Chartplotter discrepancy

[mriley]

I have an Eagle chartplotter which admittedly is a few years old, but seems to work OK. However I am noticing a discrepancy. After a trip, the trip distance will show, say, 29 nm, but the difference between the odometer reading at the start and the finish will be 33nm.

Can anyone explain this, and which is likely to be the more accurate?

Thanks (in advance).

 

[lustyd]

Over ground vs through the water maybe?
Satellite gps will be over ground as it’s where the boat has been. Speed sensor will be through the water as it’s how far the boat travelled through water. Tides and currents will make these different since against tide you’re effectively on a treadmill.
Plotters can often display both.

 

[mriley]

Sorry, should have made it clear, the odometer reading is the total mileage showing on the chartplotter. I have both total and trip distances showing on the chartplotter screen, it is between these figures that the discrepancy is (ie after a trip the trip mileage is showing 29nm but the odometer has gone up by 33nm).

 

[jbweston]

It could still be a discrepancy between the distance over ground by GPS and the distance through water from your log (if the log is networked to the plotter). If the trip is from one input and the total distance from the other they usually will have a difference.

I have exactly this issue because my log is seriously under-reading and the plotter can show distances based on either or both inputs - GPS and log - so the difference at present is large.

 

[Porthandbuoy]

Could be the trip calc has a low speed cut off, ie. anything less than, say 0.5kts, is not incremented.

 

[Dellquay13]

Could be the trip calc has a low speed cut off, ie. anything less than, say 0.5kts, is not incremented.

So all the time repeatedly drift fishing isn’t on the trip but is on the total…