Building the Salem Witchcraft GIS
Digitizing the Upham Map
H. Researching Locational Data
Two initial methods were used to locate these features on our base data, obtained from Massachusetts GIS: address matching, and searching the GNIS database.
The process of address geocoding, or address matching, involves comparing a precise postal address (123 Main Street) with a database of streets that contains address ranges for that street (100-199 Main Street). GIS software products take the known address, compare it to the attribute data for street layers, and interpolates a point on the line approximating its location. In this example, the address of 123 Main street would be placed closer to the beginning of the street with a range of 100-199, rather than to the end. An address of 149 would be placed almost exactly in a the middle.
Furthermore, the Census Bureau's TIGER database is not known for it's positional accuracy. The Bureau claims an accuracy of only +/- 167 feet.
Thus address matching is not an exact process, but it provides us with a starting point for further work in locating a building. After identifying these preliminary locations, we then overlaid the data with aerial photographs to find the exact control points for the Upham Map.
The following table shows the sites and addresses that were matched:
The address matching process resulted in this point coverage:
The GNIS system provides access to coordinates of 2 million cultural and physical features in the United States. It is not a perfect resource and, as with address matching, it provides only the starting point to locating more precise data through aerial photographs.
Users can search the GNIS database by the name of the feature, and limit the search by geographic location. Thus we could search for "Cedar Lake" and limit returns only to those features named "Cedar Lake" that are located within Essex County, Massachusetts. The search returns include geographic coordinates that can be converted to points in the GIS system.
After comparing Upham's Map with contemporary maps from the US Geological Survey, we identified these features that appeared on both sources:
There are four ponds in the area known as "Cedar Pond." Thus the returns from the search resulted in 9 points, including three worthless (to us) "Cedar Ponds." These were discarded.
The following image represents all matched points. Here, the point represented by the yellow cross indicates the "Cedar Pond" listed above.