Thread: [Gpsbabel-misc] Fwd: Realtime tracking altitude value consistently low
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From: Maarten S. <maa...@xs...> - 2008-12-23 23:00:22
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Hi, I noticed that this went to me directly, and not to the list. I have not useful answer here. Maarten Begin forwarded message: > From: "Bill Monicher" <bma...@gm...> > Date: 22 december 2008 00:41:47 GMT+01:00 > To: "Maarten Sneep" <maa...@xs...> > Subject: Re: [Gpsbabel-misc] Realtime tracking altitude value > consistently low > > Hmm, > More to this than meets the eye. > I'm sure it is not related to a map, because as I sit here, the > altitude on the GPS display varies 5 or 6 meters. The displayed value > correlates well with the value on the topo map. When I connect this > unit via usb, and use the following command: > > cls & gpsbabel -T -i garmin -f usb: -o xcsv,style=tcapture.style -F > con: > > I get a line similar to this: > 49° 10' 30.951" N 122° 39' 3.299" W 9.250700951 > 2008-12-21 08:06:51 2008-12-21T08:06:51Z 39803.33809028 > > The relevant fields of tcapture.style are: > IFIELD LAT_HUMAN_READABLE,"","%dø %d' %5.3f" %c" > IFIELD LON_HUMAN_READABLE,"","%dø %d' %5.3f" %c" > IFIELD ALT_METERS, "", "%12.9f" #altitude in meters > IFIELD GMT_TIME, "", "%Y-%m-%d %H:%M:%S" > IFIELD ISO_TIME, "", "%s" > IFIELD EXCEL_TIME, "", "%14.8f" > > The altitude displayed on the GPS unit at this time was 26m > > Another program (gpsutility http://www.gpsu.co.uk/) has a GUI that > shows me that the Altitude is the same as on the display. > Just now, when I ran it again to be sure, I noticed this line: "Geoid > sep= 17.4m" > > I ran real-time tracking using > cls & gpsbabel -T -i garmin -f usb: -o nmea -F out.nmea > > Here is an excerpt from out.nmea: > $GPRMC,230949.001,A,4910.515,N,12239.056,W,1.94,174.83,211208,,*1D > $GPGGA,230949.001,4910.515,N,12239.056,W,1,00,0.0,0.621,M,0.0,M,,*7A > $GPVTG,174.829,T,0,M,1.944,N,3.600,K*5C > $GPGSA,A,3,,,,,,,,,,,,,0.0,0.0,0.0*32 > > In the altitude section of $GPGGA we find: > 0.621,M,0.0,M > According to http://www.gpsinformation.org/dale/nmea.htm#RMC > the second value is "Height of geoid (mean sea level) above WGS84 > ellipsoid" > > I'm going to assume that the data is there, since the other program > can read it. > If that is the case then the Garmin serial/USB protocol might not be > reading it by design, or it might be mis-handling it. > My GPS might be doing something non standard too. > > Is there a facility in GPSBabel real time tracking to capture the raw > output of the connection? > If so, I might be able to determine if my unit is actually putting out > the correct information. > > --BM > > On Sun, Dec 21, 2008 at 1:26 AM, Maarten Sneep <maa...@xs... > > wrote: > >> >> The only thing I can think of is that the GPS uses information from a >> map you have loaded to find the local altitude of the geolocation. >> >> Maarten >> > |