Garmin Gps Ii Plus Set-up

[+The Red Squirrels]

We're just a litle concerned about the accuracy of our Garmin GPS II Plus and wonder if we have it configured properly having just marked a known location and checked it against Memory Map but there seems to be a discrepency.

 

Under SYSTEM MENU > NAVIGATION of our there are a number of options for HEADING; Auto Mag W005 / True / Grid E000 / User Mag E000. Could someone please confirm which we should use?

 

We currently have DATUM set to WGS84 and CDI SCALE set to +/-0.25. We believe these are correct.

 

Something else which is confusing us is using CARROCK FELL as an example, the cache sheet gives the co-ordinates as (N 54° 41.654 W 003° 00.254) NY 35371 33704 but converting the latter on our GPS (and by switching modes on Memory Map) we're given an actual BNG of NY 35370 33709. Can anyone explain this discrepency please?

[+Fangsy]

I think that the discrepancy in the BNG coordinates is due to the fact that the BNG uses a different datum from WGS and the particularly complicated maths needed to convert from one to another isnt 'perfect'. So while your GPS may convert the raw positional data into a BNG co-ord with a small discrepancy, and the system used to convert from WGS to BNG on the web page also has a small discrepancy it is possible that the two variations add up to one larger one.

 

I've also found that forcing my GPS to use the WGS datum while displaying BNG also causes a similar error.

[+The Red Squirrels]

Thanks Fangsy. Helps to know it wasn't us doing something wrong.

From our experiements today it seems to matter which way round the inputting is done as the discrepency on the ground can be quite a few metres different.

 

Do most cachers input decimal degrees rather than BNG?

 

Despite all this, we're still unsure how we should have our GPS set up as per our question above.

[+Snosrap]

We're just a litle concerned about the accuracy of our Garmin GPS II Plus and wonder if we have it configured properly having just marked a known location and checked it against Memory Map but there seems to be a discrepency.

 

Under SYSTEM MENU > NAVIGATION of our there are a number of options for HEADING; Auto Mag W005 / True / Grid E000 / User Mag E000. Could someone please confirm which we should use?

 

We currently have DATUM set to WGS84 and CDI SCALE set to +/-0.25. We believe these are correct.

 

Something else which is confusing us is using CARROCK FELL as an example, the cache sheet gives the co-ordinates as (N 54° 41.654 W 003° 00.254) NY 35371 33704 but converting the latter on our GPS (and by switching modes on Memory Map) we're given an actual BNG of NY 35370 33709. Can anyone explain this discrepency please?

For all geocaching use it's best to keep the gps set to display "DD MM.MMM" and essential to have the datum set to WGS84. I only switch it to BNG in order to plot the waypoints on to an OS map and then switch straight back.

 

The Heading setting is only relevant to bearings (ie the heading you need to follow to get to the next waypoint) and relates to how the unit displays these. There are three different "norths": True, Grid and Magnetic. If you plot bearings on a map then set to GridE000 (although I'm not sure that this is valid for OS maps - it may be a US map standard). For geocaching it is probably best to have it set to Auto mag as this will calculate and display the bearings (to the next waypoint) relative to the Magnetic Pole which means you can follow these bearings without adjustment using a compass.

 

I'm afraid this is a bit rushed as I'm dashing off to work but I hope it helps and no doubt someone else will be able to give a more detailed answer.