Field Mapping Name The grand tradition of field mapping goes back thousands of years. All you need is something to measure direction (a compass in our case) and something to measure distance (like pacing). With these simple tools, we can map small areas fairly accurately. Purpose: Become familiar with baselines (closed traverses), techniques for mapping small areas with simple tools, taking field notes, and plotting a map from field data. Equipment: provided: magnetic compass, tape measure, protractor, ruler, graph paper, not provided-bring your own: calculator Objectives: Learn how to use a magnetic compass Learn the convenient & rapid technique of the pace and compass traverse for making maps in the field Transfer field data to map format Definitions Azimuth: An azimuth is an angular distance from magnetic north measured in a clockwise direction and expressed in degrees from 0 to 360. When using a magnetic compass to measure an azimuth, be sure not to use it near metal objects that may disturb the magnetic field. Closed Traverse: A closed traverse establishes a reference frame, or baseline, on the ground used to map objects nearby. It is constructed by measuring a series of straight lines, or legs. All of the legs together are called a traverse. The last of point of the last leg matches to the first point in the first leg, closing the traverse. Each point in the traverse is called a station. Location by Radiation: This is another way of fixing the location of objects distant from the closed traverse. Simply record the azimuth and distance of the object from stations on the closed traverse.
Pacing: The length of any person s step (or pace) is quite constant if they walk naturally over even ground at a normal speed. However, the length of each person s pace will be different. We will determine the length of each person's personal pace by walking a measured distance (20 m) while counting the number of steps, repeat, and used then average of the two values. Divide 20 by the average number of paces to obtain your personal average pace length (in meters). Map Scale: The map scale shows the ratio of distance on the ground to distance on your map. It is usually expressed as a representative fraction, such as 1:1000 Step 1: Pace Length (meters per pace) Walk at a normal speed from one end of the tape measure to the other (20 meters) counting your paces. Record the number in the table. Next, reverse and walk back to the beginning of the tape measure, counting your paces again. Record the second number. You will then be able to calculate your average pace length back in the lab. Table 1: Pace (m) Paces meters / Pace Trial 1 20 Trial 2 (return) 20 Average Note: 2 decimal places are more than adequate, and 1 is also acceptable. Step 2: The Traverse Our traverse will follow the sidewalk surrounding the Great Lawn. Gather data in the following way: 1. Sketch the area to be mapped (perhaps on the back of this sheet) and label the sidewalk corners as Stations A, B, C, D, and E. 2. Begin the traverse at any point. From the first Station, determine the azimuth toward the
second Station and record it. This is called the foresight. 3. Walk to the second Station counting your paces as you go, record the number of paces. (The paces will be later converted into meters back in the lab.) 4. At the second Station, determine the azimuth back to the first Station. This is called the backsight. This measurement is taken as a check on the foresight measured earlier, it should be approximately 180 degrees opposite. 5. Continue the above procedure for each leg of the sidewalk until you return to where you started. NOTE: If the backsight is not close to180 degrees from the foresight, there is a data error and the foresight should be checked before proceeding. You might have to re-measure the azimuth. Table 2: Traverse Measurements and Calculations Traverse Leg Foresight (degrees) (paces) Backsight (degrees) Ground (meters) Map (cm) Station A - Station B Station B - Station C Station C - Station D Station D - Station E Station E - Station A
Step 3: Locate Objects We will locate several objects for your map using the traverse baseline, including at lease one corner of a building, at least one tree, and at least one sign using the location by radiation. To do this, select any tree, sign, or building corner. Walk to the nearest Station and record the Station letter in the first column. Then, determine the azimuth to the object (using the compass) and the distance (by pacing). Repeat this procedure 3 times at least, once for a tree, once for a sign, and once for a building corner. We will be able to use this information back in the lab to locate on the map. Table 3: Radiation Measurements and Calculations Object Azimuth Ground Map (degrees) (paces) (meters) (cm) Station to Tree Station to Sign Station to Corner Step 4: Calculate Map s In Tables 2 and 3, calculate the actual ground distances by multiplying your pacing length (from Table 1) by the number of paces between locations. (For example, 70 paces times 0.6 meters per pace = 42 meters)
Next you will need to convert meters to distances on the map, like we did in the first lab using a map scale. For this step, you will need to select a map scale to be used. A good scale for this map is 1:600. That is, 1 cm on the map represents 600 cm (6 meters) in reality. Simply divide your Ground by 6 to get map distance in centimeters. (This is a shortcut, normally you would convert meters to cm by multiplying by 100, then dividing by the map scale of 600) Step 5: Draw the Map (sidewalk, objects, north arrow, scale) Draw the closed traverse, and locate the building corners and trees in their correct relative positions. When locating trees and buildings, put them in the correct location, but use your imagination when rendering them on the map. In other words, make them recognizable with a little artwork. 1. Mark a suitable point on the graph paper (provided) to represent Station A. You want your data to be centered on the paper. Draw a north arrow on the map. Remember, this is azimuth 0. North is conventionally at the top of the map. 2. Using a protractor and ruler, draw the closed traverse using light pencil marks. When finished, you will probably notice that there is a closure gap, meaning the last point does not match the first. That is fine, normally we would adjust this by changing each corner a bit, but in this case leave it as is. This gap tells me that you have faithfully plotted your data, it will almost never close properly. 3. When finished, darken the lines and label the Stations (A, B, C, D, E) 4. Locate the objects you located by radiation using the proctator and ruler. Use some artwork to make them recognizable (make the tree look like a tree) and label them. 5. Draw a scale on the map, using a representative fraction. 6. Beautify! (optional) Step 6: Questions Hand in your map along with answers to the following questions: How closely did your backsight measurements match your foresight measurements? What is the purpose of taking backsights? How do you think the accuracy of you map could be improved?
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