A16.4 Surveying by offsets (polyline or polygon):
Level of expertise required for this Chapter: Intermediate; general Map Maker training
The product of a survey carried out by offsets is a polyline whose corner, A to M in the example, might represent points whose positions it is wished to plot or points on a linear feature. In this example the feature under survey, represented by a broken black line, might be thought of as the edge of a stream.
Markers are placed at Station 1 and Station 2, and their positions are determined. The line connecting Station 1 and Station 2 is the baseline for the survey.
Offsets are measured at intervals from the baseline to the feature under survey. The offsets must be at right-angles to the baseline. In the example, the first offset starts at a distance of 19.20 m from Station 1 and is 25.90 m long. The second offset starts at a distance of 34.90 m from Station 1 and 47.40 m long, and son on for the remaining offsets.
Record the results of the survey in a data table similar to that shown below.
In the example shown, when viewed from Station 1 looking towards Station 2 the baseline has been set out to the left of the feature being surveyed. When a baseline is set out to the right of a feature (when viewed from Station 1 looking towards Station 2) the lengths of the offsets must be entered into the xy file as negative numbers.
The process in Map Maker is the same as explained at 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 OFFSET and the x,y coordinates of Station 1 and Station 2, all separated by comma) > File > Save > OK
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)