Exercise 4: Distance Azimuth

Introduction

In the past few decades, new technologies such as satellites and GPS units have changed the way that geographical measurements and surveys have been taken. These technologies are crucial to many parts of our everyday lives, such as traveling to a new destination and relying on a smart phone for directions.  Although these technologies have changed the way that the modern world runs, they are simply electronics that can be unreliable and troublesome. Because of this fact, this exercise involves locating features using field methods that have been utilized for centuries. As long as the exact location of a reference point is known, accurate locations of features can be calculated using only simple distance and directional bearing measurements. This exercise incorporates old world measuring techniques with new world mapping technology to accurately locate features.

Methods

The first task in this exercise was locating a study area. An ideal location for this exercise needed to contain at least 100 unique features that could be measured for distance and azimuth. The first location that we came up with was a park since they contain many trees, light poles, benches, tables, and other features. It was also important that this location was close to campus since the winter temperatures are extremely cold in Eau Claire and we did not necessarily want to be outside for longer than we needed to be. We decided upon Randall Park (Figure 1) within the student housing area of Eau Claire. This park was a simple rectangle plot which made it very easy to decide on reference points for the measurements to be made. 

Figure 1. Randall Park is a rectangular city park located in the student housing area just north of the University of Wisconsin-Eau Claire. Close proximity to campus, many features and a simple shape made this park a great candidate for this survey.

Since this exercise involved directional bearing measurements, it is important to know the magnetic declination of the study area. Magnetic declination is the angle on the horizontal plane between magnetic north and true north. Magnetic north is the direction in which a compass points north, and true north is the direction along a meridian towards the North Pole. Magnetic declination is only about 1.07 degrees West in Eau Claire, which is insignificant due to the small scope of this exercise. This declination has such a small effect on the measurements that it is not necessary to calibrate the equipment before measuring azimuth. 

Figure 2. This figure displays the amounts of magnetic declination across the continental United States. Magnetic declination varies not only over distance but also time. The movement of the Earth's inner core causes the magnetic field to gradually shift positions and even completely reverse every several thousand years. 

In order to collect the data quickly and efficiently, we set up tables on notebook paper containing columns for feature numbers, coordinates, distance in meters, and azimuth in degrees. We also collected attribute data to distinguish each feature as a light pole or a bench or a tree, but later during the exercise we realized we could not use this data in such a way.

The instruments utilized in this exercise include a laser rangefinder and a two part radio rangefinder (Figure 3). A compass was also brought along for the azimuth measurements but the laser rangefinder proved to be much quicker and easier to use during data collection.

Figure 3. From left to right: laser rangefinder, compass, two part radio rangefinder. The compass collects azimuth measurements, the radio rangefinder collects distance measurements, and the laser rangefinder can collect both simultaneously.

Since our group consisted of only two members, we decided to have Nathan stand near each feature with the radio rangefinder receiver (Figure 4). Standing at the reference point, I would then activate the device and shout the measurements to Nathan who recorded the numbers in the table. Next, I viewed the location with the laser rangefinder in order to find the azimuth measurement and shouted those numbers back to him as well (Figure 5). To complete measurements for 100 features, we collected data from 3 different reference points in the park in a little over an hour's time. 

After the first few measurements, we realized that the two part radio rangefinder had a maximum range of about 40 meters, so the laser rangefinder was used for any distances which required it. 

Figure 4. Nathan holding the receiver of the two part radio rangefinder. This method was the most accurate but was only useful for measurements under 40 meters in distance.

Figure 5. Me using the laser rangefinder to collect data. This device allowed us to collect both distance and azimuth data quickly and efficiently.

All of the measurements were recorded into the notebook tables (Figure 6) before being entered into a digital spreadsheet (Figure 7). The information in the tables were recorded in separate columns so that ArcGIS software could seamlessly access the information. 

Figure 6. Each page contained data for a separate reference point so that the measurements didn't end up in an incorrect location.

Figure 7. This image displays a portion of the completed data table. The X and Y coordinate fields contain the locations of the three reference points, which were found in ArcMap after data collection.

With the spreadsheets completed and formatted correctly they were imported into a file geodatabase made specifically for this exercise. The data layer was immediately set to the WGS 1984 Geographic Coordinate System since the data was formatted for lat/long units. An aerial imagery basemap was added in order to properly visualize the measurements from the survey. 

The first tool that was used in this exercise was the Distance Bearing to Line tool. This tool creates line data based on distance, azimuth, and XY coordinates from the reference point (Figure 8), all found from within the imported table. The result is a series of lines originating from each reference point to the feature locations around them. The next tool used was Feature Vertices to Points, which simply placed a point at the end of each line created from the previous tool (Figure 9). Theoretically, these points should be placed at the exact location of each of the features that we surveyed. 

Figure 8. When finding magnetic bearing or azimuth, magnetic north is displayed as 0 degrees. The exact location of a feature can then be found by examining the distance and bearing from the reference point in relation to magnetic north. 

Figure 9. This aerial image shows the reference points and the corresponding feature locations from the survey. To accurately cover the most features, we divided the survey into three parts from three reference points. 

Discussion

A problem that we ran into during data collection was that the two part radio rangefinder did not operate correctly for any distances beyond 40 meters. This made data collection more difficult because it was very quick for us to have Nathan hold the receiver at each feature while I found the distance from the reference point. The radio rangefinders worked great even through thick brush, which was very common in the park between the features and reference points. Some features were too far for the radio rangefinders but too thin to target with the laser rangefinder, so Nathan would stand at the feature and I would measure him instead. Even this method proved difficult as the snow was 3 feet deep, making it challenging for Nathan to reach each feature. 

One point for this exercise that should be discussed is the importance of accuracy for the reference points. It was crucial to locate reference points that would be visible from aerial photographs. For this reason, we chose to use the intersections of the outer sidewalks as the reference points. After surveying from two of the corners of the park, we decided that we could collect the most features from the center of the park, and it would also be easy to locate the area in an aerial image as well. Once the data layer was set to the WGS 1984 Geographic Coordinate System, the reference points were placed and their locations were recorded to six decimal places. This accuracy was absolutely necessary because a simple difference of .1 decimal degrees meant a land difference of about 8 km. 

As visible from Figure 9, some of the feature locations reach out of the park and into the middle of Third Street. This occurrence can not be explained, but it may have to do with problems within the relationship of the basemap and the coordinate system. It was quite difficult to steady the laser rangefinder due to gusts of wind and shaking hands, and this fact may contribute to the inaccuracy of the feature locations as well. A solution to this problem would have been to utilize a tripod during data collection. 

The last problem we ran into during the analysis was that the description field somehow influenced the placement of the distance-azimuth line features. We had included nominal data that separated each feature as a light pole or tree or bench, but when we included this information in the tools it completely changed the locations. Because of this problem, we decided to not include the nominal data into the final analysis. 

Conclusion

While the use of a GPS would have made this type of data collection much quicker, this exercise proves that collecting accurate locational data can be done without needing advanced satellites and equipment, and can ideally be done with only a compass and tape measurer. In a dense forest, it becomes difficult for a GPS unit to receive signals from a satellite. By using the distance azimuth method, the measurements are as accurate as you make them, and cannot be influenced by any interference.