Conducting a Distance Azimuth Survey

Introduction:

Figure 1: Survey Stations along Putnam Trail

While surveying using a grid-based system can be useful for mapping smaller plots, technological advancements with GPS has allowed field surveying to become an easier, faster, and more accurate tool for obtaining and displaying field data for scales of various size. However, the use of any type of technological equipment comes with risk for technological failure while the surveyor is out in the field. Still, the job must get done! For this week's lab, students used various a variety of tools at varying locations along Putnam Trail to survey a series of select trees using the old-school Distance-Azimuth survey method. This method requires the measurement of distance and compass degree between several surveyed points (10 trees) and one pin-point location tied to a latitude and longitude coordinate pair (data collection stations) to be used later for mapping. The surveyed area and stations are pictured in the reference map (Figure 1).

Methods:

Data Collection:

To best familiarize students with a variety of equipment that could be used to obtain distance and azimuth data in the field, students divided into three groups and worked as a team to collect 10 data points from each of the three stations, using the various tools provided as they rotated through.

Figure 2: Image of TruPulse 360 

At Station 1, a TruPulse 360 range laser was used to determine the distance between the surveying pin-point and its selected surrounding data points, as seen in Figure 2. Other necessary tools included a standard compass for measuring the azimuth of the surveyed trees, and a basic GPS unit to measure the coordinates of the central point, or data collector's position. At the data collection point, the latitude and longitude pair read 44.796 deg. N and -91.5016 deg. W. While collecting the data, students alternated roles operating the TruPulse 360 range viewer and compass, measuring the diameter of selected trees, and recording the data until reaching the data collection total of 10 various points. This method was especially accurate in measuring the distances between collection point and tree. Also noteworthy of this tool's function, the measuring units displayed on the reading scope could be easily changed in the equipment's settings to read in either imperial units, metric units, (as used), or in degrees. Some challenges that arose when using this method, however, was the sensitivity of the tool's reading. In some instances, the tool tended measure the distance of a small branch that intercepted the scope on the way to the intended select tree. For this reason, many of the collected data points at this station had to be double, and triple checked for distance integrity of the actually intended data point.

Figure 3: Measuring Diameter of Trees at Breastlevel

Station 2 was the most time consuming because aside from the need for the GPS to denote a specific coordinate point, this station made due without the use of technology completely. Instead, students used a tape measure and compass to determine distances between data collection point at 44.79585 deg. N and -91.50033 deg. W and its surrounding trees. For this reason, the group did not travel nearly as far for plotted points, and the second station appears to be the more clustered data collection group of the three methods. While this method can be especially handy in case of equipment failure, it was also the least accurate of the three methods. Again, students alternated roles between holding and leading the tape measurement, reading the compass azimuth, measuring tree diameters, and recording the resulting data. Figure 3 shows a fellow group mate taking the circumference of a tree at standard breast level in order to find the diameter. Some complications the group faced in gathering data included the struggle to pick trees that were a far enough distance away to appear significant when plotted on a map, but not so far that another tree would block the tape measure's route, causing a curve in the tape and skewing the measurement's reading.

The last station surveyed used a range reader and receiver combination to record the distance. The data collector held the range reader gun at 44.795383 deg. N and -91.499388 deg. W while the person measuring the tree's diameters held the receiving end of the pair. The reader would measure and display the distance between it and the signals picked up by the receiving device. Some of the challenges that arose from this method was the occasional inability for the reader to pick up on the signal sent out by the receiver. This usually just required minor positioning adjustments so that signals would send properly without signal interruption.

Data Normalization and Mapping:

Once all of the data had been collected in the field, it was then necessary to format the data into an Excel file and normalize into a format compatible with ArcMap. A sample snapshot of the final data sheet is displayed below in Figure 4.

Figure 4: Normalized Data Table in Excel

Figure 5: "Bearing Distance to Line"

and "Feature Vertices to Point" 

Commands Location

After creating the table, the routes and distances measured between the three central data collection points and their corresponding trees were imported and plotted onto the map using the Bearing Distance to Line command in ArcToolbox under Data Management >> Features as shown in Figure 5. Figure 5 also references the location of the tool used to place the points where the trees surveyed were stationed at the end of the measured distance line. This tool could be found in the same section of ArcToolbox labeled Feature Vertices to Point. Finally, a topological image was placed beneath the resulting plotted points for reference.Figure 6 illustrates the results generated after the use of both tools in inlay of a basemap.

Figure 6: Tool's Resulting Map Image and Plotted Data

Results:

From the data points and features plotted on the map in the image above, the following map (Map 1) was constructed to further showcase the resulting distribution of each of the three methods and the corresponding trees which were selected in collecting data from each of the central points.

Map 1: Putnam Drive Survey Stations, Methods and Tree Data Points

The first station using the TruPulse 360 was the most effective tool used of the three stations for collecting distance data. It was quick, user friendly, and accurate in its measurements and could be measured with only one user. It also allowed the surveyor to collect these data points without actually having to approach any of the trees. Considering the steeply angled uphill slope of the terrain south of the trail, some of these trees were more difficult to reach by foot when necessary, as in instances when measurements were being taken at the second two surveying points. Station 3, for instance, had the convenience of using technology for measuring these distances as well, but the surveyor still needed a second person to hold the receiver at the tree's location. For this reason, most of the data points collected at the last two survey points were collected north of the trail to avoid any uphill hikes.

Conclusions:

Learning the Distance-Azimuth surveying method is an important skillset to use as a back up tool in the case of equipment or technology failure. Though the results produced are less accurate than those that may be obtained through the use of technology, the method still does a fairly good job of displaying the overall location of collected data points.

The Distance-Azimuth method can also be applied in the Point-Quarter sampling method used for determining the relative concentration of a species in a given habitat, especially those with a less defined shape as is the case with Putnam Trial. During this type of sampling, the same relative technique is used to get an estimate of the overall number of species that are within a given area. To perform sampling, a number of species in the area (in this case, trees) are sampled at random from a central point. Their correlating data is recorded and the trees are each prescribed an identifying number, just as performed in the lab detailed here. The methods begin to differ from here. In the Point-Quarter surveying method, once points are collected, a compass is used to determine and lay out four individual quadrants. The total sampled number of trees observed is multiplied by four (for four quadrants) to get the relative density of the area. This number is multiplied by the total density (calculated from the tree diameters) in order to obtain the absolute density of a species within an area, in this case, the absolute density of each tree species along Putnam Trail.

Overall, this lab equipped students with the necessary knowledge to overcome potentially critical situations that may occur in the field that will enable them to still get the job done! Despite living in an age with ever-advancing technology, learning the basics of the trade and the "old-school" methods used to collect location based data is a handy tool set to have stowed away for the occasional instances in which they just might be needed in the future.