Compass and Clinometer Surveys

To determine the astronomical potential of a horizon point we need to measure its azimuth and altitude and then calculate the declination. Where we are concerned with possible alignments set up to within about half a degree or better (half a degree is roughly the diameter of the sun or moon), this is best done using a theodolite. However, when the accuracy required is not as great as this, a magnetic compass together with a clinometer may well be sufficient.

To be suitable for the task of determining the azimuth, a compass must be of the prismatic variety, enabling the user to view a distant object, a sighting mark (such as a vertical line), and the scale all at the same time. The measurement obtained is the magnetic bearing, or clockwise angle from magnetic north, as opposed to what is needed, which is the azimuth or clockwise angle from true north. To convert a magnetic bearing into a (true) azimuth, one must know the difference between true north and magnetic north. This difference—which is sometimes known as the magnetic declination, a term that can cause considerable confusion with (astronomical) declination—varies from place to place and time to time. Values for a particular place and time can often be obtained from topographical maps, or from various web sites on the internet. It is important to make sure that the correction is applied in the right direction: if magnetic north is to the east of true north, then the (true) azimuth will be greater than the magnetic bearing; if it is to the west then the (true) azimuth will be less.

A clinometer, or inclinometer, performs a similar task for measuring altitudes except that the zero point is determined simply by leveling a bubble before taking the reading, so that no further calibration is needed. Some manufacturers produce combined compass-clinometers as a single instrument, which are extremely useful for the archaeoastronomer.

Using a compass and clinometer has some clear advantages over using a theodolite. They are light and easily portable instruments, which is a major advantage if a site is difficult of access. There is no need for preliminary setting up and no need to take supplementary readings such as timed observations of the sun, which have the added problem of being dependent upon the weather (since a clear view of the sun is needed).

However, the compass also has some disadvantages. The main one is that the reading it gives can be affected by magnetic rocks or other magnetic materials close by (steel-rimmed glasses, for example). It is desirable to take various precautions, such as keeping metal objects well away from it, and measuring alignments in both directions wherever possible—the bearings should be 180 degrees apart. Best of all is to calibrate the readings from a particular location by measuring one or two reference points whose location, and hence true azimuth, can be determined independently from maps or digital topographic data. For this, one needs to know one's own location to within, typically, a few meters; this can be determined using another portable instrument, the GPS receiver.

Many archaeoastronomers prefer to use a theodolite, while others argue in favor of a compass and clinometer. Which instruments are best in any given instance actually depends upon the problem being addressed.

See also:

Field Survey; GPS Surveys; Theodolite Surveys.

Altitude; Azimuth; Declination.

References and further reading

Belmonte, Juan, and Michael Hoskin. Reflejo del Cosmos, 25-28. Madrid: Equipo Sirius, 2002. [In Spanish.]

Hoskin, Michael. Tombs, Temples and Their Orientations, 10-13. Bognor Regis, UK: Ocarina Books, 2001.

Ruggles, Clive. Astronomy in Prehistoric Britain and Ireland, 165. New Haven: Yale University Press, 1999.

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