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Thanks to both of you, Paul Hirose and Richard Langley for giving us this very interesting piece of information: the Earth local gravity field irregularities account[ed] for most (about 95 %) of the final error (2*10**-4 in relative value) of the Mètre determination by Delambre and Méchain.

In other words - and as I understand it - at both ends of their measured Dunkerque-Perpignan segment covering about 10° in Latitude, the cumulated local N/S vertical deflections - unknown and unsuspected then - resulted in a "computed/processed Latitude difference" numerically exceeding by about 0.1' (6 arc seconds) its true value known to-day.

To say it differently, they - unknowingly - "fit" an excess of 1' (60 ") over the Pole-Equator distance, almost 2 km over 10,000 km. As a consequence their final "gauge physical determination" - i.e. the Mètre - turned out to be too short by about 2/10,000 by comparison to its intended definition.

No way for Delambre and Méchain to know the reason of this discrepancy then.

Paul Hirose referenced article also shows that the actual "quality" of their "overall processed measurements" was about 20 times better, i.e. about 10**-5 in relative value: quite an achievement indeed.

Kermit