There's been talk on the Fedi about why online maps like OSM and Google use the Mercator projection, and I think it needs a bit of clarification.
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There's been talk on the Fedi about why online maps like OSM and Google use the Mercator projection, and I think it needs a bit of clarification. (Do NOT snitch tag.)
Online maps are _mostly_ used at small scales, of a district or a city. At these scales, the earth is approximately flat, so users expect a map that is _conformal_, one that preserves angles and shapes.
Online maps are continuously zoomable from continents down to your house. They usually aren't rendered on your browser straight from vectors, but are batch-rendered offline into square tile images. If you're on a slowish Internet connection, you can see them loading individually.
As you zoom an online map, tiles get replaced with higher-resolution ones, and as you scroll, tiles get filled in to fill the gaps. You could theoretically scroll and zoom forever, so the map needs to not have a magically special reference latitude or longitude. These constrains mean that the map needs to be _equatorial_, so that no point gets special treatment, and _cylindrical_, so that meridians are vertical and lines of latitude are horizontal.
And which is the map projection that is all of equatorial, cylindrical and conformal? Mercator!
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Mercator's world map of 1569 was useful to navigators at the time because it is conformal (preserving angles) even over long distances. You could draw a line between source and destination, read off the constant bearing, and point your ship in that direction for days or weeks, and you'd arrive at your destination. This line is called a "rhumb line", or (and I love this word) a "loxodrome".
Rhumb lines are absolutely not the shortest route between two points. That would be a great circle route, but computing a great circle requires spherical trigonometry, which is something that navigators were loath to mess with (as your bearing changes over the course of traversing a great circle, so it requires constant attention). Great circles have certainly been used by navigators for a long time, particularly in aviation where carrying your fuel is a factor. Now of course, ubiquitous computing and GPS makes great circles easy, and rhumb lines are just a historical curiosity.
But because of the properties of Mercator being coincidentally convenient for online maps, we find ourselves back in the 16th Century with a default projection that is frankly bad at large scales.
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Interestingly, I see that Apple Maps (on iOS and macOS) wraps its tiles onto a sphere, and plots place names and other text live and undistorted onto the viewport. Google and OSM web are the old guard, steadfast in using Mercator.
Edit: And Google Maps has a checkbox to enable sphere projection too, TIL.
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G gustavinobevilacqua@mastodon.cisti.org shared this topic on
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@futzle I think that marble ( marble.kde.org/ ) wraps pre-rendered tiles from various sources onto a sphere without rendering the names separately and at times the effects are interesting, but then that's what you get when you use tiles that were supposed to be used for small scales
(it also offers vector maps rendered on the fly, and stuff like historical maps)
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@valhalla Someone alerted me to marble's existence a couple of hours ago. Very neat.
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@futzle yeah, that post arrived to my instance just after I had finished writing mine :D
