Sunday, June 16, 2013

Day 9: STPs and Poisson Distributions

Day 9- STPs and Poisson Distributions

The archaeological record is defined by the scatter of artifacts in their matrices near and on the surface of the earth. There are various ways to think about this record, traditionally as the site versus empty space, and thinking of sites as “natural” empirical units. Recently, however, archaeologists have begun to think of sites as a continuum of artifact densities: high to low and to zero density. This is then divided into “site” versus “non-site” occurrences. “Sites” are analytical units, created by researchers for specific purposes. The process of finding archaeological sites requires a process for investigating patterning in the archaeological record at large spatial scales. In the example of Monticello, the entire mountain serves as our site.

One type of survey is called coverage, due to the fact that total coverage of the study area is investigated in its entirety, usually using a single set of methods (e.g. Shovel Test Pits, surface walkover), and is sample based, meaning that an area is divided into multiple sampling units, only some will be fully investigated. The choice of which units is random, systematic, and purposive. There are three dimensions of variations in surveying, spatial samples, subsurface survey, and site survey. Spatial samples are surface surveys (inspections of the ground), which require some decisions as to the amount of survey exposure, spacing of surveyors, and the speed of movement and work to which the excavation takes.  Subsurface surveys dig using subsurface probes, again this require certain decisions, such as the probes spacing, the probe size (Auger holes, Shovel Test Pits, quadrats), or screenings. Site survey designates “sites” in the field, artifacts are provenienced by “site,” this is problematic because site survey is the most common in archaeology, but leaves the site to be openly interpreted. Whereas, in non-site surveying, artifacts are provenienced without regard to whether they occur to “sites,” but are mapped in regards to the STPs (Shovel Test Pits) in which they occur.

At Monticello, we use STPs on 40ft. centers, if an artifact is found, then we move into 20ft. centers, if another artifact is found, we moved down to 5x5 ft. quadrats and screen the dirt with a 1/4 inch mesh screen. The STPs are mapped on the Virginia State Plane for record keeping purposes. The Monticello Plane Survey uses the coverage survey, subsurface survey, and on site surveys. The non-site survey, however, has its own advantages over site surveys. The act of creating “sites” is purpose driven and transparent for non-site surveys, non-site also has the possibility for others to evaluate the results, and the possibility to do it multiple ways. Defining a “site” otherwise gets you as a researcher and archaeologist into a grey area.

Modeling a Subsurface Surveys requires calculating how close the STPs should be, in order to do this the probability of intersecting a site, and the probability of finding one or more artifacts )given that you intersected a site) must be taken into account. In order to model a subsurface survey you must assume “sites” are “non-sites,” the probability that an STP will go into a site (site size and spacing) as well as the probability of finding one or more artifacts given the intersect site. With this the artifact density (mean or variance), screens, and STP diameter are a part of the equation necessary to find this probability. In order to solve for the probability of finding artifacts in STPs, a couple of equations are necessary, which again for math’s sake I will not go into detail about. The equation’s names are the Poisson Distribution, which is used for rare events, and as the mean increases, the Poisson becomes the Gaussian Distribution.

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