Geophysical data require interpretation to make them more widely accessible. To assign ‘meaning’ to a very large assemblage of data points (a 1 ha square of magnetometer data recorded at 0.25 m x 1 m resolution already contains 40,000 measurements) an interpreter needs to understand the nature of geophysical data and their possible causes, as well as the archaeological context and the resulting restrictions on reasonable feature attributions. Data interpretation is hence subjective and the caveats of postprocessual theory apply. It may be tempting to escape this uncertainty by restricting data interpretation to the delineation of geophysical anomalies, hoping that somebody else will make a judgment about their meaning. However, such a separation of geophysical and archaeological analysis will not harness the full potential of the carefully collected data. The archaeological interpretation of geophysical data may change over time as new information is revealed elsewhere and it is hence necessary to keep the measured data for possible future re-interpretation (see Reasons for archiving).
Data interpretation should be accompanied by clear diagrams that highlight relevant geophysical anomalies, possibly by delineating them with an outline polygon. They should be annotated with labels that can be used in the textual description; simply referring to an anomaly as “the strong magnetic anomaly in the north of the survey area” is far too ambiguous. Many different labelling schemes are currently in use and have their individual merits. They may even include site numbers and indicators of the geophysical method (e.g. [1m3]) to make them unambiguous in the report of a large survey area.
The textual description and interpretation can follow two different styles. The first style has two clearly separated stages, starting with a geophysical interpretation of anomalies according to the geophysical characteristics of the data (e.g. “Earth resistance anomaly [r1] could be caused by a confined low resistivity feature of ca. 2 m diameter at approximately 1.5 m depth.”, or “The positive magnetic anomaly [m2] is very broad and appears to be caused by a deep feature.”). The second stage is then the identification of these geophysical anomalies as archaeological features (e.g. “Anomaly [r1] can be interpreted as a buried pit of dimension…”). This requires prior knowledge about the site, its geology and related archaeology. Also the overall spatial morphology (square, rectangular etc.) and extent of anomalies may help with their archaeological interpretation in line with similar approaches found in aerial photographic interpretation. The second textual style combines the geophysical and archaeological interpretation for each feature and may often omit the geophysical analysis. This is far easier to read, especially for a busy reader who just wants to know “what has been found”, but it is more difficult to follow for experts who may wish to understand why a particular interpretation was made.