Saturday, September 03, 2005

Playing with Log files

If you select the Data tab of GPSDash, Pocket Topo! and some other GPS packages you will be able to watch a continuous stream of data sent by your GPS card.

Something that looks like this:

$GPGGA,185621.155,3659.6803,N,12157.7357,W,1,07,1.5,12.9,M,,,,0000*2E $GPGSA,A,3,23,20,16,13,27,04,24,,,,,,2.9,1.5,2.6*38 $GPRMC,185621.155,A,3659.6803,N,12157.7357,W,0.00,,030905,,,A*67 $GPGGA,185622.155,3659.6803,N,12157.7357,W,1,07,1.5,12.9,M,,,,0000*2D $GPGSA,A,3,23,20,16,13,27,04,24,,,,,,2.9,1.5,2.6*38 $GPGSV,3,1,09,23,69,007,32,20,55,178,33,16,45,070,37,13,43,313,34*70 $GPGSV,3,2,09,27,29,251,35,24,18,243,31,04,15,285,31,25,06,058,26*72 $GPGSV,3,3,09,03,05,123,*46

These are NMEA Sentences, the standard proposed by the National Marine Electronics Association and used by GPS devices and vendors to describe geographical positions.

To understand this talk you need to grab a copy of the NMEA 0183 standard specification or check the web for FAQ's.

For example, the first line above


Uses one of the available sentence types GGA (Global Positioning System Fix Data) and if I'm reading this right, it seems to be telling me among other things that:

This measurement was taken at 18:56:21 UTC (Universal Time Coordinates); at latitude 36 degrees 59.680' North and longitude 121 degrees 57.735 West with a GPS Fix obtained from data of 7 satelites, at 12.9 meters above sea level.

The other sentences will give you different types of information. $GPGSV for example will tell you which are the satellites in view and $GPGSA gives you data on GPS DOP (Dillution of Position) and active satellites.

Notice that some GPS receivers can generate log files in a different standard like Sirf, maker of GPS chips.

"Hacking GPS" from Kathie Kingsley-Hughes has among other things a chapter that describes NMEA sentences and the use of VisualGPS.


With a package that supports reading and writing of log files you can track your trips and generate logs with data received from your GPS receiver to replay it later.

Some of these packages provide ways to generate and analise GPS data in NMEA format. Among them you will find GpsDash described in previous posts and VisualGPS with free versions available for the PocketPC and Windows.

VisualGPS CE won't show you a moving map to help you track your current position but it provides textual information about it and detailed status of the satellites data.

As soon as a fix is obtained, start a new log file [Log | Start] by selecting where VGPS should save it. Go about your trail and select Log | Stop when you are done.


You can replay the log file with VisualGPS in the Pocket PC or transfer it to its companion package in a Windows box. Visual GPS has a pretty interesting Survey window that you can use to analise your GPS data.

Check this site for some really cool "drawings" made with data from GPS log files.


A quick way to generate some interesting "drawings" is provided by GPS Visualizer. You upload one or more log files, select the background for your map, output format and the site will generate a combined image of your tracks with image from satellites or USGS maps.

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