Hi David, No idea as to why you'd be getting this sort of error.
But to answer your other question, yes you can check whether or not the position has been differentially corrected if you have access to the raw NMEA messages. If you look through your NMEA log file you will find three or more sentences for every position captured. The typical ones are $GPGGA, $GPGSA, $GPRMC, $GPGSV, but there are many more depending on your grade of GPS. These hold a coma separated list of parameters. The one which holds differential correction data is $GPGGA and the parameters you are interested in would be 6, 13, 14. ---------------------------------------- Element Description Typical Data ---------------------------------------- 0 Message type $GPGGA 1 UTC Time 023215.000 2 Lat 4253.1475 3 Lat Indicator S 4 Long 14719.4391 5 Long Indicator E 6 * Fix Status 1 (see below) 7 Num Satellites 05 8 HDOP 5.4 9 Altitude 10.6 10 Altitude Units M 11 Geoidal Sep 12 Geoidal Sep Units 13 Diff Correction Age 14 Diff Correction Station ID 15 Checksum *0F ---------------------------------------- * Fix status: 0 - Invalid fix 1 - Valid GPS fix 2 - Valid GPS fix in differential mode ---------------------------------------- If you really want to get into the nitty gritty of NMEA, these links could be of use: http://www.kh-gps.de/nmea-faq.htm http://www.commlinx.com.au/NMEA_sentences.htm http://www.nmea.org/ Cheers, Peter Zyczynski -----Original Message----- From: Eagle, David A [mailto:[EMAIL PROTECTED] Sent: Tuesday, 28 October 2003 9:29 PM To: [EMAIL PROTECTED] Subject: MI-L Off topic GPS Question - Scotland This is a vague question because I have very vague information at the moment and its posed at those of you who deal with GPS surveying...The surveys I am questioning are in Scotland, Uk. Situation: A vehicle survey with GPS tracking its position has been done in the Central Belt of Scotland (Glasgow, Edinburgh region) and the returned results when converted from Lat/Long to BNG were spot on. A similar survey further north near Dundee produced converted BNG coords with a southerly 10-15m error. Then moving further north still to a point near Aberdeen the coords are out to the south by 15-20m. The errors are consistent and seem progressive as you move north. My question to the masses is, how would you explain this and what would you do about it? I don't expect miracles as I haven't provided much technical detail... I am inclined to believe some sort of calibration may have been done in the south and not altered for the two northerly surveys...is there any way of checking in the NMEA message string whether the data is being corrected differentially? Any thoughts greatly appreciated...sorry about the sketchy info, its being fed to me slowly. Best Regards, David --------------------------------------------------------------------- List hosting provided by Directions Magazine | www.directionsmag.com | To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] Message number: 8919
