Friday, December 18, 2015

Behind the Scenes at The State Museum – Mapping the archaeological record at Fort Hunter

To continue our discussion about Fort Hunter data collection, processing and usage we will take a look at one of the most important factors in understanding and preserving an archaeological site. This factor, as is commonly stated in the real estate industry is, “location, location, location”. In order for archaeologists to understand the landscape of an archaeological site we must know where everything is in location to one another both horizontally and vertically. The reason it is so important to record the location of artifacts, features and structures, is that once they are removed from the ground there is no way put them back in their exact place again. In addition, maps depicting the exact location of different types of artifacts are necessary to identify artifact patterning and activity areas. The excavation methods employed by trained archaeologists insure that the entire archaeological record of a site is properly recorded during excavation as archaeology is a destructive science.
In order to preserve this locational information, sites such as Fort Hunter, are excavated based on a grid set from our datum (a known fixed point). This allows archaeologists to go back to a site, whether it is from year to year or twenty years from now, and re-establish the grid. With good documentation and a re-established grid, archaeologists can determine what areas had been previously excavated. The grid also provides the horizontal locational information of artifacts and features that have been removed from that area. At Fort Hunter our grid is in 5 by 5 foot square increments, which is termed as a unit. We name our units using the northing and easting (for example N90E10) of the southwest corner of a square. This designation allows us to easily reference that unit and keep track of the artifacts or features.

Overview of Fort Hunter excavations with stakes and string line indicating the grid, Fort Hunter 2007

Once a grid is established, we begin excavating units in levels in either arbitrary levels of a predetermined measurement (for example 3 inches or 5 centimeters etc.) or based on soil layers, which are indicated by changes in soil color and texture. The layers are often given an alpha designation based on the soil type. Identifying the same types of soil throughout the grid allows us to see how the soil layers slope and change over the landscape. These anomalies can indicate different geologic/climatic processes as well as point to the activities of people on the landscape. Within these natural layers, we then excavate in arbitrary levels. These levels and layers are measured below the set datum elevation, which provides the vertical location information of the artifacts found within that level.

As mentioned in our last blog, “…unique catalog numbers are assigned to each provenience.” The provenience mentioned here is the locational identity of the artifacts based on the horizontal and vertical measurements discussed above. It is with the locational information and the well-developed catalog that we are able to know how the artifacts and features are related to one another.
Now that we have explained how we use the grid, we can look at how we layout the grid, take measurements and how we manipulate the data in the lab. The basic idea of establishing a grid is to create accurate 90 degree angle squares and in order to do this archaeologists use a transit, tape measures and some basic geometry. A transit is an instrument that sights straight lines and different angles. The transit is also used with a stadia rod to measure the depth of a level.

Staff member using transit, just beginning to set up grid, Fort Hunter 2010

Today we use a newer technology called a total station. A total station is an electronic transit which can also sight straight lines and angles as well as use a laser and prism to collect precise horizontal and vertical measurements of a point on our grid. Using the Top Con Data Collector (handheld attachment to the total station), we are able to easily store and look up point information while in the field and also download and convert the data into a spreadsheet format.

Staff member using Top Con total station, Fort Hunter 2014

Staff member holding prism to take measurements using total station, Fort Hunter 2014

Example of collected data in spreadsheet format

With the data collected, we are able to then create useful maps, which allow us to analyze the relationship between features, structures and artifacts. It is also possible to use unit and artifact data to create distribution maps. Common programs used to create such maps include Golden Software’s Surfer and Autodesk’s AutoCAD.

Example of a feature map, showing relationship of several different features

Example of an artifact distribution map

Example of a profile map

With today’s technology, and the detailed information we collect, there are many different mapping options including those above as well as the ability of creating 3-D images. Knowing the relationship of artifacts and features on the landscape provides the foundation that archaeologists use to develop explanations for how past humans were living on and using the landscape. Creating these maps provides a useful visual comparison of how features, artifacts and structures are placed on the landscape. Finally, maps also provide a great way to interpret an archaeological site and how we present different ideas of the past to others.  

We wanted to take a moment to remember a longtime volunteer, Sheila Dunn. Sheila was a dedicated volunteer who put a lot of time and effort into collecting data and creating Fort Hunter maps for us. Using her training and past experience in watershed surveys she was always ready and willing to help out in any way and put in great effort to create some of our first maps of the Fort Hunter excavations. Thank you, Sheila.  

Sheila Dunn

We hope to see you all in the new year at the 100th anniversary of the Pennsylvania Farm Show running from January 9-January 16, 2016. Look for us in a new location this year directly off of the Maclay Street entrance near the children’s carousel. From all of us in the Section of Archaeology at The State Museum of Pennsylvania - Have a happy and safe holiday!

For more information, visit or the Hall of Anthropology and Archaeology at The State Museum of Pennsylvania .

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