Figure 3: Satellite View of Owen Park, the study area for the activity. Image provided by Google Maps.

Figure 4: Owen Park, with its many trees, provided a number of survey points for this activity

Figure 5: Owen Park

Using the TruPulse, the distance, azimuth, and height of fifty different trees were calculated and recorded. A copy of the table of the data can be seen in Figure 6, showing the values of distance, azimuth, and height, as well as the coordinates for the four starting points.

Figure 6: Data Table with distance, azimuth, and height values

Once the points were collected, they needed to be imported into ArcMap for data manipulation. The Excel table that the data was input into was brought into a file geodatabase, brought into the program, and the Bearing to Distance Line tool was used to show the positions of the trees digitally, with the result being illustrated in Figure 7. To ensure a truly accurate reading, the lines were reprojected into the WGS 1984 UTM Zone 15 for maximum accuracy. Once this was done, the Feature Vertices to Point tool was used to attach points to the end of the lines, to better illustrate where the trees were supposed to end, as seen in Figure 8.

Figure 7: Survey Line positions acquired from the data collected in the park, and using the Bearing to Distance Line Tool

Figure 8: Feature Point to Vertices tool being used to plot where the trees should be, compared to actual position on satellite image

Discussion:

Unfortunately, with most methods of data collection, a margin of error exists that needs to be corrected for. The first potential error that occurs in this process is magnetic declination, which is the difference between True North and Magnetic North, which the laser rangefinder uses to produce accurate readings. Luckily, in Eau Claire, this phenomenon's effect on the readings by the TruPulse is as minimal as possible, so compensation is not necessary. An error that did take place, however, was the lining up of the data points with physical trees. By examining the image, it is evident that there is a bit of an accuracy issue from the data collection of the points. Unless the satellite image being used as a basemap is older, and does not show the current location of all trees, it appears that there are some points on the map that do not belong to a tree, suggesting that there is a source of error in this process, be it the satellite image not being temporally accurate, the projections not lining up, or a combination of other factors. 

Conclusion:

This experience with the TruPulse laser rangefinder was invaluable, and introduced us to an entirely new and technologically advanced way of surveying a landscape. While the rangefinder was more high-tech than survey grids, error still can occur while using it, which is evident in the inaccuracy of the location of the points surveyed. But this error could be attributed to collection by us on the ground, with the exact distance or azimuth values being recorded wrong, or the coordinates of the start point being incorrect by using an aerial photo instead of a GPS. But these kinds of error happen, and experiencing it in the field allows us to learn about the drawbacks of survey equipment, and choose the best one for the job, being fully educated on its positives and negatives, and any error that may occur when using it.