Date:         Fri, 4 Aug 2006 12:09:22 -0600
Reply-To:     Gregor Brandt <gbrandt@MARIONETTE.CA>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Gregor Brandt <gbrandt@MARIONETTE.CA>
Subject:      Re: Reasonable cruising speed?
Comments: To: Loren Busch <starwagen@GMAIL.COM>
In-Reply-To:  <86476e250608040859m92add5bobaf054c27a37787f@mail.gmail.com>
Content-type: text/plain; charset=US-ASCII; delsp=yes; format=flowed

RE: GPS

The GPS is highly accurate on speed and distance.  The problem is
that the GPS samples at discrete intervals, typically once per
second.   This means that if you are going around a corner at any
speed, the GPS will not see this as a curve but as a set of points.
To calculate distance the GPS draws straight lines between these
points and add up those distances.  This causes it to be off and
lower than most odometers (that are reasonable accurate).

Gregor


On 4-Aug-06, at 9:59 AM, Loren Busch wrote:

> Interesting point and one that was discussed quite a bit a couple
> of years
> ago.  Assuming you are correct then that would say that GPS
> indicated speed
> is only correct when traveling a straight road.  And what does that
> say
> about the ground track trace most, even older GPS units, store?
> And what of
> the newer GPS units , especially those designed for use in
> vehicle?  Do they
> provide more accurate data, or, like my older hand held unit
> (Magellan 4000)
> give bad info on the odometer function?
> BTW, not trying to get an argument going here, just really curious
> about
> other peoples real world experience with hand helds vs. GPS units
> sold for
> use in vehicles vs. laptop software/GPS combinations.
> Guess I'll have to set up both the handheld and the laptop and run
> them
> together for a hundred miles or so and then compare.
>
> On 8/4/06, Ken Lake <ken.lake@alereon.com> wrote:
>>
>> RE: GPS distance.
>>
>> This is because the GPS calculates the straight line distance between
>> the starting and ending points.  Although it calculates its exact
>> location several times a second and uses that to determine its
>> speed, it
>> does not store the incremental distances between locations, which it
>> would need to calculate total distance.  Hence it can plot your trip,
>> give you the speed, current location, but distance from starting is
>> always wrong (compared to how you got there)
>>
>>
>>