eTrex10 north references

Good evening, I don't understand what factors to consider when choosing which north reference to set on my Garmin eTrex10. I will be using this, French IGN TOP25 maps and UTM co ordinates in the Pyrenees later this month. Which north reference should I use? Grid, True or Magnetic?

When walking in the UK and using British National Grid references my eTrex10 is currently set to Grid; is this correct?

I hope someone can help!

Regards,

Henry

 

Yes UTM maps are called grid as they resemble a spread sheet grid with easting's going along the bottom and northing's going up the left. (ie always across and up)

True and Magnetic are only used when using Lat and Long because magnetic of the earth is constantly twisting from true map north (only a small amount each year).
(incidentally they go from the equator north or south and from the Greenwich line east or west ie up or down first then east or west)

Strictly the grid north moves relative to the compass too but the distances travelled are rarely enough to cause an issue.

Kevin H