Conducting a Distance Azimuth Survey

Introduction
This week I missed the actual field activity because of involvement with a research project in another class, so much of my understanding of the activity comes from the posted description of the assignment, as well as talking to classmates and looking at past blog posts.  The purpose of the activity was to learn how to map locations of features without use of a GPS, using the concept of slope distance and azimuth.

The Azimuth compass starts at 0 degrees as true north, 90 degrees is east, 180 degrees is south, and 270 degrees is west.  Slope Distance takes into account the curvature of the Earth in its measurement, making it more accurate then normal distance.  To conduct a distance azimuth survey, you have a central location (the position of the surveyor) and then you collect information on the distanance of each point from it, as well as the azimuth degree.  You can take measurements of the locations of many different features as long as you have a way to collect this information. Knowing the precise location of surveyor is one of the most important peices of information in the survey. If this point is off by even half of a meter, the same error will be transferred to all of the other points. 

This type of survey is useful because in the field you need to be prepared for a variety of situations.  If you are stuck in a situation where you are not able to use GPS, this method is a simple way that a relatively accurate survey can be completed. 

Study Area
The study area is the a section of the UW Eau Claire Campus located at 105 Garfield Avenue, Eau Claire, WI 54702.  Since I was gone for the activity, I emailed the members of the group that I was listed in on D2L.  The groups were adjusted, but I contacted Professor Hupy who said it was fine to use data collected from Aaron Schroeder's group.  This groups used two stationary surveyor points so that a large area of the northern part of the campus mall could be surveyed.


Figure 1.  Map of study area

Methods
A TruPulse 360 Laser Rangefinder was used .  The device allows you to take a reading of a stationary object and immediately get its slope distance and azimuth from the starting location.

Figure 2. Picture of a TruPulse 360 Laser Rangefinder

The two stationary places were chosen because they were determined to be places that could be easily seen easily in an aerial photo, and therefore easy to find the correct location. A tripod was used to set up the TruPulse in the stationary places.  Features were shot, and the distance, azimuth degree, and type of feature were recorded and later placed into an excel document.



Figure 3.  Portion of the distance-azimuth degree survey excel document

One concern that is necessary to take into consideration for distance-azimuth degree surveys is magnetic declination.  A video from the National Geophysical Data Center shown below is helpful for understanding what magnetic declination is.

Essentially, Magnetic declination is the difference between magnetic north and true north.  The NGDC has magnetic field calculators that can be used to determine the amount of magnetic declination in your geographic location at the time of the survey.

Figure 4. Magnetic Declination shown for the Eau Claire area

The next part of the activity would be mapping the location of these features in ArcMap.  Before doing anything else in ArcMap, the first step was to create a geodatabase which I named distance_azimuth.  This was important for being able to organize and keep track of all the data and layers produced.

I tried using the Bearing Distance to Line command to import the data into ArcMap, but I got error messages when I tried doing this.  I looked at other blogs and found that some of the blogs mentioned using a Table to Table Command to before using the Bearing Distsance to Line Command.  The Table to Table Command, found in the ArcToolbox under conversion is a way to take an excel worksheet and turn it into a geodatabase table.


Figure 5. ArcMaps's description of the Table to Table  tool

When I had the data as a table in ArcMap, I was able to use the Bearing Distsance to Line Command found under data management tools under features.  This allowed me to take the data in the table and plot it as lines on a map.


Figure 6.  ArcMaps's description of Bearing Distance to Line command
 

When I first performed the bearing to line command, I set the x field as new x and the y field as new y.  I had thought that these were the most updated values, but when I looked back at the excel document I saw that these rows contained values with less decimal places (refer to Figure 3.)  This meant that these values were not precise enough which is why I got values that were in the wrong areas, as I am reletively sure that the group did not set up a tripod in the river.

Figure 7. Input of the Bearing Distance to line command shown on the left, output of the command shown on the right.

After checking the table I realized that I should use the origin x and origin y values instead, and when I used these the surveyor points moved to more realistic areas.

Figure 8. Input of the Bearing Distance to line command shown on the left, output of the command shown on the right

 The next step was to get points by turning the vertices of the lines into points.  This was done using the Feature Vertices to Point too under data management under features. 

Figure 9. Input of the Feature Vertices to Points command shown on the left, output of the command shown on the righ

 Results

Using the layers generated in the methods section it is possible to make a map that overlays the recorded location of the features with an imagery background.  Below are two maps, one that show the locations of just the feature points, and one that shows the locations of the feature points with the lines. 

Figure 10.  Location of features collected from survey 

Figure 11. Location of features shown with lines.

 

 

It is also easy to generate other maps that contain informations about the feature points collected.  If a join is performed between the points feature that was generated, and the original survey table, than the information can be used to make maps on the locations of features with certain characteristics.  Two examples of this can be seen below.


Figure 12. Map showing the location of trees in the survey area

Figure 13. Map showing the location of lightpoles in the survey area.

Discussion
The biggest issue that I saw when making maps using this data, was that there was a large level of inaccuracy.  For example,the picture underneath show an aerial view of zorn arena.  When I looked back at the table, the points that you can see on top of zorn were listed as being taken on the edges of the building.  One of the furthest points in was close to 20 meters away from the edge of the building, meaning that in some areas the measurements were very noticably off.

Figure 14.  Aerial view of Zorn arena with feature points from the survey.

Since I was not present for the actual data collection, I don't feel like I can speculate how much of it came down to things like human error or miscalculation.  I would say however, that the points were at least relatively close, and if the features being measured were larger, the proximity to the actual locations probably wouldn't be as apparent.  Also I would assume that with more practice, the errors that arise when using this method would be decreased. 

There was another concern that I saw when reviewing other people's blog posts, and that is the concern that this technique, at least in the way it was used in class might not be that useful, because it relies on an expensive laser to work, and therefore is less likely to be useful in an emergency situation where a GPS isn't working. However, the same method could still be used with a different way of measuring distance and angle, so it is still useful to learn the process even if you don't have access to this specific technology.
Conclusion
This week's assignment was different for me because I was not present for the actual field activity.  Despite this, I still feel like I was able to learn about the surveying methods and gain an understanding of how it could be useful.  The assignment was well explained, which helped greatly when I was going through the indivdual parts of the assignment, and reading through blog posts from last year was also very useful.

Understanding how to conduct a distance azimuth survey is a useful skill to have in circumstances where you do not have access to a GPS.  It may be more time consuming, and more difficult to get to a similar level of accuracy, but it is still a good skill to have when out in the field collecting data.