Michel
Joined: 10 Jan 2006
Posts: 966
Location: Norway
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 Posted: Thu Mar 16, 2006 12:20 pm Post subject: relative vs. absolute position WAS: GPS Elevation Readings |
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On Mar 16, 2006, at 8:08 PM, JeffFowler(at)aol.com wrote:
| Quote: | They are measuring the shift of the Earth's plates with it for
earthquake
studies.
|
Amazing, isn't it, Jeff?
Yet we have to differentiate between absolute and relative positioning.
Until the satellites came to exist, maps were drawn from positions
derived solely from a sextant and a chronometer. Each country has its
own co-ordinates standard. With the GPS came the WGS-84 standard. To
adapt to satellite navigation, maritime charts have now a correction
table, sometimes up to 200 yards offset - as I experienced here, in
Norway.
Before the GPS, I sailed with a Decca receiver, a British technology
invented for the D-day, in 1944. While not being too accurate in
absolute position (i.e. the latitude and longitude) it was accurate
within a few yards to repetition (relative position), which enabled me
e.g. to put fishing pots at sea, then come back the day later, and pick
them up, from my previously plotted position, even in the thickest fog.
Likewise, if you use a reference point, the GPS can put you within
maybe less than a yard from where you were previously. This technology
is used e.g. by the Norwegian snow ploughs working in the mountains,
keeping the passes between the east and west country open at all time.
But they have first to drive all the way, on a nice day, plot their
track, and come back when it snows and everything is whited out. Note
that they also use differential GPS, i.e. a correction from a known
place is constantly transmitted.
Cheers,
Michel
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