Garmin tide data discrepancy

[Erin]

Just curious as to whether anyone else uses the tidal charts on their Garmin.

I use it regularly on the pc at home with Mapsource & Homeport as I know that my boat starts floating when the tide is at 7m above CD and I can then work out the best time to head down. I've found the time calculated by Homeport to have been pretty accurate, perhaps +/-15 mins or so most times. However, and this is a pretty fundamental 'however', the Homeport program uses exactly the same tidal atlas data off my CD yet it gives this weekends figures as being roughly 40 mins earlier for 7m of tide. What confuses me is that both programs agree on the time of high and low tide for Saturday, but they give different readings for the 7m height (which is just over half tide locally). Has anyone else come across this? I presume Homeport must be using a different algorithm or something and bending the curve a bit but I now don't know which one to trust. I guess I would experience the same on my new plotter compared to the old.

Any thoughts?

[karlT]

Yes, use it on the Garmin 2010 Chartplotter regularly to check for 5.5m to scrape in to get fuel at Gorey. Usually accurate to within a few mins.

Are the heights of both high and low the same on both sets of data ??

[BogMonster]

There was an interesting article in RIB mag about tidal ready reckoning - there do seem to be different methods and the advice was not to completely trust any of them!

I use the Firefox extension called Whatsthetide which gives a little onscreen display but I don't think it is very accurate - but it doesn't need to be for me all I need to know is up/down and which way it's going. Never compared it to the Garmin tide stuff on the plotter.

[Smithyyy]

We chartered a yacht for last weekends round the island race - it had a Garmin plotter and the first thing the navigator said was "the tide times from the plotter are different from those taken manually from the Almanac". So yes there seem to be differences, personally I'd go with the Almanac

[Channel Ribs]

Mapsource on the PC shows 17:40 for 7m at St H.

Neptune Tides, also on the PC, shows 17:50.

And the eTrex, using the data from Mapsource, gives 17:20.

[Erin]

Originally Posted by karlT

Are the heights of both high and low the same on both sets of data ??

Yep, times and heights for high and low water are identical, it is just the mid tide data that differs. As I said the strange bit is that the base data comes from the same Garmin 'Atlantic tides v5' CD but the two Garmin programs obviously interpret this differently.

Smithyyy - I agree that the high and low times and heights are slightly different from those in a printed almanac, but in the past I've found the Garmin to be more acurate locally.

I will have to get out my old plotter and see how it compares to the new plotter. And see how the new tidal data on my new map cartridge compares with the old.

[Erin]

Now that's very interesting. Just done a check of the tides using my office PC with Mapsource and it weirdly agrees with the Homeport on my home PC not Mapsource. The only difference between Mapsource at home and work is that I allowed my home version to update itself a little while back. It looks like the update has screwed up the curve data.

Any chance someone could check the 7m height for St Catherines, Jersey on Sat am. I make it about 0506 and 0958 give or take a few mins.

[Channel Ribs]

Originally Posted by Erin

7m height for St Catherines, Jersey on Sat am. I make it about 0506 and 0958 give or take a few mins.

0545 and 1040 with Neptune

0542 and 1043 with Mapsource

Both BST

[Erin]

Have you ever been looking at the tide data for the wrong day? I have
I think I've worked out the error, and it's user not Garmin . When I realised Saturday was the 2nd not the 1st it all started to make sense.

[Channel Ribs]