Does magnetic variation matter when creating a KMZ?

[Pat Macleod]

To some extent this is somewhat academic here in the UK, with magnetic variation at about -0.6 degrees, and reducing, but I've been wondering whether for purposes of complete accuracy we need to take it into account when generating a KMZ. My very brief experiments suggest that if you are exporting a KMZ from OCAD , OCAD handles it, and the map is accurate in Google Earth. On the other hand if you are going via a JPG and the maprun KMZ utility - eg from Open Orienteering Mapper - then it does matter, and you need to rotate the map to grid North before exporting to JPG. Can anyone elaborate or correct me on this?

[Peter Effeney]

I believe that the KMZ file will be correct independent of magnetic variation. That is, a location on the map will have the correct Lat/Lng.

What is dependent on magnetic variation is whether the rectangular outline of the map - and any North lines on the map are parallel to each other.

Orienteering maps traditionally have had magnetic North aligned vertically on the map. When producing a Map in OCAD, OCAD only allows you to export an image or a KMZ in a rectangular box that is aligned with the screen of your computer. So if you don't rotate the the map (including the grid reference system) in OCAD so that magnetic North is aligned vertically, when you export a rectangular map from OCAD - magnetic North will not be vertical on the map. The map would still be valid (unless you've drawn you North lines and North arrow incorrectly).

In Australia where magnetic variation is 11 degrees, if you import map content from systems that are grid-aligned (eg Google Maps), and then draw your North lines and North Arrow, these will be a 11 degrees to the vertical. So to produce an Orienteering Map with these vertical on the page, you rotate everything by 11 degrees. Any rectangular "map" produced from this will then have Magnetic North vertical.

However, KMZ's you produce before rotation and after rotation are equally valid... but will necessarily cover a different patch of the map (as the rectangular export will always be vertical rectangle in OCAD).

As an aside, when we use systems like www.oomap.co.uk in Australia, the map produced is grid-aligned and magnetic North is not actually shown. Anyone checking with a compass, would find that the grid North on the map is 11 degrees off Magnetic North. These maps are typically only used for StreetO and this factor is accepted as "how it is".