[gps-talkusers] Re: GPS mathematics
- From: Michael May <MikeMay@xxxxxxxxxxxxxxxx>
- To: gps-talkusers@xxxxxxxxxxxxx
- Date: Thu, 20 May 2004 10:12:46 -0700
John,
You are absolutely correct that longer distances are tricky to calculate.
We have made improvements in this in version 3.
Mike
At 09:41 AM 5/20/2004, you wrote:
>Greetings,
>
> Interesting you should ask this question. I did some distance
>traveling with the BN GPS last week and found some interesting things,
>some of which may very well be related to the mathematics of GPS.
>
> First I should say that I don't have an answer to your question.
>What I do know is that you can't find the distance in an X y coordinate
>system for a latitude and longitude. This is because the earth is not
>flat. Hence, if you are at the equator you'll find that longitude lines
>are a particular distance apart (I don't know the specific distance, but
>if you know the circumference of the earth and you divide accordingly,
>you'll figure it out. Anyway, as you travel north from the equator the
>longitude lines will get closer and closer together. Eventually, they
>all meet at the north pole.
>
> So, in order to compute the distance between two points you need
>to account for the curve of the earth. For short distances,
>particularly in computations near the equator, any error introduced by
>not doing this may be fairly minor. For longer distances, particularly
>as your distance from the equator increases, these errors will be
>dramatic.
>
> So, as to the interesting thing I saw last week.
>
> I was in a car. We traveled from Arlington Massachusetts to
>Stamford Connecticut. On the trip down, the BN GPS measured the
>distance at about 200 miles. I felt this was not accurate. I'd always
>believed the distance to be more like 180 miles. On the return trip,
>taking the same roads, the car clocked the distance at 177 miles. My BN
>GPS crashed about three times on that trip so I couldn't clock the trip
>again.
>
> At one point though, we were driving east on I-90 (the Mass
>pike) when the driver saw a sign saying 26 miles to I-95. From that
>sign, to the I-95 ramp, the BN GPS clocked nearly 36 miles. This is a
>pretty substantial error. Clearly such a substantial error cannot have
>existed throughout the entire 177 mile trip otherwise BN GPS would have
>reported a distance that was 68 miles in error. So I'm wondering if BN
>GPS is more inaccurate when traveling east/west then north/south because
>the earth's curviture is not accurately being accounted for.
>
> Since the Mass Pike is a bit north of Longitude 42N it is nearly
>half way to the pole. Is it possible that BN GPS isn't accurately
>dealing with this fact?
>
>John
>
>-----Original Message-----
>From: gps-talkusers-bounce@xxxxxxxxxxxxx
>[mailto:gps-talkusers-bounce@xxxxxxxxxxxxx] On Behalf Of Matthias
>Weingart
>Sent: Thursday, May 20, 2004 10:47 AM
>To: gps-talkusers@xxxxxxxxxxxxx
>Subject: [gps-talkusers] GPS mathematics
>
>
>Hi,
>
>maybe I am asking in the wrong group (this is a group for GPS users, and
>not developers?), but I would like to know something of the GPS basics.
>In fact I would like to calculate the distance and angle between 2 GPS
>coordinates (or position in a x-y diagram, where one coordinate is
>(0,0)). (starting from the NMEA-string, result should be meters).
>
>I would be very happy, if somebody here could point me to some good
>ressources.
>
> Matthias
Michael G. May
CEO Sendero Group
Developers and distributors of BrailleNote GPS
Now distributing BrailleNote, VoiceNote, Miniguide, The Tissot Silen-T
tactile watch and the ID Mate, bar code reader
MikeMay@xxxxxxxxxxxxxxxx
http://www.SenderoGroup.com
(530) 757-6800, Fax (530) 757-6830, Mobile (530) 304-0007
Sendero Group, LLC
1118 Maple Lane, Davis, CA 95616-1723, USA
Latitude, 38 33 9.239 North
Longitude, 121 45 40.145 West
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