"Blunkett u-turn on data privacy plans"

[Roland Perry]

In message <1165408936-6078119@espace.net>, Fearghas McKay 
<fm-lists@st-kilda.org> writes
>> >Don't forget specialised GPS receiving
>> >stations are used to track minute movements (down to the order of 100
>> >microns) in e.g. earthquake research.
>>
>>I have never heard of such a thing. Most claim even differential GPS is
>>only good to a metre.
>
>it is differential movement that is being measured not absolute
>positioning. You have two antenna and you measure where thwy are in
>relation to each other. Similar tricks have been used on ships for years as
>it allows the replacement of gyroscopes for measuring roll and yaw.

So what you are saying is that you put two normal GPS receivers a short 
distance apart and measure the difference between their positions, that 
difference increasing slightly if a fissure develops in the ground 
between them?

And you can detect a difference of 100 microns? Surely that would 
require each receiver to have a readout significant to 100 microns, even 
assuming they drifted and wobbled absolutely in time with one another.
-- 
Roland Perry