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Comment by nickdothutton | Hacker News Reader

I thought everyone learned the Mercator use case in school? It is for marine navigation. A line of constant bearing will always appear straight on it. If you want to see the relative size of continents that's a different use case and you need a different projection.

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Never knew that and millions are so.

Why?

Because it is not used just for marine navigation. It is mostly used for everything related to maps and apps with maps.

Apps with maps use a specific and different Mercator projection which produces a nice quadtree for dynamically loading data. This is possible with other projections too, but reprojection and training users to expect a different looking map is expensive.

Almost nobody plots courses of constant bearing on a map. Almost nobody uses a map to measure area either.

99% of the use cases for maps these days are for local navigation where artifacts are minor. The differences in map projection are basically irrelevant to users in these cases.

Measuring area is a different use case to looking at the world map to get a sense of relative sizes - of continents, countries, regions, etc.

This in turns influences how you think about history, population density, travel times, etc.

But what if I'm looking at a world map to get a sense of where different cities fall on different latitude lines to gauge their climate?

Sure that’s a valid use case, 100%.

A line of constant bearing will always be a great circle. A straight line on a mercator projection of the earth does not constitute a great circle (in fact only the equator line is a great circle and each vertical line is half of a great circle).

> A line of constant bearing will always be a great circle.

That's not right. Imagine heading due West, and keeping your compass pointing West every step anywhere not on the equator. That traces out a longitude line, not a great circle.

If you follow a constant compass heading you will spiral toward the poles in a helix, not travel in a circle around the globe. To travel along a great circle, your compass bearing must constantly change as you move.

Seems I got terms confused. Thanks for clearing that up.

Spend some time in Flight Sims :-)

I mean even for marine navigation, you use digital solutions now. But paper charts people would use before aren’t exactly zoomed out to show the whole world. So I feel like this whole world Mercator projection hasn’t been very relevant for a long time, even for navigation

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