Distance/Azimuth Survey

Introduction:


The Field Activity for this week was once again upset by the ever changing weather conditions of Wisconsin in the spring. The original plan was to survey tree species in Putnam Park on the University of Wisconsin Eau Claire Campus however a mix of snow, rain, and sleet, changed the activity. The alternative was the collection of tree species points on the campus mall, to the West of Phillips Science Hall as shown in the map inset in figure (figure 8). This study area contained a small stream, walking paths, and several trees planted, of various ages, along the stream area. The area of study can be seen in a photo shown from the perspective of the anchor point for the data in figure 1. The goal of this particular activity was to collect points with a spatial reference without relying on a GPS to take points and specific information on that GPS device. Essentially, if there is a technical difficulty in the field, how can an accurate survey still be conducted.

Figure 1: A view from the anchor point towards Phillips Hall and the trees being collected as points.

Methods:


The survey method employed is called a Distance/Azimuth Survey. To run the survey one point is collected with an exact location. From that point, all other points are simply a reference. In order to transfer the desired points into something with a spatial reference two things are needed, the distance to the point to be collected and the azimuth. Distance was collected in meters using Sonic Combo Pro (figure 2) to shoot a sound wave to a collecting beacon held at breast height on the tree being measured. Azimuth is a attribute collected in degrees using the angle away from 0 degrees, or North, the azimuth is an angle measurement from the anchor point. To collect azimuth a TruPulse 360B(figure 3) was used. Seventeen different trees were surveyed with the following information, distance in meters, azimuth in degrees, circumference at breast height, and species. The location in decimal degrees was collected for the position of the anchor point. Together, the collected attributes provide a location of the tree, and the physical information on the tree provides an idea of size and appearance.
The collected data was transferred into a "Share" Excel file and was then converted into X and Y Data points in ArcMap. The decimal degrees measurement was converted into longitude and latitude for mapping purposes and combined into the attribute table in figure 4. The repeat in points seen in the table will be addressed later. "Bearing distance to line" was the first tool used with the purpose of using distance and azimuth to create lines from the anchor point to the collected tree points, the effects of this tool can be seen figure 5, the ESRI definition for this tool is in figure 6. Then the "feature vertices to points" tool added points onto the end of the lines created, the lines were subtracted and only points remained. The ESRI definition for how this tool functions is in figure 7. This data set had to be checked for spatial reference and was reprojected into the GCS_WGS_1984 projection.
The resulting point feature class was placed onto a basemap of the UWEC campus and a map was created to show results (figure 8). The result is a fairly accurate tree survey.

Figure 2: Held to chest height and shot at a receiver the Sonic Combo Pro collected distance in meters.

Figure 3: The TruPulse 360B could be looked through like a mono-scope and a pulse was sent to what was in the crosshairs to measure distance and azimuth.

Figure 4: An attribute table in ArcMap of collected points.



Figure 5: The resulting lines from the bearing distance to line tool.

Figure 6: The ESRI definition of the Bearing Distance to Line tool.

Figure 7:The ESRI explanation of how points are placed with the Feature Vertices to Points tool.

Discussion:


Distance/Azimuth surveying is not without its difficulties or its errors. In fact is was clear to see how errors could occur quite easily. The rain occurring during surveying could have scattered the laser and sound waves damaging the integrity of distance and degree measurements. The point data is also not perfect, as visible in the resulting map there are some species of trees that appear to be growing inside of Phillips Hall, it seems that the farther away from the survey tools the trees are, the less accurate point collection is. There were also several problems when converting the data from a field notebook to an excel file and into ArcMap. At first points were placed in the ocean and data was in decimal degrees not meters. The problem was that the X and Y fields in the Excel file had been switched and was placing the data in the southern hemisphere. A simple issue to fix once the problem is realized but certainly a good lesson in paying attention to the most basic of geospatial methods. The result was a doubled attribute table as stated earlier, this was able to be ignored as quantity was not measured, just location.

Figure 8: The resulting map from the Distance/Azimuth survey of trees by Phillips Hall.

Conclusion:


This is not a perfectly accurate way to survey anything. It certainly has its errors but it still a reliable way to get relative points onto a map. There would be ways to conduct this survey again to make for less errors and difficulties but it would never be as perfect as a survey grade GPS.
If done again more points would be collected at varying distances from the anchor point to get more variance in accuracy. That data set would then be compared to points collected at the same location with more accurate equipment so that the error margin could be studied.
This field activity was helpful for seeing how there are more than just one way to collect points when technical difficulties arise in the field.