Different datums on your GPS

ssr56220

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Hi first post so go easy if the answer to this is obvious.

On my Garmin GPS i have set the datum to the charts i am using, at present all ED50. What happens when you sail off one of those charts and onto a WGS84 datum chart. I have entered any waypoints from the new chart after changing the datum on the GPS to WGS84. Does it matter what datum is set on the gps when you manualy enter waypoints? And I assume you should you be changing the datum of the GPS to the chart and waypoints you are currently using?

Having written it down now i think i know the answer, but any advice welcome
 

tome

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You need to enter waypoints in their native datum so if using ED50 make sure the Garmin is also set to ED50 before entering them

When you move onto a OSGB36 or WGS84 chart, change the Garmin datum to suit and hey presto you'll see all the waypoints are automatically adjusted
 

Boathook

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Personally, it doesn't bother me. Using the main datums only seems to put the waypoint a 100 yards out plus the inaccuracy of a pencil line whilst bouncing around along with parrallel rules I seem to always be "out"- regardless of datum used. If approaching rocks etc I try to err on the side of caution or slow down even more !!
 

graham

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100yards could be a long way off approaching a harbour entrance in fog. OK you SHOULD be not relying only on one method of navigation but....
 

Signed Out

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I'd be surprised if any recent UK chart is not WGS84, but if it is, OK, just enter the coords in the datum of the chart and change datum of GPS as you go, as mentioned above.

However... I'd be truly impressed if anyone spends the time it takes to plot their position on a chart from the GPS as they get towards a harbour in fog. Personally, I think I'd rather be out at the helm, watching out and listening.
 

jimbaerselman

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[ QUOTE ]
I'd be truly impressed if anyone spends the time it takes to plot their position on a chart from the GPS as they get towards a harbour in fog. Personally, I think I'd rather be out at the helm, watching out and listening.

[/ QUOTE ] Plotting on a chart, agreed. But at this stage one would be navigating by way points using the GPS screen . . . whether hand held or viewable from the wheel.

And the way points would have been entered from the chart (or a list), so the datums used must be corrected to be the same between GPS and chart.
 

Oldhand

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I keep a list of all my waypoints with 2 sets of co-ordinates, WGS 84 and native datum of the chart they are plotted from, if not WGS 84. This requires using a datum change calculator of some sort or the data offsets on the chart, if provided. I then enter all my waypoints into whatever in WGS 84 so that my GPS datum can always be left on WGS 84. I found it to easy to forget to change GPS datum when necassary and the above procedure overcomes this.

There is an excellent Dutch free download program called PCTrans, which does very accurate datum and other geodetic transformations.
 

tome

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Be careful using PCTrans or similar software, especially if applying 7 parameter shifts. There are 2 conventions in use for rotation - left and right hand. The signs of the rotation parameters have to be reversed to switch between the 2

I have never seen datum shifts qualified with which convention is assumed, and it is one of the most common errors we see in surveys. To prevent these errors, we issue a set of control co-ordinates within the survey area in both datums and test our proposed transformations using these co-ordinates before applying them to any uncontrolled waypoints/positions

The errors resulting from a sign reversal can be large
 

Slow_boat

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[ QUOTE ]
Try 400 yards datum difference and tell us it really doesn't matter

[/ QUOTE ]

Okay. Ahem.

IT DOESN'T REALLY MATTER

Using a hand bearing compass for a three point fix (remember them?) in a lumpy sea I would be happy with 400yds accuracy and assume to be in the most dangerous position in the cocked hat.

400yd accuracy on a running fix (even less will remember them) when a couple of miles off are normal.

Entering harbour is pilotage, not navigation and I'm not going to be doing it in fog anyway, GPS or not!

I also have mk 1 eyeball, ears, echo sounder and log.

I use GPS to check the accuracy of other methods of navigation/pilotage but I know I'm old fashioned/over cautious/reckless (delete as appropriate)
 

Signed Out

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Agree with Duncan here, entering a harbour is definately a case of eyes and ears, and if foggy, avoid if possible. Wouldn't use GPS to go in at all.
 

tome

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Few of us would. But if you have a navaid as a backup with a potential accuracy of less than 10m, why risk a 400m degradation just because you have it set up incorrectly? The mind boggles!
 

Boathook

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400 yards I was take a lot of care but have found in the past 100 yards to be the norm with different datum on charts. As someone has commented I prefer to see items with the eye rather than totally rely on GPS. There is also the depth sounder but around Christchurch it just flashes shallow water !!
 
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