Excuse the use of map inserts rather than screenshots, I haven't got much time at the moment.
Look at an old Ordnance survey map of the uk and you'll see it's covered by a thin, blue lined grid. On recent map systems such as Google Maps and OSM there is no grid. My discussion today argues that we should bring the grids back.
Grids No Longer Necessary? To locate yourself on an old map such as OS a grid allowed you to use grid references. Web maps produce points at the right place without such a grid so because a grid adds to the visual clutter of a map, we're best rid of them?
Landmarks and Scale: A grid is helpful because it gives a sense of scale to the map. If you're looking at part of the antarctic
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you have no sense of the size of what you're looking at. Fix a grid of 1km spacing over the top and you can immediately judge distances.
Point Landmarks. To locate yourself on a map your brain looks for known features and fixes your position in relation to this point. Here in London that means that Londoners often describe an area in relation to underground (=metro) stations because that's how most of us get around. The problem with this is that in tests it's found that people tend to position points that they remember by a landmark too close to the landmark. So if my house was 1 mile away from Clapham South underground station if I was tested I'd tend to draw it closer, say 3/4 of a mile.
Line Landmarks Another way we locate points is in relation to line features such as a major road or coastline. This has problems too. Consider the question, which is further west of these UK cities: Glasgow or Cardiff?
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Glasgow is towards top of map in Scotland. Cardiff is due west of London and Bristol, you may have to zoom out or in a little
Without looking at a map most Brits think that Cardiff is further west. That's because they have a mental map in which the East coast of the UK runs north south whereas it actually runs NNW-SSE and they warp space to accomodate this misconception.
Grid Solution Because a grid runs exactly north-south and east-west it gets over the Glasgow-Cardiff problem. Also, it minimises the underground station problem as it represents a continuous set of landmarks rather than discrete points (although this point is worthy of more testing to prove this is the case). So a grid can deliver both a sense of scale and a landmark system which allows people to fix positions on a map and remember their positions correctly.
When to use Grids: I suspect that its more useful to use a grid in an area without a strong set of landmarks (desert, tundra) rather than somewhere with good landmarks, if you put a grid over US cities with their right angled street systems I imagine users would ignore the map grid because the street system provides such a good system of landmarks already.