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If you've ever wondered why those plastic bushwalking compasses have a rotating bezel, this would be why...
So you can correct for magnetic variation/declination by moving the bearing bezel the appropriate amount left or right as appropriate for the variation in your area & then once applied you can read off the true bearing without having to do any calculations.
Note, variation 'varies' dependant upon your geographic location... there is an area in WA where the variation is zero... Variation also can vary for the same reason over time, it usually progresses in the same direction... ie: if say it was 5* east, it will precess in an easterly direction... so a few years on it may become 6* east & so on...
This is one (among many) very good reason that nautical charts are updated very regularly.. in addition to applying any newly discovered features that may be hazardous to navigation, it also updates magnetic variation to be applied to the area that chart covers. Survey maps are the same... when used professionally, updates are released regularly (nautical charts via monthly Notice to Mariners), don't recall the publication used for survey (land) maps.
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