Navigating by true tracks
 

Just a question regarding flying up north:

In the higher lattitudes you are required to fly with reference to TRUE track rather than magnetic (as we all know). In order to do so you are required to switch to a true heading source (ie. DG-mode which disconects the flux valve and allows the gyro to move freely and to no longer compensate for magnetic variation)- are you with me so far?

My question is: what source of reference do you use to set the HSI? Obviously it would be possible to set it to the true tack of the runway once lined-up on center-line prior to take-off, but then how do you compensate for precession and gyro drift once you're in the air? Or, better yet, how do you ensure your HSI is set to the correct track if, for instance, you departed an airport from a more southern airport and then entered the Northen Domestic Airspace and had to set your DG to a true heading?
Anyone have any input or did I make the question too complicated?

Years ago, before GPS, we used to set the reference to Grid North (basically a random direction printed on the chart) and keep it updated from the sun by use of a sextant. I'm not sure I could do this any more!

The B742 used an INS, this system " thought in True " but dispalyed (usually) magnetic. If you flicked the True/mag switch to true, this then annuciated "True" on the HSI and on you went.... easy!

(from memory, you could display variation, which changed rapidly at these Lat's)

You need an inertial heading reference system. Or an astrocompass....!

Although in reality, with a GPS you don't need either, but it is awfully prudent.

BTW, same problem down south too! :E