Consistent linear scale is only possible with large scale (small extent) maps (or maps of worlds that do not have complex curvature). A map of a large continent or whole world is going to have distances distorted, though how it is distorted varies with the projection. The Equidistant Cylindrical projection for instance preserves distances along meridians, but distorts distances in all other directions. At the poles you have a single point stretched out the the width of the entire map.Originally Posted by Dark Huntress
So, if you want to put a meaningful scale indicator on a map, it needs to be a map of a small extent, and it needs to be in an appropriate projection for that extent.
Similarly, a Compass Rose or Rhumb lines imply that bearings (Compass directions) are preserved. Again, this is possible for small extents, if the projections is centred on that extent, and for the very specific Normal Mercator projection, under which it is true for the entire map (Although areas are massively distorted) If bearings are not preserved, then you shouldn't use a compass rose. A simple north arrow can be used to indicate that North is preserved: many projections preserve North without preserving Northeast for instance. In some cases, even north can't be preserved: polar projections put a pole at the centre, so north is either toward or away from the centre.
The final major component of a map that many people treat as decorative, but which has specific meaning, is the graticule. This is the grid of latitude and longitude lines. This has the most information in it and so is the most complex to deal with. Simply using a grid of squares implies you are using the Platee Carre projection (A special case of the Equidistant Cylindrical projection) This is simple, but the distortion takes the form of some very ugly looking stretching near the poles. Drawing a map in this projection that lacks such stretching means the map would look pinched if projected on a globe.
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