photograph, and figure 11-3 shows the line mapmade by the use of overlays.are all used for measuring unknown angles anddistances and/or for laying off known angles anddistances.BASIC SURVEYING INSTRUMENTSMAGNETIC COMPASSMost fieldwork done by an Engineering Aid(especially at the third- and second-class levels)is likely to consist of field measurements and/orcomputations that involve plane surveying ofordinary precision. This section describes the basicinstruments, tools, and other equipment used forthis type of surveying. Other instruments used formore precise surveys will also be described briefly.Surveying instruments come in various forms,yet their basic functions are similar; that is, theyA magnetic compass is a device consistingprincipally of a circular compass card, usuallygraduated in degrees, and a magnetic needle,mounted and free to rotate on a pivot located atthe center of the card. The needle, when free fromany local attraction (caused by metal), lines itselfup with the local magnetic meridian as a resultof the attraction of the earth’s magnetic NorthPole.Figure 11-3.-Line map made by overlays from the aerial photograph in figure 11-2.11-12
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