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Like Ferdinand Hassler, George Davidson was an immigrant. He was born in Nottingham, England, in 1825. His family came to Philadelphia in 1832, seeking a fresh start. In 1841 he enrolled at Central High School, a uniquely ambitious scholarly enterprise founded by Alexander Dallas Bache, a great-grandson of Benjamin Franklin. Bache could spot talent, and Davidson had it. Soon he was employed in Bache's astronomical observatory at the school, and then later in Bache's magnetic observatory, the first in the country.

Davidson was a success, and he seemed bound for a scholarly and sedentary career as an astronomer. But in 1843, Ferdinand Hassler of the Survey of the Coast died in action, tumbling off a hill in New Jersey as he tried to protect one of his instruments during a dramatic lightning storm. After some maneuvering, Bache succeeded Hassler as head of the Survey. He brought his best students with him, including Davidson.

The constraints of the Survey and its work changed Davidson's life. Instead of building a career at an observatory that doesn't move, he soon became the greatest expert in the Coast Survey on how to turn any given spot anywhere into an observatory for the purpose of determining the latitude and longitude of that specific spot. Originally this required two different instruments for finding latitude and longitude. Eventually Davidson combined them into the Davidson Meridian and Equal Altitude Instrument, a legendary telescope. It was small enough and humble enough to be carried by mules on a trail, but it allowed a Point of the Beginning to be established almost anywhere on earth.

The Coast Survey Goes to War
The Coast Survey was a civil scientific agency, but many of its key staff members were politically liberal or even radical. The native-born American members were Abolitionists, while many of the scientifically trained immigrants were social progressives who quickly took up American causes. Davidson and his compatriots worked vigorously along the West Coast during the 1850s, but when the Civil War loomed, the entire organization decamped for the East to assist the Union in the struggle to come.

The basic Union naval strategy was to impose a blockade on the entire coast of the Confederate states, to prevent movement of ships and supplies. Then the Union armies and navy would fight their way up the Mississippi and other strategic rivers.

Bache was a member of the secret committee in charge of the naval blockade. In preparation for the battles ahead, he had commissioned a set of secret documents to be compiled and published, called the Notes on the Coast. The Survey had participated in a long tradition of publishing coast pilots, sailing directions, and notices to mariners. These guides described how to get from here to there. The Notes on the Coast broadened this to encompass what the strategic significance of there was--how there related to railroads and fortifications, for example.

Much of the work that Davidson and his compatriots performed during the Civil War amounted to ordinary practices of coastal mapping performed under extraordinary circumstances. The coast is, famously, where the land meets the water, and the Coast Survey measured and mapped both domains at once. On land they mapped the immediate nearshore environment, always, and then gradually mapped inland, particularly the terrain that was visible from a ship. The network of triangles provided positions for the land features. These were mapped onto t-sheets, "t" standing for topography.

In the water they mapped hydrography, meaning the water above the shape and composition of the bottom. Their two major tools were, first, another set of triangles, between the survey boat and two flags or signals on shore, and second, a lead weight on a hemp line. By triangulation to the signals they could determine the position of the boat, which they then correlated with the depth of the water gauged by the lead line. They entered this data on a map on the boat, the boat sheet. Once back in camp or on the steamship, the data was corrected and cleaned to produce the smooth sheet. The smooth sheets for the water were combined to make h-sheets (for hydrography) and then merged with the t-sheets to create the master for an engraved chart covering water and land. Especially for the war effort, photographic transfers of simplified versions of the engraved charts were produced rapidly and cheaply by lithography. By the end of the war, Coast Survey personnel were the finest cartographers in the country.

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