Azimuth & Distance Survey

Introduction

For this exercise surveying using distance and azimuth was implemented to collect point data. A distance azimuth survey starts with a surveyor measuring the distance and azimuth, angular measurement, an object is from the surveyor. This method is a useful technique to know and have ready in case the power runs out on more sophisticated techniques. The UW-Eau Claire campus mall and a nearby street were surveyed collecting point data for street signs, light poles, major trees, bike racks, benches, tables, and garbage cans. These features were chosen for how numerous and spread out around the study area they are. A total of 100 points were collected from three different origins.

Study Area

The area of UWEC was chosen because of its close proximity to those involved in the surveying and for our general knowledge of the known features. UWEC is located along the Chippewa River but the area surveyed is flat with a small decline in elevation near Little Niagara Creek which runs through the campus. A newly constructed campus mall was finished in 2013. The new mall is surrounded by many university buildings. The section of road surveyed runs alongside a university building. Figure 1 below shows an aerial view of UWEC.

Figure 1. UWEC lower campus located along the Chippewa River in Eau Claire, WI

Methods

Points were collected using two different instrument; a compass which determines azimuth and a laser device which determines both slope distance and azimuth. The two different instruments provide a comparative analysis angle to the survey since both determine azimuths. By using both azimuth and distance an accurate survey of the study area was created. Before the points were collected a phenomenon called magnetic declination was investigated.

Magnetic declination is the angle between magnetic north and true north. Magnetic north is the direction the compass will point to while true north is the direction along the Earth’s surface towards the geographic North Pole. It is important to understand the degree of declination because without understanding what it is any survey using a compass without adjusting for magnetic declination will be inaccurate. NOAA has an application that will calculate the degree of declination for any location. For Eau Claire, WI the degree of declination is approximately 1.36 degrees W (negative). It’s negative because the location of Eau Claire is west of the line-of-declination. Anything easy of the line is positive. This means that 1.36 degrees was subtracted from every azimuth collected from the laser and compass.

Using the laser and compass the 100 total points were collected over two days. 50 points from the first location, 25 from the second and third locations respectively. The information was recorded into a field book then transferred to an Excel spreadsheet (figure 2) using six different fields; Distance (meters), Azimuth (angular degree), Compass (azimuth from the compass), Notes (type of feature), X (x coordinate of the surveying point), Y (y coordinate of the surveying point). The surveying point refers to where the surveyor stood to collect the point data.

Figure 2.  A section of the completed spreadsheet with 6 fields and an ID column. 

The X and Y coordinates were found by using AntiMap, a smart phone application which records point data and creates a spreadsheet capable of being imported to Excel. The coordinates for each of the three surveying points were entered into the X and Y fields in the spreadsheet for the points collected from those three surveying points.

Once all 100 points were entered into a spreadsheet with all relevant information ArcMap was used to display the points. First, in ArcMap, a geodatabase was created to store all of the feature layers that would be created. Next, a basemap from USGS was displayed for the area of UW-Eau Claire from 2013. Then the spreadsheet was imported into ArcMap. A model (figure 3) was created to display the data from the spreadsheet as both point and line data. Figures 4 and 5 show the locations of the tools used to display the data.

Figure 3. The model with the two tools is ran twice. Once to get points and lines derived from the laser and again to get points and lines from using the compass. The model can be saved to be edited later with different inputs and outputs.

Figure 4. Location of the Bearing Distance to Line. This takes the distance in meters from the laser and creates a line from the surveyor point to the feature point on the angle of the azimuth from the laser/compass.

Figure 5. Location of the Feature Vertices to Points. Creates a point of a feature from the azimuth and distance recorded by the laser/compass.

The model significantly decreased the amount of time spent finding and using the tools individually. Since it was able to be saved and used later with different inputs it drastically increase productivity.

An issue discovered when running the model was that when there were different X and Y coordinates the model would fail. To remedy this three separate tables were created; one for each surveying point. This did lead to using the model more than expected. An issue that was encountered by previous class members was that the tools would not work correctly unless the X and Y coordinates had six decimal places. This was an easy change and made the tools run perfectly.

The feature layers created from the model were saved to a geodatabase and the WGS 84 projection was used since latitude and longitude was being used instead of meters. This allowed for the points and lines to be displayed correctly on the basemap instead of not even on the map if using a projected coordinate system.

Results

Once the points were displayed with lines from the surveyor point we noticed that this method of collecting point data is fairly accurate. On a small scale map (figure 6) the points seem very accurate lining up well with where the features really are but on a large scale map (figure 7) the points have a certain amount of error.

Figure 6. Small scale map showing the location of the points and azimuth lines derived from the laser. Note how some points fall on nearby buildings showing there is a lack of accuracy in those areas either from the surveyor or from the basemap. 

Figure 7. Large scale map showing the location of the points and azimuth lines derived from the laser. From this scale most of the points appear to be very accurate with minimal error. Most of the inaccurate points are along the large building in the south west portion of the survey area.

When comparing the points from the laser to those collected from the compass there is a noticeable difference in both large and small scale maps. Figure 8 below shows the compass points compared to the laser points.

Figure 8. Large scale map showing the locations of all points derived from the laser and the compass. If both of the tools recorded the same azimuth for each feature there should be no difference in the locations of any of the points on the map. 

Our X and Y coordinates for the three surveying points are very accurate because of how open of an area we surveyed is. Any error associated with the placement of points on the map may be due to the basemap being improperly georeferenced. Even though this would account for a small change in the location it may end up making a large change in the location of points especially those near buildings.

Discussion

Overall this surveying method of using azimuth and distance is very accurate and compared to more modern techniques such as GPS and survey stations which are used by professionals in the field. This method is reliable in all weather conditions and in virtually any type of environment. One main issue with measuring distance that may have accounted for a majority of error is if the laser actually bounced off the feature or if it bounced off something else. Light poles from a distance are hard to hit with the laser but our results of the locations show that the laser was very accurate in recording the distance.

When comparing the compass to the laser we found that the compass routinely differed by a few degrees for each point with very few being within a degree or two of the laser derived azimuth. A reason for this error may be magnetic disturbances but that should have affected both tools equally unless the laser has something built into it that takes that into account. 

Conclusion

Recording point data can be performed many different ways but using azimuth and distance is one of the more reliable methods. Being able to collect data in a way that is quick, simple, and easy to process makes for increased productivity. We were lucky to have warm weather when we collected the data but even if it was snowing we would have been able to perform the survey. This ability to work in multiple conditions allows for azimuth and distance to be a preferred method of collecting points if more sophisticated methods are not available. If we did not use the laser to find distance we could have used a tape measure. The tape measure distance and the compass azimuth would allow for this entire survey to be completed without any battery power required. To perform a more accurate survey multiple measurements of the features should have been taken to find an aggregate azimuth and distance but since we only had one week to perform the survey and process the data the results we achieved are still fairly accurate.