Thank you all for your responses to my GPS query... ********************* My problem:
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. ********************* I had a number of responses that has since started me looking in all the right places if nothing else. Jon and Peters responses summarise well the issues I am up against... Thanks to Peter Zyczynski for this, just what I was looking for: ********************* 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.kh-gps.de/nmea-faq.htm> http://www.commlinx.com.au/NMEA_sentences.htm <http://www.commlinx.com.au/NMEA_sentences.htm> http://www.nmea.org/ <http://www.nmea.org/> ********************* Thanks to Jon Gramm There are many possible reasons for this to occur. Different equipment being used. One data set being corrected an the other not being corrected (Differentially or RTK) If the data was corrected differentially it is possible that the reference station coordinates were input incorrectly. Different postprocessing software with differing datum transfomation parameters. A Molodenski transformation routine will yield different results from a Bursa-Wolff transformation routine etc. The key to isolating the discrepancies is to reprocess all of the data with the same processing package then check for the range of errors. I am not sure if there is a string in the NMEA code that denotes whether or not the data is being corrected differentially or in Real Time Kinematic. Other things that could contribute to the differences would be Single frequency receivers versus Dual frequency receivers. IF a static network was run for one network then how was it adjusted and which points were held fixed, the list goes on and on. ********************* Many thanks to you all. David This email and any attached files are confidential and copyright protected. If you are not the addressee, any dissemination of this communication is strictly prohibited. Unless otherwise expressly agreed in writing, nothing stated in this communication shall be legally binding. --------------------------------------------------------------------- List hosting provided by Directions Magazine | www.directionsmag.com | To unsubscribe, e-mail: [EMAIL PROTECTED] For additional commands, e-mail: [EMAIL PROTECTED] Message number: 8958
