This error you saw has been fixed in the next release of the GPS software due out later this summer. Charles. 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 Charles M. La Pierre CTO Sendero Group, LLC Lat. 37 15' 25" N Lon: 121 53' 04" W