The following was presented at the Society for Historical Archaeology 2012 conference in Baltimore, as part of the symposium People Who Lived With Glass Houses: The Archaeology of Gardens and Scientific Agriculture in Early America.
“Here was a field for industry and enterprise, strongly inviting:” using GIS to identify scientific gardening and agriculture on plantation landscapes
Scientific Gardening and Landscapes
Much has been written about gardens and gardening, the explicit and implicit reasons for their existence, and gardens as a product and perpetuator of ideologies. Scientific gardening, the application of experimentation and the scientific method to the botanical world is one such ideology. Such a concept would have found itself right at home among the planter elite in colonial America. Not only did practically all of their homes have adjacent gardens, but the business of these planters and their plantations was large-scale agriculture. Many kept copious notes and records regarding this endeavor and tried to understand best how to run their farms by experimenting, applying the scientific method, and bringing to bear the weight of the principles of the Enlightenment. Gardens can be seen as a kind of laboratory in which planters worked to understand and refine the science of agriculture. Washington, Jefferson, and Madison are only the best known of colonial America’s scientific gardeners. While scientific agriculture was an ideology of the elite, the physical labor that made it possible was supplied by enslaved Africans and their descendants. Because the very act of gardening requires physical alterations to the landscape, it is possible to recover this activity archaeologically; however, before we can use archaeology to address scientific gardening, we must first be able to locate these remains on the landscape.
The use of computers has fundamentally changed archaeology. Artifact catalogs and databases, artifact density mapping, ground penetrating radar, and 3-D modeling are all specific examples of how computers allow archaeologists to work with vast quantities of data that otherwise would be unfathomable. In recent decades, a new computer technology, Geographic Information Systems, or GIS, has given researchers the ability to map, view, and manipulate spatial data which in turn, can facilitate and inform archaeological excavations. A GIS is “a computer-based system to aid in the collection, maintenance, storage, analysis, output, and distribution of spatial data and information.” GIS programs enable researchers to organize and display multiple layers of spatial data simultaneously. On an archaeology project, these layers might consist of a base-map derived from satellite imagery, an AutoCAD file showing test unit locations or feature outlines, artifact density distributions, the results of remote sensing surveys, and a digital elevation model or contour map showing site topography. All of these data sources characterize a different aspect of the site being investigated and by combining them in a GIS program, the researcher can display and explore the relations between them and even generate new spatial knowledge from them. One such data source that benefits immensely from and contributes to being incorporated in a GIS database is LiDAR.
LiDAR, or Light Detection and Ranging, is a three-dimensional mapping technology that can be used with GIS as a part of an archaeological research project. LiDAR uses pulses of light to create a three-dimensional computer model of a ground surface. This digital elevation model can be incorporated into a GIS and used to provide an extremely accurate representation of the topography across the landscape.
In order to create a LiDAR dataset, a sensor is placed above the area to be surveyed on an airborne platform, typically an airplane or a helicopter. A pulse of light is fired at the ground, reflects off the surface to be mapped, and returns to the sensor platform. The time between the initial and return pulse is recorded and because the speed of light is a known constant, this time can be converted into a distance. The GPS coordinates of the sensor platform are also recorded and along with this distance and the angle of the beam, the exact three-dimensional real-world coordinates of that point on the ground surface can be recorded. This process is then repeated hundreds of thousands or millions of times across the surface to be mapped. The resulting dataset can be used to create a digital elevation model in a GIS program that represents the ground surface. The absolute resolution of this method (that is, how close any given point is to where it is theoretically supposed to be on the earth’s surface) is under a meter while the relative position of these points with respect to each other (that is, the actual shape of the digital elevation model) is on the order of only a few centimeters. The strengths of this method is that it is a relatively inexpensive way to map extremely vast landscapes relatively quickly with an accuracy and precision that would otherwise be impossible to achieve. The LiDAR pulse is also capable of penetrating leaf cover in forested areas to produce a bare-earth ground model. This is extremely beneficial when mapping areas that are shaded beneath tree-cover like portions of many formal gardens.
Between 2002 and 2006, the state of Maryland commissioned a LiDAR survey of the entire state. This data has been made available to researchers and the public, free of charge through the Department of Natural Resources. In their 2006 article in American Antiquity titled “LiDAR for Archaeological Landscape Analysis: A Case Study of Two Eighteenth-Century Maryland Plantation Sites”, James Harmon, Mark Leone, Stephen Prince, and Marcia Synder successfully show how this LiDAR dataset can be used in the context of an archaeological research project, including the location of large garden features.
The current Wye House was built in the early 1780s for Edward Lloyd IV. Located in Talbot County on the eastern shore of Maryland, it is a seven-part “Roman Country House” and is an example of late Georgian/early Federal architecture. The plantation has been owned by the descendants of Edward Lloyd since 1658. Surrounding the main house is a surviving 18th century formal landscape and North America’s only surviving 18th century greenhouse. Archaeology in Annapolis has been excavating at Wye House since 2005, both in and around the greenhouse and on the Long Green, an area of the plantation where enslaved laborers lived and worked.
One technique used to aid archaeology at Wye House was to import an oblique historic aerial photograph and a tracing of an historic map into a GIS program and georectify them, that is, stretch and warp the images in such a manner so that the photograph essentially becomes a bird’s-eye-view photograph and both are tied to known coordinates on the ground. In this manner, features seen in this early 20th century photograph and historic map but which no longer exist can and have been found in the ground. The same process can be used to locate and characterize landscape features associated with scientific gardening, like the formal gardens at Wye House.
Once this historic aerial photograph is warped and stretched, the features seen in the photograph now correspond to their real-world locations and the gardens seen in the photograph can be traced and mapped. The same can be done with the historic map. The features depicted on the map can be traced and projected onto any other spatial dataset we choose, in this case, LiDAR derived elevation data.
Georectifying this map depicting the core of Wye House plantation and its gardens as well as this early 20th century aerial photograph depicting the same creates a map which allows us to do two things: first, we are able to take this map into the field and use it to locate on the ground features of archaeological interest; and second, we can superimpose topographic data derived from the Maryland LiDAR survey and use it to teach ourselves what the topographic signature of a 18th century formal garden looks like. Armed with this knowledge, we can locate and characterize formal gardens at other plantations which may not have the same historical documentation that was used at Wye House.
At this point, it is important to understand the palimpsestic nature of landscapes and its affect on LiDAR datasets. The metaphor of the palimpsest, a piece of parchment on which the written words are literally scraped off and reused but with the indentations and memory of the previous configuration, is commonly used to describe the manner in which historic landscapes evolve. While landscapes are rarely scraped clean, they are constantly changing and evolving, both physically and culturally while people interact with their components, build new ones, and tear down old ones. So while the most recent configuration of a landscape is both physically and temporarily distant from its previous iterations, it has been shaped by and still retains many of those previous inscriptions if one knows how to look for them. In fact, this is one of the underlying premises of the discipline of archaeology. As such, the LiDAR dataset does not actually record 18th century landscapes or landscape features. Everything seen through LiDAR is a 21st century landscape which may or may not have 18th century antecedents. It is the job of the landscape archaeologist to understand the genealogy of the landscape being studied and determine not just what is old and what is new but also what has been erased.
After the Civil War, the economic basis for the plantation system was radically redefined. As such, those aspects of the landscape created by and supporting slavery became obsolete and a painful memory of the past to be forgotten. Across the South, gardens and homes fell into disrepair and slave quarters were removed from the landscape.
In order to better understand what the topographic variation seen in the digital elevation model, additional research needs to be incorporated into the study. Historic maps and photographs are only two such sources. Fortunately, many of these plantation landscapes have rich documentation that enable researchers to piece together the changes that have occurred, and in doing so, piece together the layers that comprise the palimpsest that are historic landscapes.
Ratcliffe Manor was built in 1747 for James Hollyday and his wife Anna Marie Robins. Consisting of a two and a half story central block and an adjoining wing, Ratcliffe Manor Located just outside Easton in Talbot County, Maryland, as described in the 1930 Homes of the Cavaliers, Ratcliffe Manor “has been fortunate in many things, but in none more so than the disinclination of the three families who have owned it to ‘make improvements.’” It is “virtually the same, that is to say, but with the addition now of…prodigal growth of boxwood which has claimed an entire garden for its own instead of girdling the flowerbeds primly with its green.” In the 82 years since this passage was written, the boxwood at Ratcliffe Manor has all but died, the flowerbeds have turned to grassy lawn, and no trace of the original garden remains to be seen by the casual observer save the formal garden terraces. Because it appears that no substantial human modification to the garden at Ratcliffe has occurred since it was originally designed, the palimpsest that is this landscape appears far easier to read.
The garden at Ratcliffe is comprised of a series of falling terraces. The location of these terraces is obvious and the placement of the path and garden beds or outdoor rooms can be surmised from a basic understanding of 18th century formal garden design. What is not apparent is the configuration of these planting beds. Fortunately, elevation data generated by LiDAR survey reveals features within these spaces which are not visible to the naked eye.
Of interest are the following features identified through the use of LiDAR. 1) Rectilinear bed feature; 2) Use of converging lines; 3) Potential walkway and planting bed; 4) Bed outline and potential garden feature; 5) Potential garden feature; and 6) Potential garden feature.
It should be strongly cautioned that the upper limit of the vertical resolution of the Maryland LiDAR dataset has been reached in attempting to identify these garden features. As such, extreme care needs to be exercised when interpreting these results as random noise can be mistaken as a landscape feature with cultural significance. As argued by Harmon et. al., using a dataset with higher spatial resolution may help alleviate this problem; however, the topographic variation between the landscape features being teased out here is so small that current airborne LiDAR technology is stressed to resolve them.
While no excavations are planned for the garden at Ratcliffe Manor, this methodology could be applied at other plantation sites and serve as the starting point for a project investigating scientific gardening there.
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