Tangential, but one thing I recently learned about telegraphy was that it was one of the final tools, for establishing an accurate global map. Taking your long

Tangential, but one thing I recently learned about telegraphy was that it was one of the final tools, for establishing an accurate global map. Taking your longitude requires knowing what time it is. An hour's error is 15 degrees of rotation or more than a thousand kilometres. Just one second error is several hundred metres. Before the telegraph there was no standard time, not as we use it today, where it's the same second in New York and London. And synchronizing clocks used to be very difficult. In the 1850s, just before the cable, the US Coast and Geodetic Survey, would routinely ship chronometers between London and Boston every few weeks, to keep the American time standard synced with London, so that American ships could use the London-centric longitude definitions. And of course, this was back when clocks weren't terribly accurate, and a quality chronometer might lose or gain a few seconds a month. In the 1840s, American surveyors started using long-distance telegraph links to make longitude measurements in different locations at the same time. The distance between the coasts was established within a few hundred metres by the late 19th century. Some more reading: https://adsabs.harvard.edu/full/1897AJ.....18...25S https://adsabs.harvard.edu/full/1897AJ.....18...25S "The telegraphic longitude net of the United State and its connection with that of Europe, as developed by the Coast and Geodetic Survey between 1866 and 1896"