[LINK] limits of technology in finding someone

Stewart Fist stewart_fist at optusnet.com.au
Sat Dec 9 15:35:10 AEDT 2006

Roger writes:
> I've asked a couple of people to use GPS to nail down the height
> difference between my house and the trig on the ridge about 250-300
> metres away as the crow flies, and 600 paces in 5 mins 40 secs +/- 10
> secs.

What do people think about the accuracy of GPS -- in the best circumstances,
and with the best gear.

I remember when the USA had dithering on the system (to deliberately reduce
the accuracy to a few hundred metres, maybe even more).  At that time they
boasted that their military accuracy, with no dithering, was a matter of

In fact, I distinctly remember one claim that it was measurable in
millimeters, and that it was more accurate for house-block surveying than
the conventional terrestrial triangulation methods.

The question of exaggerated claims to accuracy in these matters is
interesting because of the claims to millimeter accuracy in measuring the
sea-levels from satellites.  The satellites are perturbed by gravitational
anomolies they pass over, and they are pushed around by solar winds, and
they are flying over an Earth which is spheroidal measuring the distance to
water which has a surface changed by waves, surges, tides, atmospheric
pressure lifts, etc. and land masses which are rising and falling tidally,
as well as tectonically.

I just can't see that they could possibly have accuracies in their satellite
sea-level measurements better than a hand-span or so -- but I've never been
game to say so in print because I don't have any direct experience or

I remember back in the Cuban Missile crisis the US claimed that their
satellites could read the headlines of a newspaper or the number plate of a
car from space.  And most people believed them.

It later turned out that they best photos they could ever produce made the
bodies of the missile look like fuzzy maggots on a sheep carcass
photographed by a box brownie at 100 paces.

I suspect that false claims of accuracy are endemic to the US military, NASA
and to NOAA.

Stewart Fist, writer, journalist, film-maker
70 Middle Harbour Road, LINDFIELD, 2070, NSW, Australia
Ph +61 (2) 9416 7458

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