Invisible wrote:
> The point is *totally* to have a PK that never changes,

Well, yes, that's the desire. But you can only achieve that by having 
meaningless PKs on everything, which I find to be undesirable if a 
meaningful PK can be found.

If your database is likely to be big enough that an incorrect initial PK 
is 
going to be catastrophic, then you can go for meaningless PKs.

However, in Tom's case, the GPS coordinates being wrong means *everything
* 
is wrong for that pole, so changing the GPS coordinates might as well be 

treated as deleting the pole in the wrong place and creating the pole in 
the 
right place.

> OK, well I'm gonna have to say they must be doing something more precis
e 
> than "stand in the general vicinity of the intersection, take down the 

> GPS coordinates, and we'll just use those".

I don't know.

>>> GPS is only accurate to a few hundred yards of course,
>>
>> Uh, no. What are you smoking?
> 
> Hey, the satellites are 20,000 km up in the sky. It's astonishing that 

> GPS can even figure out which city your in, never mind fixing your 
> location to a few hundred yards.

"""
The phase difference error in the normal GPS amounts to 2–3 metre
s (6.6–9.8 
ft) of ambiguity. CPGPS working to within 1% of perfect transition reduce
s 
this error to 3 centimeters (1.2 in) of ambiguity. By eliminating this er
ror 
source, CPGPS coupled with DGPS normally realizes between 20–30 c
entimetres 
(7.9–12 in) of absolute accuracy.
[...]
However, 2 millimeter accuracy requires measuring the total phase—
the number 
of waves times the wavelength plus the fractional wavelength, which requi
res 
specially equipped receivers. This method has many surveying applications
.
"""

> So, what, you pick a GPS coordinate at random that's approximately in 
> the right area? (So long as its unambiguously not near any *other* 
> intersection.)

There's a certain level of error of a couple of meters. You pick a standa
rd 
place to measure from (such as the center of the manhole that you're 
interested in) and call that your location.

When you go back to find the intersection, are the GPS coordinates given 

good enough that someone else can tell what intersection you're talking a
bout?

It should be noted, btw, that they're not really interested in the 
intersections, but in the wires running under the roads. The same PK can 
be 
used to track the wires running under or over not-roads as well.

>> The point of using GPS coordinates is that they are guaranteed not to
>> change.
> 
> Sure. Unless the GPS coordinate system moves, which can't possibly 
> happen. Oh, wait...

Have latitude and longitude changed since we invented RDBMs?  I don't thi
nk so.

-- 
Darren New, San Diego CA, USA (PST)
  "How did he die?"   "He got shot in the hand."
     "That was fatal?"
          "He was holding a live grenade at the time."

Post a reply to this message