Introduction:
Unfortunately, field work does not always go as planned—weather may be a hindrance or perhaps the field equipment necessary is unavailable or not functioning. In order to work around these situations, a back-up plan must be at hand. In this exercise, we are using an alternative, “back-up” mapping technique—azimuth and distance data collection—so we may be prepared for any problems with more advanced equipment. The general idea is to find a base point and map all other features in relation to this base point using distance and azimuth. There are a couple of ways to approach this type of mapping based on the different tools used. One approach is to use a compass and distance finder while the other is to use a laser device that will measure both distance and azimuth. This information can later be mapped as point features in ArcMap or another GIS. To keep it relatively simple, we are only mapping point features in a 50 meter by 50 meter area somewhere near/on our campus.
Methods:
Concepts:
Azimuth: Azimuth
is the measurement of an angle on a spherical plane or in a spherical
coordinate system.
Magnetic Declination: Magnetic declination from true north is often a factor when collecting data using a compass. True north, or magnetic north, is not always in the same direction as compass north. The declination measures the angle between true north and compass north from a certain point on the globe. It varies with geographic location and also changes over time. If the declination is not accounted for while collecting data, the location of the features that have been mapped will not match up with the actual features in the real world. Luckily for us, the magnetic declination for the city of Eau Claire is insignificant. Magnetic north in Eau Claire is 59 minutes west of compass north; therefore, we did not have to account for the declination while using the compass.
Data Collection:
Measurement
techniques:
Before we were
able to begin our own projects, we had to learn the advantages and
disadvantages of the measuring techniques and how to use them. With both of the
methods, a point of origin must be established from which to measure the
distance and azimuth. The first technique includes using a compass to find the
azimuth from the point of origin and a distance finder to measure the distance.
By holding the compass up to our eyes while also looking at the object, it was
fairly simple to find the azimuth. The distance finder was also simple to use.
It has two parts—a receiver and a “shooter”. The receiver should be held near
the object that is being mapped and the shooter will compute the distance by
pointing it at the receiver. The second technique is using a laser gun that can
record both the azimuth and the distance of the object in question. It only
needs to be set to the correct setting and pointed toward the feature.
Both
of these measuring methods have advantages and disadvantages. The first method
using the compass and distance finder can be inaccurate because it relies
heavily on human judgment. It is also more difficult because it requires two
tools rather than one and at least two people to measure the distance. However,
it requires the least amount of technology and can be used in many different
situations. The second method, involving the laser gun, is much more convenient
because it only relies on one tool and can be done with only one person. The
downsides to the laser include a greater dependence on technology and it can be
harmful if pointed at the human eye.
Once
we had learned about the techniques, we headed out to collect a few points
outside of our science building. We chose a tree as our point of origin from
which to measure the rest of the points. We were in an area that had a few
sculptures and trees placed between the building and a large parking lot. We
tried out both techniques on these points and found that they had slightly
varied measurements—likely due to human error in finding distance and azimuth.
Area:
The first step in
this project was to select an area near or on campus at our university that has
a general size of about ¼ of a hectare (50x50 meters) with a significant number
of features to be mapped. We chose a small area behind one of the dorm
buildings that is part of a disc golf course (Figure 1). We chose this area because it
seemed to have an ample number of features to be mapped. The area is near the
edge of a large hill with a steep drop off and has a triangular shape. Though
it is covered in snow this time of year, in the summer most of the area is
covered by lawn with trees spread throughout. It has a small picnic area and a
couple holes for the disc golf course.
Figure 1: This is a photo of the disc golf course area we were surveying with the azimuth-distance method. This photo was taken from the perspective of our origin point.
Data Collection:
Next, we had to
actually collect the data. Instead of choosing only one method of data
collection, my group and I paired up with another group and did both techniques
for the same area. We thought it would be interesting to compare the two and
see the geographic differences. The point of origin was established at the back corner of
one of the buildings framing the area. With the compass and distance finder, we
had one person go to each point and hold the receiver so we could record the
distance from the point of origin and had another finding the azimuth using the compass provided to us by our professor (this compass did not need to be adjusted for the magnetic declination as mentioned previously). A third
person was in charge of using the laser gun which recorded both distance and azimuth, and the last
person recorded the data for both methods. This took about an hour to complete
as we had to get a bit imaginative in finding a sufficient number of points.
Data Management and Mapping:
After going out
into the field, we had to enter in our data into an Excel file and ArcMap. The
Excel file contained five categories: point number, X and Y coordinates,
distance, azimuth, and point description. The description was kept fairly
simple—most points being trees (Figure 2). The X and Y coordinates for every point were
the decimal degrees of the point of origin. We found the coordinates using
Google Earth, though this is an imperfect method of finding the coordinates. In
ArcMap, we uploaded the Excel file and saved the data point of the origin as a
feature. Once this was accomplished, we used the Bearing Distance to Line tool
in the Data Management toolbox to create lines with vertices ending where each
data point should have been in relation to the origin. This tool takes the
distance and azimuth field and creates a line from the origin at the
appropriate distance and angle for each point (Figure 3). Then, we used the Feature
Vertices to Points tool. This tool simply creates a multipoint feature with the
points being at the vertex of each line. Lastly, we added a basemap of our area
so as to see the relation between the real-life features and the point features
we mapped in the field.
Figure 2: These Excel files contain all of the data for the points we collected in the distance-azimuth survey. The table on the right has the data collected using the laser technique while the table on the right used the compass and distance finder.
Results:
The
results of this survey were not as accurate as I had hoped. Most of the points
are relatively close to their features; however, for a map with this large of a
scale, it is not sufficiently accurate. There was also a lot of discrepancy between
the results for each method. Though some pairs of the points were close, there
were others that are extremely off (Figure 4). This is likely due to the human
error aspect, though some blame can be attributed to the fact that the two
people collecting the points were standing next to one another rather than in
the exact same spot. We also had some difficulty with the distance finder for
certain points because they were blocked by other features—it is possible that these
points may be off due to interference with the other objects.
Another major issue was the
accuracy of the X and Y coordinates for our point of origin. With the image
provided by Google Earth, it was difficult to find the exact spot that we were
standing and using for our point of origin. On top of that, the X and Y
coordinates found using an online map are surely not as accurate as if we had a
reliable GPS unit with us on the ground. These factors likely influenced the
overall accuracy of the rest of our data points as their locations are entirely
based off of their relationship to the point of origin.
We were
not able to collect the 50 points that were asked for either; we could only
find 32. There just weren’t enough features in the area that could have been
mapped and we were stretching it as it was. We collected points like the sign
on a fence or the handrails for the staircase that led down the hill to fill in
for some of the points.
Another
issue we came across was the available basemap in ArcMap. Unfortunately, the
basemap did not have the resolution needed to create a clear map at the scale we
were working on. The resulting map is very fuzzy and it is difficult to make
out any of the features we recorded.
Figure 4: This is the final product of our distance-azimuth survey. The red points are the data points collected using the compass and distance finder while the green points represent the data points collected using the laser.
Conclusion:
Overall,
though the results were not as accurate as we were hoping for, the project was
extremely informative and useful. Knowing how to use distance and azimuth to
map points is an invaluable skill that will surely be utilized sometime in the
future. With every project or field study, issues will arise and it is important
to have a back-up plan so as to keep the project on its feet. Using two
different techniques, my group and I were able to compare and contrast various
tools within the distance-azimuth method of mapping data. We found that the
laser gun was more accurate than the compass and distance finder; however, the
compass and distance finder technique may be the only available option for a
project and the tools necessary are much easier to come by.
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