Precision and Accuracy

These two terms, which mean similar things in common usage, have distinct meanings to archaeologists and others who undertake surveys as part of their fieldwork techniques. Suppose that we use a magnetic compass to measure the azimuth of a prominent mountain peak on the horizon. The compass, we find, is graduated in degrees but it is possible to estimate the angle to the nearest half-degree. When we take the same reading three or four times we get readings consistent to the nearest half-degree. Half a degree, then, is the precision of our compass reading.

Accuracy, on the other hand, refers to how close that reading is to the truth. We may have forgotten to take into account the fact that compasses point to magnetic north rather than true north. There may also be a local magnetic anomaly. As a result, it may be that the reading we obtain is actually four degrees higher than the true value. The accuracy of our reading, then, is only four degrees. Great precision is no guarantee of great accuracy.

Overprecision can be very misleading. If we use a theodolite with an electronic distance measurement (EDM) capability to measure the distance between, say, the center of a stone circle and an outlier, then we will be able to read off the result to the nearest millimeter. However, standing stones are generally large and irregular in shape and may have shifted somewhat during the millennia since their original construction. The circle may not be exactly circular and its center may be ill-defined. To measure, precise to one millimeter, the distance between two points which are themselves arbitrary to within several centimeters or more is completely pointless. Worse, it can be misleading: there is a danger that such "falsely accurate" measurements will be used subsequently, say, in calculations that explore the possible existence of a precise unit of measurement.

Finally, we may also speak of the precision with which prehistoric people set up an astronomical alignment. Thus it could be that a group of people in Bronze Age Britain constructed a row of standing stones aligned upon the solstitial sunrise to a precision of about half a degree (thirty arc minutes); that a team of surveyors in 1999 measured the alignment to an accuracy of a few arc minutes; and that the results were published in 2001 quoting declinations to a precision of one arc minute.

Compass and Clinometer Surveys; Field Survey; Theodolite Surveys.

Stone Circles.

Azimuth; Declination.