Re: GPS speed indications

Larry G wrote:
On Sep 6, 4:27 am, Nate Nagel <njna...@xxxxxxxxxxxx> wrote:
Rich Piehl wrote:
Larry Sheldon wrote:
This is a pure question without preconceived notions as to what the
answers might be.
What I think I know is that the unit-in-hand locates satellites and
does timings to determine where it is in three dimensions.
What I don't know is how often they do that.
Seems like if the interval is very long it would not know about
altitude changes, ziggy-zaggity twisty roads and while it would know
position accurately it might not know exact distance traveled between
fixes and thence would not be able to calculate speed accurately.
I don't exactly how often it updates. But I was watching my unit on two
lane roads and freeways where the posted speed limit changes. My unit
displays the road speed limit and dead on where the the posted speed
limit was my unit showed the speed limit. This occurred numerous times.
The only way it could know I was passing that point is by updating
every half second or so.
Additionally my unit shows the speed I'm driving. As I adjust my speed
the unit adjusts my speed almost immediately.
I'm guessing it updates its data 3-4 times per second
Hmm, that's one thing I was disappointed in with my Garmin - the speed
limit display is very inaccurate - e.g. near my house it displays 25 MPH
when the posted is actually 35, same thing happens near my work, it
displays 40 when it's really 45... yes, I updated the maps as soon as I
had the opportunity, it was useless until I did so as there are so many
new roads and reconfigured interchanges around here - especially in
areas that I'd really rely on it (places I'd have to go for work, that
I'm not really familiar with)

ALL map and other location-specific data is out of date and can (and
does) take several update cycles to get current info.

Part of this is because the DATA in GPS units comes from third-party
suppliers and so they have their own update cycle - which is
invariable not synced with the GPS units that are being released -
even new ones.

At some point.. GPS units will morph into "connected" units and
updates will be done similarly to the way that computer updates are
done... but we're not there yet.

Right, I understand that, I was just pointing out that the speed limit data is more out of date and/or less reliable than the basic map itself, at least in my area. If it's out of date, and not simply inaccurate, I don't know how out of date it is because I've lived here for several years now, and don't remember ever seeing a 25 MPH speed limit sign in the location I mention above, for example.

I'm not really complaining, because it's the navigational data that is really crucially important, but it would be nice if I had the same faith in the speed limit data, because it has the potential to be a handy feature.


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