GPS Surveys

Global Positioning System (GPS) receivers use signals from satellites to determine their position on the ground. Given a reasonably clear view of the sky, a standard GPS receiver can usually locate itself in the horizontal to within a few meters. This information is clearly useful in locating or relocating sites of interest, especially in remote areas, but it also has some uses in the course of archaeoastronomical field survey. Sometimes we wish to know the azimuth of a feature on a distant horizon, such as a mountain peak, in order to assess its potential astronomical significance. Then again, we might wish to know the azimuth of a reference point such as a church spire in order to calibrate a compass and clinometer survey. Given that the distant point is readily identifiable on a topographic map, one way to do this is to determine its coordinate position from the map, and to determine the coordinate position of the observing point using a GPS receiver. The azimuth of the distant point can then be deduced.

Standard GPS receivers are not accurate enough to be used for measuring the azimuth between two points (that is, determining the orientation of an alignment) unless the points in question align, in turn, with a distant landmark whose azimuth can be determined with the aid of a map, as already described. Trying to determine the azimuth of one point from another simply by placing a GPS receiver at each of them would, given a ten meter uncertainty in the position of either, require the two points to be something like two kilometers (one and a half miles) apart in order to achieve half-degree accuracy in the result. Differential GPS receivers can be a good deal more accurate, perhaps determining a location to within one or two meters, but even then the two points would need to be several hundred meters (up to half a mile) apart for this technique to be of any use.

Carrier-phase GPS receivers relate to standard GPS receivers as FM radio does to AM radio. Although still prohibitively expensive for most users, they have the potential to transform field survey techniques generally in the near future. A carrier-phase GPS receiver has the capacity to determine a position to within a centimeter or two if not better. This should dispense with the need for heavy surveying equipment such as a theodolite in many cases, although even this sort of GPS receiver will experience dif ficulties if there is not a clear view of the sky, for example because of buildings or trees.

See also:

Compass and Clinometer Surveys; Field Survey; Theodolite Surveys.

Azimuth; Declination.

References and further reading

Ruggles, Clive, Frank Prendergast, and Tom Ray, eds. Astronomy, Cosmology and Landscape, 179-181. Bognor Regis, UK: Ocarina Books, 2001.

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