Thousands of Westchester County’s earliest deeds used trees, stone piles, and streams to mark property lines. As Westchester County developed, and the property within it increased in value, the need to more properly determine boundary lines was clear. The solution was a county-wide Control Survey, conducted ca. 1934-1941, with “chaining crews” working in the field, the construction of “Bilby” towers, and human “computers” doing thousands of mathematical calculations.

The Control Survey’s 1939 report provides a short history of the project, while also detailing, with words and photographs, the work of the office and field crews, and explaining the triangulation process used to establish the horizontal grid of elevations throughout the county resulting from the survey, which continue today to serve as “known points” to surveyors mapping property. The report also lists every individual from the county involved in the Control Survey. In addition to this 1939 report, the Westchester County Archives holds additional photographs from the Control Survey as well as some of the original calculation records.