# How did Henry Kater measure distances down one part in $10^5$?

Wikipedia says that in 1817, Henry Kater was able to measure distances accurately enough to get at least five significant figures in a measurement of $$g,$$ suggesting that he could measure a distances on the order of one meter to an accuracy of one hundredth of a millimeter. Indeed, Wikipedia claims he made this measurement to an accuracy of $$2.5 \;\mathrm{\mu m}.$$

This would be an absolute distance measurement, not detecting changes in distance (as one would do with an interferometer). How did Kater achieve this accuracy?