Field Activity 6: Distance Azimuth Survey

Introduction

                An accurate and sufficient survey of an area can sometimes be accomplished using a grid based approach, but this method is not always ideal. Often the survey area is too large to effectively create an accurate grid for spatial sampling. An effective alternative is the use of a GPS receiver in conjunction with a total station, a surveying instrument containing an electronic distance meter as well as an electronic theodolite. A total station is capable of very accurate horizontal and vertical angles, sloping distance, as well as calculation of coordinates of an unknown point from a known point. While this combination of technology is a very accurate and efficient method to survey large areas, it is expensive and not always reliable. The price of a total station starts at $3,500.00 and a GPS is also usually needed to determine at least one known coordinate. If a person is fortunate enough to have access to such equipment, it should not be completely relied upon. Technology tends to fail on occasion and it is important to have an alternative survey method. The distance and azimuth method is one such alternative that can be done using less complex equipment. This method requires at least one known coordinate point and data is collected in relation to the known point; this is known as implicit data. With known coordinates of one point, the distance azimuth method can be achieved using only a compass and a measuring tape. Accuracy and efficiency does however improve as quality and availability of equipment increases.

Methods

                This lab was performed in order to gain experience using the distance azimuth survey method. The study area was a section of Putnam Park, on the UWEC campus (figure 1). A recreational grade, Bad Elf brand GPS was used to determine the coordinates of three known points within the study area. The implicit data gathered in this survey included azimuth and distance; from each given point groups used a lensatic compass to determine azimuth and a TruPulse laser rangefinder to find distance in meters to various trees in relatively close proximity. Other attribute data included tree species and diameter at breast height (DBH). To efficiently collect data while also allowing for each group member to utilize the equipment, the group delegated tasks and rotated between them. One group member selected a tree and measured DBH, one found azimuth, another measured distance, another recorded the data, and tree species identification was a group effort.

 In order to compile data sets from all three coordinate points the class created a shared excel spreadsheet in which all groups could enter their collected data. The appropriate fields within the spreadsheet were set to a numerical data format and the table saved as a csv file to facilitate the import into ArcMap. To begin mapping the survey in ArcMap a new geodatabase was created and the table was imported into it. The table was then added to a map document as XY data and then converted into a point feature class using the WGS84 projection in order to coincide with the GPS coordinate values. When displayed over a world imagery base map, two of the three points were not located in their expected locations. Point three was located near interstate 94 and point two was about 30 yards north of its actual location. The points were corrected using the base map for reference and editor to move the points to a more accurate positions. In order to map all data collected, data management tools found in Arc Toolbox were used. First, the Bearing Distance to Line command was used to create lines from each origin to the surveyed trees (figure 2) and then the Feature Vertices to Points command was used to create points at the vertices of the lines created. The resulting points were then symbolized based on tree species (figure 3). Finally, the created features were used to compile a map layout depicting all three study areas.

Figure 1. Location of study area

Figure 2. Result of Bearing Distance to Line command

Figure 3. Result of Feature Vertices to Points command

Results/Discussion

                A few issues to were encountered during the initial import of data into ArcMap. After creating a point feature class of the origin points, a world imagery base map was added and zoomed into the study area`s location, but the points were not visible within the study area. The zoom to layer feature of the point feature class revealed the points to be located off the west coast of Africa, a fellow classmate`s further investigation indicated the X and Y values had been transposed. After this error was corrected, the remaining inaccuracies could most likely be explained by a GPS error. The study area was at the base of a large ridge to the south, which most likely interfered with the GPS. Previous groups participating in a similar lab used Google Earth to determine origin points and also encountered the trivial errors found in this lab after the X and Y coordinates had been transposed to represent the correct values collected. The error was corrected with the same level of ease as in this lab, by simply editing the point locations before running the other data management tools.

The features created from the data management tools were compiled and displayed over a world topographic base map to create a final map layout depicting the origin of each survey within the study area as well as the distance and species of each tree surveyed (figure 4). The data was also used to create a graduated symbols map based on DBH of the trees (figure 5). The final results are quite accurate, but point three is slightly south of its actual location and some of the trees appear to be in the middle of the road. This could have been more accurate if the initial points had been edited using the world topographic or the streets base map instead of the world imagery base map.

Figure 4. Layout depicting tree species

Figure 5. Layout depicting tree DBH

Conclusion

                In real life surveying situations, often the sample area is too large for a grid based survey. In these instances, surveyors often utilize expensive, high-tech equipment; such as total stations. However, not all surveyors have access to this equipment and when it is available it may fail to function properly. The distance azimuth survey proves to be an acceptable alternative approach when the study area is too large for a grid based sampling method and resources are limited. As seen technical reports from previous classes, even when beginning a survey without known coordinates the distance azimuth method can still be utilized. This just requires one be able to locate their origin(s) on Google Earth or some other application in which the coordinates of the location can be extracted.