EX 4: Conducting a Distance Azimuth Survey

Introduction:

As one of the main goals from the University of Wisconsin - Eau Claire, to graduate as a critical thinker, today's lab was an important variation of that idea. Often times the unexpected can limit your capabilities by having your equipment fail, weather complications, or other unforeseen complications. Without the proper motivation and/or creativity, these problems could delay the survey and waste valuable time. That is why having other tools at your disposal is vital to completing the objectives for whatever project you may be involved with.

Today's project was designed to facilitate creativity and teach the method of distance azimuth, specifically its use in surveying. Given the event that your GPS battery dies in the field, distance azimuth could be used to collect points. As long as the location of your survey point is known, as a reference point, the distance and azimuth--the angle from true north (0°) measured clockwise--can be used to find the relative location of additional points.

Methods:

Magnetic declination, sometimes called magnetic variation, is the angle between magnetic north and true north. Declination is considered positive east of true north and negative when west. Magnetic declination changes over time and with location. Figure 1 shows a map of magnetic declination for the United States in 2010. Magnetic declination for Eau Claire Wisconsin (Latitude: 44.7985°, Longitude: -91.482444°) is -1° 5' West, declination is NEGATIVE.



Figure 1. Created by the National Geophysical Data Center (NOAA).
Magnetic Declination for Eau Claire, WI is negative (-1° 5' West)

Figure 2 Using a tripod for the range finder was an effective method
for dealing with the user error that could have resulted when shifting
from side-to-side during the 1.5 hours it took to find the distance
and azimuth of the trees, signs, and buildings from this central location.

Using distance azimuth is an effective way to collect data; however, this method largely hinges on the use of high accuracy and precision when all the points are calculated from a central location. That is why knowing the exact location of surveyor is of the utmost importance. If this point is off by even a foot, then all the other points collected will have the same error. In addition to knowing the correct position of the surveyor, keeping the location constant is important. If the surveyor is taking steps from side-to-side while collecting data the azimuth with change. Even a eight inches could change the azimuth by a degree or two, and over 100 meters this could result in a much larger error. In short, distance azimuth relies heavily on consistent technique and high accuracy records concerning surveyor position. Figure 2 shows how the group tried to deal with keeping a consistent location for all 100 data points collected as well as a prominent location which could be found on an aerial image (corner of tennis court used to find the lat./long. of the surveyor).

Figure 3 Shows the interface for the Table to Table
Tool. This tool allows the importation of a table
into an existing geodatabase.

After surveying in the field, the rest of the process was focused on creating a feature class of points. This point feature class would represent the objects 'shot' during the survey. The first step was creating a table of the survey in excel. Next, that table had to be imported into a geodatabase (Figure 3 shows the import process). Although the work in the field was carried out as carefully as possible, the values were slightly flawed.

Figure 4 An illustration of Earth's shifting poles. Note the discrepancy between magnetic north and true north
or the "north pole."

Due to the complex processes within the Earth's core, magnetic fields are produced. We use these to navigate with our current day compasses. However, every 200,000-300,000 years the poles switch. This is not an over night event, but a slow fluctuating process. This is

Figure 5 The field calculator was used on the azimuth
field to adjust for the switching of Earth's magnetic poles.
-1° 5' west was the magnetic declination adjustment used
because of the survey location in Eau Claire, WI.

the reason for the previously described phenomena described in Figure 4 with the magnetic declination. For Eau Claire, WI (latitude: 44.802536°, longitude: -91.500164°) the magnetic declination was -1.083333. This value was then used in the field calculator to adjust the azimuth of each angle. Figure 5 shows the equation used to correct for the changing of the poles.

Figure 6 shows the interface for the Bearing Distance to Line
tool. It is important to add the correct fields for the x- and y-
fields. For this survey the latitude (y-field) and longitude
(x-field) at the corner of a tennis court was used.

This was important because the laser was giving the azimuth for magnetic north; and, for surveying true north is desired. True north can be find by adding/subtracting, depending on your location, the magnetic declination from the azimuth. Since the excel table only had fields for ID, distance, and azimuth, the data was still useless. Without a reference point the data had no spatial relationship to the rest of the world. This was corrected by adding x- and y-fields which would represent the latitude and longitude of the reference point for each individual surveyed point. For this study, only one reference point was used. However, in real world examples a clear line of sight will not always be available. If this was the case the x- and y- fields would contain different values depending on the latitude and longitude from which the survey was conducted. In a future study it would be wise to add the spatial location into the excel table during the field work/before importing the table into the geodatabase. That way if there was multiple survey locations the points would already have their own spatial reference point and the risk of error created during the data management phase could be taken out of the process.

Finally the data was in proper form to be used in the Bearing Distance to Line tool (Data Management -Toolbox, Features -toolset). Figure 6  shows the interface for the tool, Figure 7 shows the result of the tool. Note the inputs required of the tool for it to run properly. Now you can have a better understanding of why the table had to be so meticulously prepared before its use.


Figure 7 shows the result of the Bearing Distance to Line
tool. With 100 inputs this feature class has limited use
for analysis. You can see where each line stops; however,
points close to the reference point are difficult to see
where their respective line segments stop. As such, additional
tools must be used before the representation is complete.

Figure 8 shows the interface for the Feature Vertices to Points
tool. Note the help file to the right and the method used for
this feature class, "END." This means a point will be created
at the end off all the vertices, but NOT at the origin/start
of the vertices.

The final step in the exercise was to create a point feature class from the output of the previously used tool, bearing distance to line. This was accomplished with the Feature Vertices to Points tool (Data Management -Toolbox, Features -toolset). Figure 8 shows the interface for the tool, as well as the inputs. Note the option "END" was used to produce this particular feature class. The decision was made because this feature class was going to be used in combination with the output from the bearing distance to line tool. Figure 9 shows the result of the Feature Vertices to Points tool.


Figure 9 shows the final results of the survey. Note
the origin of all the green lines. This is the reference
point were all the data was collected from using the
laser range finder.

Discussion:

Now that the survey and feature classes have been completed, the analysis of accuracy can be completed. Looking at Figure 9 every yellow point should have its correlating tree, parking sign, house, or bridge support pillar. When the points don't match up, a question should be asked. Why isn't there a feature under that point? There are many reasons for this. The image used might be temporally inaccurate. To get a better estimate of when this image was taken, an analysis of the surrounding area was needed. If you are familiar with the University of Wisconsin-Eau Claire you would quickly recognize some discrepancies. There has been a lot of construction as of late with one student center being demolished, another being built just to the South, and a new education building close by. Only the beginning stages of the student center are visible in this aerial. The official date of construction started on March 7th, 2011. As you can see by Figure 10, construction has just started to begin. Only the preparatory stages have occurred at the time of this photo. Suggesting the date to be around early spring time. Other reasons could stem from different projections used, distortions during the mosaicking of images, or user error in the field.

Figure 10 Construction on the new Davie's Center began March 7th, 2011. You can already see a trailer and
disturbed ground. Figure 11 shows the Davie's building during mid-construction.

Figure 11 The W.R. Davies Student Center took just over
2 years to build. The aerial photo above shows the building
during mid-construction, likely during mid-summer of 2012
using the color of the leaves as clues, and the knowledge
that the previous building was demolished in August of 2012.
The old building is still standing in Figure 11.

Conclusion:

In order to understand distance and azimuth, many aspects of surveying had to be researched. Phenomena such as the shifting of Earth's poles, magnetic declination, operation of a compass, etc. Although each piece of the lab was not individually a keystone piece of information. Collectively they created a deep understanding being the principles of distance and azimuth. At the end of the lab I felt very confident I knew what I was doing and I could lead others in the field where distance and azimuth could be put to use.