Activity #4: Distance Azimuth Survey

Introduction 


Figure 1 - Visualization of azimuth

This activity was designed to introduce our class to surveying using the distance and azimuth method. This is a fairly simple technique that can be employed when more accurate technology fails or is too expensive. The concept behind this method is simple: locations of features or objects can be accurately recorded and mapped using only slope distance from the surveyor and the azimuth.

Azimuth is an angular measurement of where something is in relation to true north and the observer (Figure 1). For example, north is 0° or 360°, south is 180°, east is 90°, etc. Slope distance is the distance from one point to another along a slope. Think of it as the length of a right triangle's hypotenuse as opposed to the triangle's length (Figure 2). As long as we have accurate coordinate location for where we are standing when taking the readings, we can use slope distance and azimuth to chart anything within range.

Figure 2 - Difference between slope distance and horizontal distance

In today's world, there are other faster and more accurate was of doing surveys, but sometimes that isn't an option. GPS devices rarely work when buildings or other structures obstruct the sky, people forget to charge batteries, equipment can break, weather conditions may restrict what kinds of technology can be used, and on and on. Understanding and knowing how to employ these "low-tech" methods not only comes in handy in a pinch, but also helps us understand how different types of equipment and technology work. After a brief demonstration of the equipment we would be using, we were put into groups and instructed to find an area to survey.


Figure 3 - TruPulse 360 Laser Rangefinder

Methods

Equipment: TruPulse 360 Laser Rangefinder (Figure 3)
The TruPulse rangefinder comes equipped with many functions, but we only need azimuth and slope distance. When looking through the viewfinder, there is a display along the bottom informing the surveyor what measurement is being displayed, and along the top is the actual measurement. On the left side of the device are arrow buttons used to switch modes. "SD" indicated standard distance (in meters) and "AZ" indicated azimuth. On the top of the device is a "fire" button which must be held for a few seconds to get accurate readings. Firing the laser once will collect all the data, so distance and azimuth can be recorded by only firing the laser once per object. 

Magnetic Declination

One issue when finding azimuth is the magnetic declination. This refers to the angular difference between true north (geographical north) and magnetic north (compass needle north as found by using magnetic field lines). If magnetic north lies to the east of true north, it is considered positive declination. The declination is negative if to the west. Some areas can vary drastically, others very little. Before going out into the field, it is always a good idea to check magnetic declination for a study area to ensure measurements are accurate, or can be adjusted for if needed. The magnetic fields of the Earth slowly change over time, and that means magnetic declination does as well.



Figure 4 - Enter zip code to find location,
then calculate to get magnetic declination

There are ways to manually find magnetic declination, such as comparing magnetic compass reading to Polaris (North Star), which is within a degree of true north. However, to save time and not have to wait for a clear night, we can also use websites like NOAAs. For this, we simply have to input a zip code to find out the magnetic declination for that area (Figures 4 and 5).

Figure 5 - Magnetic declination results: 1° 4' 53" W


 





For Eau Claire, we have a magnetic declination of -1° 4' 53". This is a relatively small declination, so we didn't make any adjustments.







Survey Area

Our group wanted to survey an area on campus. In order for this to work, we needed to pick places to survey from that we would easily be able to find accurate coordinates for. Using a GPS device would be too inaccurate. The best thing to do would be to survey from places that would be easily spotted from satellite images (building corners, street corners, etc.), find them on a base map in ESRIs ArcMap, and get the lat/long coordinates from that.



Figure 6 - Red shows areas of campus that are out dated in this
image. Area in blue is the study area we settled on since it was
relatively unchanged.

When we found campus in ArcMap on the aerial imagery base map, we found that the image was very out of date. Campus has changed drastically since the image was taken. In order to get accurate coordinates and have the items we surveyed show up, we needed to pick an area that was mostly unchanged.

In figure 6, I have areas that have completely changed since this image was taken boxed in red. The area in blue is our study area (also in figure 7). This seemed to be a large area that was relatively unchanged that had many features that we could survey.





Figure 7a - Area on Campus to be surveyed. 



Survey

Figure 7b - Locations where we would survey from.

To do the survey, we used three different locations that could be spotted in figure 7 so that we could use ArcMap to find the lat/long coordinates. For each location, we stood in one spot and gathered data from 25 different features. At each location, a different member of our group operated the device while another wrote down the readings that were called out. This way we all got experience using the TruPulse. For each feature, we documented item number, type of feature (bench, light pole, tree, etc.), slope distance, and azimuth (Figure 8).

Figure 8 - Documentation of TruPulse data

  Location #1 is right outside the north entrance to Schofield Hall, at the bottom of the stairs. Location #2 is right in front of the corner of the metal railing at the base of the stairs leading to the foot bridge. Location #3 was on the opposite side of the footbridge, at the corner of the railing at the bottom of the bike ramp. At all locations, we faced inward toward the area directly north of Schofield Hall. Because of this, many features were surveyed from each position but had different distances and azimuths, depending on where it was surveyed from. This way, we would be able to see how accurate our strategy was. Figures 9-12 show the view from each location, as well as many of the features we surveyed.

Figure 9 - Location #1, looking northwest

Figure 10 - Location #1, looking northeast

Figure 11 - Location #2, looking south

Figure 12 - Location #3, looking southeast

Figure 13 - Zach using the TruPulse at location #2

Importing Data

Figure 14 - Excel spreadsheet with our field data ready
to be imported into ArcMap

Figure 15 - Coordinate data displayed in ArcMap
showed Location #1 and Location #2 had the same decimal
degree location

Once we completed our survey, it was time to enter all of the data we wrote down into an Excel spreadsheet that we could import into ArcMap (Figure 14). The table consisted of six fields: Point Number, Distance (m), Azimuth, Point Data (what type of feature it was), X, and Y. The X and Y fields were the longitude and latitude of the three points we collected data from. These coordinates were actually the first problem we encountered during the lab exercise. We planned on using the aerial images from ArcMap to find the coordinates of the points we collected data from. When we finished collecting our data and went to ArcMap, it was not precise enough. The coordinates for Location #1 and Location #2 were the same (Figure 15). If we used those coordinates in our spreadsheet, the data we gathered in the field from Locations #1 and #2 would be displayed from the same point, and would be incorrect. Instead of using those coordinates, I went to Google Earth to collect the coordinate data (Figure 16). The problem here was that Google Earth uses degrees/minutes/seconds for measurement. We needed our data in decimal degrees. Using this website (Figure 17), we converted the Google Earth values to decimal degrees, which gave us much more precise coordinates (six decimal places).

Figure 16 - Google Earth displayed more precise coordinate
locations, but not in decimal degrees

Figure 17 - Website used to convert degrees/minutes/seconds
measurements from Google Earth to decimal degrees

Once the spreadsheet was completed, it was ready to be imported into ArcMap. First, though, we needed to create a geodatabase for all of our data to reside in. A geodatabase is a way to store, organize, and easily access spatial data. To create a new geodatabase, go to Catalog in ArcMap, right click on a folder, and select 'create new file geodatabase.' It was here that all of the data could be saved. To import the Excel table, I right clicked on the new geodatabase in the Catalog and selected 'import --> table (single).' Then I just had to select the sheet (not the whole workbook).

Using the table that I just imported as the input file, I ran the "Bearing Distance To Line" tool. This can be found in the ArcToolbox under 'Data Management Tools' then 'Features' (Figure 18).  This tool used the starting coordinates, the azimuth, and the distance to create a new line feature class (Figure 19).

Figure 18 - Bearing Distance To Line tool in ArcToolbox

Figure 19 - Line feature class created using Bearing Distance To Line tool

Figure 21 - Feature class created with Feature Vertices To
Points tool, as well as the line feature class from figure 19.

 Next I ran the Feature Vertices To Points tool in 'Data Management Tools,' then 'Features' (Figure 20). This tool creates a point feature class at the ends of the line feature class specified, in this case the feature class I created in the last step.

Figure 20 - Feature Vertices To Points tool

Figure 21 - Example of using the Search tab

It is also worth noting that there is an easier way to locate these or other tools. ArcToolbox contains many useful tools, and it can be difficult to remember where all of them are. By clicking on the 'Search' icon on the toolbar, the 'Search' tab is opened. Typing in keywords brings up tools that you might be referring to, as well as where they can be found in ArcToolbox (Figure 21).

Results

Figure 22 - Final results of Bearing Distance To Line tool
over a satellite imagery basemap

 Our survey turned out pretty accurate. Figures 22 and 23 show the results overlaid on a satellite imagery base map, and Figure 24 shows the most notable errors circled in red. The points that we took the survey from look slightly off, meaning all the other points are just slightly off as well. There were a couple of points that were very off. One line ends in the Chippewa River, and two lines are on the library.

Figure 23 -Final results of Feature Vertices To Points tool

Figure 24 - Major errors in final results (circled in red).

Discussion

 Overall, I thought the survey went well. The starting points could be slightly wrong simply because the coordinates we used weren't exact. If we changed the X and Y fields, all the of the data could be improved. Also, I don't know how sensitive the TruPulse is when measuring slope distance. For example, would the reading be very different if the surveyor is pointing at the base of a tree or the middle of a tree? Perhaps it would have been more consistent if only one person used the TruPulse the whole time, as opposed to trading off?

The two points in the library that seem like errors could actually be features that are hidden by the large overhang above the main entrance (like a bench or a garbage can), and so not errors at all. Lastly, the point in the river could simply be human error. The measurement could have been said wrong, written wrong, transferred to Excel wrong, or the button wasn't held down long enough. All of these are possibilities.

 On the plus side, it was a fairly nice day outside. Amiable weather makes taking accurate measurements much easier. In our first lab, I felt that our data collection might have been more accurate had it not been windy and sub-zero. Double-checking becomes less of a priority when you can't feel your fingers. For this lab, however, we could concentrate on the exercise. 

Conclusion

 I can see how this tool would be very useful in the field, though probably not on it's own. It is good to have as a back up, but doesn't seem as accurate as would be needed when working in the field professionally. It is always a good idea to know how to complete a task multiple different ways. When in the field, you can't always count on technology. This method is relatively low-tech and easy, so can be used anytime. Not only have I learned a new skill, but it also helped me to understand a little more about survey techniques. This method is very basic; pick a point that you can confirm is correct, then find where everything else is relative to that point. If your first point is correct, than all the other points should be correct too. Even without a laser rangefinder, this strategy could be put to use in many situations.