> Hmm, but doesn't the local value of "horizontal" affect the precision > also ? > You would really need to know the direction of local gravity for this > to be any good... An interesting problem. How do you measure the direction of gravity? How far off does it get? With modern equipment, I'd expect something like comparing time with observations of the stars would be a good approach. What did they do before they had atomic clocks? Several years ago at the local USGS open house, a guy had a neat topo map of the SF peninsula. The lines were gravity, magnitude not direction. They were trying to estimate rock density. They measured gravity and corrected for elevation. We asked him how they got the elevation that accurately? He smiled and said from the sewer people. -- The suespammers.org mail server is located in California. So are all my other mailboxes. Please do not send unsolicited bulk e-mail or unsolicited commercial e-mail to my suespammers.org address or any of my other addresses. These are my opinions, not necessarily my employer's. I hate spam.