Aside from all its other uses: the telegraph gave a way to synchronize clocks. And accurate time is accurate measurement of distance.
> [...] The latest determination in 1892 is due to the cooperation of the McGill College Observatory at Montreal, Canada, with the Greenwich Observatory. [...] The final value for the longitude of the Harvard Observatory at Cambridge, as adjusted in June, 1897, is 4h 44m 31s.046 ±0s.048.
-- https://adsabs.harvard.edu/full/1897AJ.....18...25S
71.12936 W; give or take about 2 metres: https://www.bing.com/maps/?v=2&cp=42.38148%7E-71.12936&style...