Case Study
Sample Density and Resolution: Cahokia
The data set below was part of a larger methodological
study performed at Cahokia Mounds Historic Site. Electrical resistance
data were collected at very high sample densities, revealing small-scale
detail in the archaeological record. The sample density of this
subset of the resistance survey data was experimentally "thinned"
by removing alternate lines of data (other processing parameters
are identical). The relationship between sample density and feature
detection may be readily observed.
Figure 1. Subset of resistance survey data. The survey
area is located to the northeast of monks mound. Although other
portions of the resistance survey revealed more spectacular large-scale
rectilinear patterning, this subset was chosen because of the unexpected
detail revealed at very high sample densities (16 s/m). Darker shades
represent higher resistance.
Figure 2. (below) Major components of (presumed) cultural
patterning are indicated by colored lines. Other less distinct patterning
has not been marked. The rectilinear or trapezoidal patterning indicated
by yellow lines is thought to be associated with wall-trench architecture.
The patterning indicated by the dashed yellow line is fainter, but
individual postmolds within the wall trench may be visible. Such
small-scale features might be better resolved by a narrower mobile
probe spacing (a 75cm spacing was used to resolve deeper features).
Larger-scale resistance lows thought to be associated with archaeological
features are marked in orange. Numerous resistance highs (not marked)
may be caused by pits or other features, although their patterning
is more dificult to interpret. North-south striations visible throughout
are caused by plow scars.
Figure 3. (below) data sample density has been reduced
to 8 s/m. Wall trenches have become largely, if not entirely, unrecognizable.
Discrete resistance highs are less distinct. The large-scale resistance
lows in the southeastern portion of the plot are still fairly distinct.
Figure 4. (below) Data density has been reduced to
2 s/m, very commonly employed on prehistoric and historic sites.
Most cultural patterning in this data plot has become unrecognizable,
although this sample density has successfully mapped large-scale
cultural patterning elsewhere on the site.
Comments: While high data densities are relatively
time consuming (and therefore costly) to collect, the level of detail
obtained may be very rewarding. Sampling strategy should represent
a pragmatic compromise between resolution and cost. Very high sample
density was rewarding on this site, not because of the ephemeral
nature of the features, but to resolve the features in the complex
physical context created by long-term intensive occupation.
Sample density
and resolution summary page
Sample
density and resolution: 3030 Winchester
