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[aprssig] APRS resolution

Keith VE7GDH ve7gdh at rac.ca
Mon Oct 1 21:36:53 UTC 2007


Tapio OH2KKU wrote...

> There's a difference between accuracy and resolution...

More eloquent than my "it doesn't bother me if anyone wants to transmit with
extra precision" when I should really have been saying "it doesn't bother me
if anyone wants to transmit more resolution"!

> Another thing entirely is whether typical uses of APRS need more
> resolution than two decimals of minute. Most probably don't...

My thoughts exactly. I know that some of the SAR types (and I am ex SAR)
will say "more precision can indicate which side of the river I'm on" but it
still comes down to the accuracy of the position as indicated by the GPS
receiver. More resolution doesn't necessarily mean more accuracy. I think
that 2 decimals of minute is "good enough" for the average APRS user. If
someone is reporting a position either verbally or via APRS, if there is an
insurmountable obstacle like a raging river or a deep ravine right beside
the position, and others had to navigate to that position, it would be more
than worthwhile to indicate to them "which side of the river or ravine" they
were located on.

The average APRS user might argue that "more precision" (or as you
pointed out, really more resolution... not necessarily more precision)
could make a difference if dispatching an ambulance to a motor vehicle
accident on a divided highway. Again, any verbal report in that instance
should include which side of the divided highway the accident took place on.

If someone has access to a differential beacon, or they on a hilltop with a
dozen satellites in view and a good WAAS signal instead of in a canyon, it's
great if the higher resolution is permitted. I just think that for average
users, it isn't needed and is meaningless. I only had a GPS receiver for
about the last 3-4 years that I was involved with SAR. Even with SA
(selective availability for the new guys) with a possible 100 metre error
plus errors introduced by the GPS receiver itself, having any kind of a
position provided by the GPS was a huge step up from trying to
determine a location with just a map and compass with poor visibility!

73 es cul - Keith VE7GDH
--
"I may be lost, but I know exactly where I am!"





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