A16.3 Surveying by close-compass traverse (polyline)
Level of expertise required for this Chapter: Intermediate; general Map Maker training
The procedure is very similar to the open-compass traverse (see Annex 16.2), but enforces the creation of a polygon. The CLOSE statement in the xy file forces Map Maker to close the polygon. It is good practice to check the geometry of polygons by first plotting them without using the CLOSE statement. This will allow the magnitude of any surveying errors to be assessed.
The product of a closed compass traverse is always a polygone. The corners of the polygon may represent points whose positions it is wished to plot, or the polygon may enclose a landscape feature (as it does in the example below). The magnitude of surveying errors may be assessed by plotting the polygon in Map Maker without using the CLOSE statement in the XY file (see below).
Set a marker on the ground at Point A. From Point A take a compass bearing on Point B, then measure the distance between Point A and Point B. From Point B take a compass bearing on Point C, then measure the distance between Point B and Point C. Repeat for the remaining legs of the polygon, ending at the starting point.
Record the results of the survey in a data table similar to that shown above. The bearings shown in the table of survey data are grid bearings. (Magnetic compass bearings must be converted to grid bearings by compensation for local magnetic variation. If magnetic north is west of grid north in the area of survey the variation is deducted from the magnetic bearing. If the magnetic north is east of grid north the variation is added to the magnetic bearing).
The process in Map Maker is the same as explained in Annex 16.2:
Main menu > Utilities > Text editor > Save as type: Map Maker survey data (xy) > Select folder > Specify file name > Save > Enter survey data (as shown below, starting with line COMPASS TRAVERSE, ending with CLOSE) > File > Save > OK
When plotted the survey data should describe a polygon that closes perfectly. In practice this never happens, there is always an ‘error of closure’, a gap between the starting point and the ending point of the polygon.
The CLOSE statement is used here to tell Map Maker to close the polygon. Map Maker does this by sharing the error of closure between all of the corners of the polygon.
It is good practice to check the geometry of polygons by first plotting them without using the CLOSE statement. This will allow the magnitude of the inevitable surveying errors to be assessed.
Main menu > Utilities > Vector utilities > Import files > Map Maker survey (.xy) > Import Map Maker survey data,step 1 window: Choose source xy file > Select folder and XY file from previous step > Open > Next step > Accept or specify file name > Finish > Yes > Confirm save
Load the file in Map Maker (Add Layer)