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.

Address Matching

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:

Existing Sites Depicted on Upham Map
address city zip name Upham no
149 Pine Danvers 01923 Rebecca Nurse House 95
171 Holten Danvers 01923 Sarah Holten House 93
4 Elerton Danvers 01923 Daniel Rea House 48
199 Hobart Danvers 01923 Nathaniel Ingersoll's Ordinary 148
238 Conant Danvers 01923 Bridget Bishop (Edward Bishop)House 68
67 Centre Danvers 01923 Parsonage 149
Essex & North Salem 01970 Jonathan Corwin House 151
Essex & Washington Salem 01970 Meetinghouse of the first church of Salem 160
Forest & Hobart Danvers 01923 Salem Village Meetinghouse (1692) 173
Hale & Dane Beverly 01915 John Hale House 107
Hanson & South Salem 01970 Gallow's Hill 172
Hobart & Centre Danvers 01923 First Church 174
Rt 1 & Rt 62 Danvers 01923 Joseph Putnam House 39

The address matching process resulted in this point coverage:

GNIS Matching

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:

Contemporary Features Identified on Upham's Map
contemporary feature feature Upham site county latitude longitude
Danvers State Hospital hospital Hathorne's Hill Essex 42.58111 -70.975
Humphreys Island island Island in Humphrey's Pond Essex 42.52139 -71.00278
Wills Hill summit Wills Hill Essex 42.59667 -71.02417
Cedar Pond lake Cedar Pond Essex 42.51944 -70.98611
Wenham Lake lake The Pond or Great Pond Essex 42.59306 -70.89306
Whipple Hill summit Thorndike or Whipple Hill Essex 42.57083 -70.95833

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.