Georeferencing a 1892 Sanborn Map of Contoocook, NH

While researching historical maps I came across the treasure trove of Sanborn maps that are in the public domain and have been digitaized by the Library of Congress. The Sanborn Map Company (a company that still exists today) produced detailed fire insurance maps of cities and towns across the United States. The earliest available maps are from the late-1860s although the majority come from the 1880s through the 1950s.

Contoocook is a small town located in the Town of Hopkinton in Merrimack County, New Hampshire. It has a covered railroad bridge that was first built in the mid-1800s and is a historic building that is included in the National Register of Historic Places.

As a small town with a historic past and, like many towns in New England, a center where many original houses and buildings remain, it is an interesting case study for georeferencing a historic map to compare with the present day.

The first step was to take the historic map and georeference it, which means assigning geographic coordinates to points on the Sanborn map. I used satellite imagery from Google to identify the reference points. The result is in the first map below.

The next step in the process was to digitize the features on the map (seen in the map below). This creates independant features which can be named and assigned their own data, and it allows the removal of the historic map for a comparison of the historic structures to the present day town.

The map below is the final result. In this map we can see that the areas along the river and around the railroad station have changed the most significantly. Most of the railroad buildings have been removed or replaced with the exception of the passenger station. Along the river, the mill buildings and shops are almost entirely all gone.

The georeferenced map is quite accurate as can be seen by the paths of the roads and railroad tracks. The path of the old railroad tracks are especially interesting to note as their former route can still be seen in the satellite imagery even if the tracks are no longer there. To the east a slight, curving gap can be seen in the trees, and to the northwest there are noticeable differences in the concrete paving where the tracks were likely removed and paved over.

The covered railroad bridge can be seen but the wooden bridge for the road has been replaced. Many present day buildings in the town match up closely to the historic map although there are clearly buildings that have been rebuilt with a different footprint on the same plot of land.

There are a few key things to point out about slight inaccuracies in the alignment of the digitized structures and their present day counterparts. There are several possible reasons for these discrepancies including: the satellite imagery being at a slight angle; slight measuring or scale errors in the hand drawn historic map; a difference in the projection (CRS) used in the satellite and historic maps; or buildings being demolished and rebuilt on the same site. Further investigation would be needed to identify the exact reasons.

What appears to be an inaccuracy in the north arrow on the historical map is actually a difference in orientation. The historical map is oriented towards magentic north as opposed to true north.

To conclude, Sanborn maps offer a detailed vision of the way cities and towns in the US used to be and when overlaid onto a present day map or satellite imagery offer an engaging way to see what remains, what is new (or at least newer) and what glimpses of the past we can see in the landscape even when things have changed and the original structures no longer remain.