Sample Density and
Resolution: 3030 Winchester
The data set below was part of a magnetic gradient
survey performed at the 3030 Winchester site (results are discussed
in more detail here). Data were collected at 0.5 meter transect
intervals, with 8 samples collected per meter along each transect
(sample density = 16 s/m). The sample density was experimentally
"thinned" by removing alternate transects from the data
file (processing and display parameters are otherwise identical).
A transect interval to 1m (which has been very common practice on
both historic and prehistoric sites) was found to dramatically
reduce the detectability of hearth features.
Figure 1. Subset of magnetic survey data. Darker
shades represent higher (positive) values. Five Prehistoric hearth
features plainly appear as circular magnetic highs. When these
features were excavated, a correlation was found between the amplitude
and geometry of the geophysical anomalies and the volume and composition
of the excavated hearths.
Figure 2. (below) The data set plotted above with alternate lines
of data removed to simulate survey at 1m transect intervals (8
s/m). At this sampling interval, detection of hearth features
is hit-and-miss. Only one of the hearths is plainly detected,
and correlation between feature size/composition and anomaly strength/geometry
is lost - in fact, the only hearth that is plainly detected (N15/E6)
is the least substantial of the features within this area. While
these hearth features create very distinct anomalies on an otherwise
magnetically "quiet" site, their radius of detectability
is quite small, and they are easily missed if the transect interval
is too great. While 1m transect intervals can be appropriate where
features of interest are large or substantial (e.g. many historic
sites). Spatially small and very subtle anomalies require closer
sampling intervals. Cost-effectiveness is best served when the
sampling strategy is optimized to address specific research interests,
feature composition, and site conditions with budgetary constraints.
