As one of my more ridiculous spare-time endeavours, I am tinkering around
with historic (1870, 1893, and 1908) maps of Dana, Massachusetts. I'm
trying to overlay scans of the maps onto a modern coordinate system so, for
example, they could be viewed in Google Earth. Under ordinary circumstances,
I could simply match up features like roads and rivers, which don't change
much over time. But in this case I want to rely heavily on latitude and
longitude because much of the land around Dana is gone.
Most of you who have picked up a USGS topographical map are probably familiar with NAD83 coordinates. They are similar to the WGS84 coordinates used by GPS systems—but WGS84 coordinates are always on the move due to measurement error and (believe it or not) plate tectonics. The NAD83 coodinate system is linked to the North American plate and defined with reference to metal surveyor’s benchmarks planted in the ground all over the continent.
Between 1927 and 1983, the standard datum was NAD27. Now I knew that NAD27 was based on a simple elliptical model of the Earth, and that the entire continent had to be surveyed by hand in order to figure it out. But until tonight I did not know that for 56 years the geodetic center of the Earth was defined as… Meades Ranch, Kansas. (Apparently the ranch is a nerd-tourism spot. With permission from the owners you can take pictures of the benchmarks set into two rocks at the center of the world.)
Back to my question: what was the datum in use prior to 1927 when my maps were drawn? I still haven’t found the answer to my question. Perhaps the accuracy of surveying at the time makes it a moot point. I’ll probably have correlate my maps by brute force.
Now about the land. If you haven’t been to Dana, Massachusetts before, I’m hardly surprised. It doesn’t exist. The town was disincorporated, dismantled, and shipped away in 1938 when the Swift River was dammed to create the Quabbin Reservoir. Today the area is closed to traffic and filled with about 412 billion gallons of water (155 million gallons of which flow into Boston’s faucets every day). That’s not to say you can’t visit Dana today—but that will have to be the subject of a future post.