What datum do you use?

[subzero]

WGS84

The only minor problems with Magnetic Compasses is that their can be local anomolies to the variation showing on the charts. Variation 011° E (01/04), now closer to 12° in our neck of the woods. (If you go online, your GPS Almanac will be updated automaticly if updates are available). Charts are not updated so frequently and do not have the changes built in for the future. The Poles continue to shift.

Your compass will also be affected by other induced magnetic fields such as the metal of your console, radios and other electrical equipment nearby and most frequently Mobile Phones, Transistor Radio and even Keys and Torches being placed beside or near the compass. (Deviation). For the permanent built in errors of your Compass (Deviation), you can go the the compass adjustment bouy up at the mouth of the Brisbane River and "Swing your Compass". This will tell you how far out your compass is on the different points of the compass in relation to what it should be. (Then you can allow for it).

For local navigation, the differences are not that significant, what is important is that if you are going to operate of both paper and gps, use the same method, either True or Magnetic, not a mixture of both. Check that any instructions you give or recieve to someone else, you are using the same method. (I once had the pleasure of watching 2 senior crew deteriorate in to a row over the lack of both of them to agree on a heading when Navigating screens up via Radar, Plotter and GPS/compass only. Navigator was calling true headings of a plotter/gps, helmsman was steering magnetic of a compass.... it was very funny to watch as they didnt twig to what was going on... I had to stop it in the end before it got to heated).

If you use publications such as Beacon to Beacon, any headings are given in Magnetic

At the moment, the difference between True North, and the Magnetic North Pole, are actualy about 560 miles apart.

Here is an interesting article from Garmin on how to use GPS with Paper Charts.
http://www.garmin.com/manuals/UsingaGarminGPSwithPaperLandMaps_Manual.pdf

This subject is a very awkward one, taking 1 part in isolation from the rest, It is something very difficult to do justice to and make it understandable. Hopefully I wont have to get to involved in this and this answers your questions just a little... like someone said earlier, bring back Kerry, this is not my best subject

Cheers Lloyd

[Mantaray]

so does selecting magnetic or true affect the actual gps position?

[subzero]

If you are talking about Magnetic/True taking you on the same heading to the same place, 180 deg Magnetic will take you in a different direction to 180 degrees True.
The position you are stationary at is not dependant on heading, ( True/Magnetic where you are going), it is the specific location of where you are at the time of interogation of the satelites. Two different functions that the GPS performs.

If you are talking about actual navigation using a GPS or paper charts I would have to say yes. The charts do not correct themselves they leave that up to the user and just like the euphinism of annologue on an ordinary radio, the band you will pick them up on is quite wide and drifts, digital is very specific... if you use the compass rose on a chart and a magnetic compass, user error is guaranteed because of variation, deviation, movement whilst planing or checking your course, perception or interpretation of the rose and the compass etc

I am not an expert on GPS so I do stand to be corrected.

Kind Regards
Lloyd