Is a TomTom accurate on a marine chart?

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HenrikL

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Hi everyone!

I hope to be able to pick the brains of some of you experts in this forum - and I would like to think there is a simple answer to my question.... I just don't know it myself!

I am going on a sailing holiday in the South Ionian Sea in Greece and would like to bring my TomTom 730 just to verify my position (Lat and Long) and plot it in the chart.

The chart I wiil be using in the Ionian is using the WGS84 datum and as far as I have been able to gather from other posts so is the TomTom.

Is that correct, and does that mean I should be OK using the TomTom accurately on that chart?
I know the rest is then down to how accurate the chart itself is! :rolleyes:

Regards,
Henrik.
 
Should work fine.

I've 'hacked' an old car GPS (ViaMichelin running WinCE 5.0) to run OziExplorerCE for use as a mini chart plotter and it works. All you'll be doing is just using the GPS part to give you lat and long. It'll be WGS84 I'm sure - but you can check it against the chart when you get on the boat and, assuming you tell exactly where said boat is, see if it shows the right position on the chart.
 
We are in the Ionian and had a good laugh at a guy who had a nokia phone with a "plotter" app on it, it showed his position on the wrong side of an the island and inland. So I would save your money and buy a chart IF accuracy is what you're after.
 
Marsupial said:
We are in the Ionian and had a good laugh at a guy who had a nokia phone with a "plotter" app on it, it showed his position on the wrong side of an the island and inland. So I would save your money and buy a chart IF accuracy is what you're after.
Click to expand...

You've misread the OP - he only wants it to give him Lat & Long, like using a Garmin 72 or such like, he'll then plot that on a paper chart.
 
HenrikL said:
Hi everyone!

I hope to be able to pick the brains of some of you experts in this forum - and I would like to think there is a simple answer to my question.... I just don't know it myself!

I am going on a sailing holiday in the South Ionian Sea in Greece and would like to bring my TomTom 730 just to verify my position (Lat and Long) and plot it in the chart.

The chart I wiil be using in the Ionian is using the WGS84 datum and as far as I have been able to gather from other posts so is the TomTom.

Is that correct, and does that mean I should be OK using the TomTom accurately on that chart?
I know the rest is then down to how accurate the chart itself is! :rolleyes:

Regards,
Henrik.
Click to expand...
Henrik
Yes, even road nav TomTom's use WGS84 as the datum, as this is the datum that the satellites use, and hence all sat navs, for their lat long. (Some gps and apps can be adjusted to use different datums for different onboard charts etc, but) basically your TomTom 730 will use WGS84, so just take the lat long and use on your WGS84 chart, and you'll be fine. If you use non WGS84 charts, then you'll need some way of offsetting the difference, as the 730 probably won't be able to provide lat long with different datums.
 
Not sure if this is that relevant but may help. I have a TomTom Bluetooth "puck" (originally for use with Pocket PC and TomTom) that I occassionally use as a back up to 2 other GPS inputs (fixed Furuno GP32 and Bluenext Dongle) when running both MaxSea and Open CPN and have never had a discrepancy between any of the inputs.

tomtom-bluetooth-gps.jpg


Shouldn't be a problem IMHO.
 
as I said, all satellite based satnav use WGS84 as standard, and only a few specialist (eg marine gps units) allow you to change the datum used, but they still use WGS84 as input and output before it's varied
 
The GPS in it is likely to give you just as accurate Lat & Long readings at sea as it does on land. The problem in some areas is the charts themselves. They may be to WGS84, but most charts where I sail in Turkey are based on 19th century surveys, and can be as much as half a mile out in places. I can correct my position on my chartplotter when in harbour, for example in the small harbour at Bozburan, and the chartplotter will show the boat as being either on land or well out to sea when I tie up to jetty in the next bay.
 
Muddy Paws said:
You've misread the OP - he only wants it to give him Lat & Long, like using a Garmin 72 or such like, he'll then plot that on a paper chart.
Click to expand...

No you're not getting it, as Norman E says GPS AND a Chart out here is as good as a choco teapot. I have a set of waypoints and routes that have been constructed by traveling the route and marking it as I go - no chart involved in as much as there is no correlation between the GPS and the charts - because they dont match up! BUT by using the GPS ONLY I can find the same position on the earths surface time and again - just like using DECCA back in the dark ages.

The point is we need the chart, compass and the eyeball and daylight to do the trip in first place, we cant trust the "electronics" on its own and at night - in places we have not been before.

AND another thing, two days ago my upto date chart (paper) suggested we had 25mtrs of water - we had less than 10 - the electronic one had no data for that area - so navigation out here is more "hands on", nothing like the well documented UK shores. In some bays we use a lead line off the bow as we approach very slowly. - I knew it would come in handy one day.
 
GPS interference

Marsupial said:
No you're not getting it, as Norman E says GPS AND a Chart out here is as good as a choco teapot. I have a set of waypoints and routes that have been constructed by traveling the route and marking it as I go - no chart involved in as much as there is no correlation between the GPS and the charts - because they dont match up! BUT by using the GPS ONLY I can find the same position on the earths surface time and again - just like using DECCA back in the dark ages.

The point is we need the chart, compass and the eyeball and daylight to do the trip in first place, we cant trust the "electronics" on its own and at night - in places we have not been before.

AND another thing, two days ago my upto date chart (paper) suggested we had 25mtrs of water - we had less than 10 - the electronic one had no data for that area - so navigation out here is more "hands on", nothing like the well documented UK shores. In some bays we use a lead line off the bow as we approach very slowly. - I knew it would come in handy one day.
Click to expand...

As stated, the charts can be less than accurate (and do say so on them).

Another issue is the fact that just off Gaios on Paxos there is a reef and my GPS/depth gauge have frequently gone down in 50+m of water for no apparent reason. Very disconcerting...
 
BrendanS said:
Henrik
Yes, even road nav TomTom's use WGS84 as the datum, as this is the datum that the satellites use, and hence all sat navs, for their lat long. (Some gps and apps can be adjusted to use different datums for different onboard charts etc, but) basically your TomTom 730 will use WGS84, so just take the lat long and use on your WGS84 chart, and you'll be fine. If you use non WGS84 charts, then you'll need some way of offsetting the difference, as the 730 probably won't be able to provide lat long with different datums.
Click to expand...

I think you are wrong Brendan but its a bit of pedantry on my part. The satellite signal is basically a time signal. The GSP converts this into a position on the geoid and its the gps that does that calc in terms of WGS84 or another datum . In other words the lat and long numbers he gets out of his GPS will vary according to what datum there is in the program in the TomTom..
 
TOM TOMs in the distance.

We took my wife's tom tom with us up the thames, as a trial and also to amuse/occupy the minds of grandchildren and wife. Navigation was done visually of course but it was spot on, as much as I could see. The instructions as to where to go to get to St katherines dock was hilarious.
 
paulm299 said:
I have a via Michelin X970T which I cannot get updated maps for . Would you care to share the hack :) ?
Paul
Click to expand...
Mine's an X960 but I'm fairly sure that the principle will be the same. I'll put some info together and PM you later. BTW it still gets used in SWMBO's car. The hack is non-destructive as far as the original function of the GPS is concerned.
 
Thanks for all your replies!

It seems there are things to watch out for.
I will bring it along and see how it works out, and checking it at a number of known positions to assess its accuracy is a good idea!
It will not be used as primary navigation tools - my eyeballs Mk.I with chart and common sense will be used for that!

Best regards,
Henrik.
 
Bosun Higgs said:
I think you are wrong Brendan but its a bit of pedantry on my part. The satellite signal is basically a time signal. The GSP converts this into a position on the geoid and its the gps that does that calc in terms of WGS84 or another datum . In other words the lat and long numbers he gets out of his GPS will vary according to what datum there is in the program in the TomTom..
Click to expand...

You can be as pedantic as you like, but WGS 84 is the reference coordinate system used by the Global Positioning System, and satnavs natively deliver output in WGS84 and convert to others on the fly. So lat/long will nearly always be to WGS84, unless some on the fly conversion has been enabled.

He asked a simple question, and it has a simple answer, without needing lots of little technical points to be raised.
 
If you are using Admiralty charts just make sure that you read the note on the chart that tells you how enormous the error is for positions plotted from GPS data. We has a flotilla holiday a few years back and some of the charts stated errors of up to 1.5 nm.
 
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