[LINK] limits of technology in finding someone

rchirgwin at ozemail.com.au rchirgwin at ozemail.com.au
Sat Dec 9 17:32:40 AEDT 2006

Stewart Fist wrote:

>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
>What do people think about the accuracy of GPS -- in the best circumstances,
>and with the best gear.
It's a good point, and one which deserves to be remembered in the 
context of this story.

The guy's GPS may have been quite accurate enough to place his car 
within a few tens of metres. Accurate placement is meaningless if you're 
placed on an inaccurate map ... so it's actually not GPS accuracy but 
the completed units we're discussing.

So what of the maps?
Google Maps, Street-directory.com.au, Whereis, and Multimap all find my 
Google Maps shows a one-way section of Lilyfield road which hasn't 
existed for years, and shows Lilyfield Road crossing the Hawthorn Canal 
(it's closed). It doesn't know about the cross-city tunnel, either.  So 
it's drawn from data which is too outdated to be used as the basis of 
navigation (but hang on, isn't Google the new "best at everything" that 
we all admire?)

Whereis and Multimap don't identify one-way streets online, so just how 
good can the directions be?

Street-directory gets Lilyfield Rd right, and then when I try to 
renavigate, its Website gives the error below my signature.

And yes, Stewart, the general attitude to technology capability is "if 
it's in the press release it must be true" ... I agree entirely. If you 
want a measure of just how credulous people can be who count themselves 
as informed and sceptical, try suggesting that RFID press releases 
overstate the capability of the products ...

Richard Chirgwin


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>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.

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