Change in Lat and Long from GPS to Plotter

[yoda]

I am currently taking an NMEA feed from my Garmin 12 and using it to provide position data on my laptop which is running Maptech. The strange thing is that the positions are showing as slightly different between the two machines but the boat location on the screen seems perfect. Is this something to do with the charts used by maptech or something else? Both machines are set up for the same chart datum.

Yoda

 

[discovery2]

Just a thought -
Is the Garmin & laptop both set to the same datum?
I understand that WGS84 is the most common datum for electronic charts.
I experienced a similar problem when the plotter was set to the Euroean1950 datum and indicated the boat was berthed in the car park and not in the marina!

This may, or may not help!

 

[nigelhudson]

One may be displaying decimals and the other minutes and seconds.

 

[Stoaty]

One may be updating the display faster than the other, giving the impression they are not in sync. I use maptech and have a furuno GPS. It took a lot of careful watching and rapid eye movements before I worked out what was happening.

 

[Bilgediver]

One may be displaying decimals and the other minutes and seconds.

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Yep this is the usual reason......just set both to read decimal or both to read minutes and seconds..

 

[wooslehunter]

How slightly is slightly? What are you actually seeing? If it's very small, there may be a limitation in resolution in the data that's sent out over NMEA. I assume that there's some way to view the NMEA sentence in Maptech. If the NMEA data is truncated, the G12 may show a higher resolution position than Maptech & hence appear as an error.

Either way the NMEA monitor should give you a clue. You should see a $GPRMC........ or some other sentence. You can decode these sentences manually by looking the format up on the web.

If the data's getting through, then I'd look to see how Maptech interprets it.

If on the other hand it's 100s of meters, it'll be something else but at least you can check the data.

 

[Danny]

I reckon David probably has the right answer. The GPS may be set to display position in some other datum than WGS84 (possibly OSGB36?), but it'll still output the coordinates in WGS84 in its NMEA sentences. The actual position would, of course, be the same in both cases.

 

[brianrunyard]

The original post stated that both were set to the same datum.
More likely one displaying seconds and one displaying decimals.

 

[yoda]

That sounds like it, will check at the weekend. Thanks very much.

yoda

 

[Danny]

My apologies Brian (and Yoda). You're right - and I thought I'd read the whole post carefully! Guess the brain cell switched off before the last sentence. I plead age related insanity.