Why online maps stop before the poles

A map made from reusable squares

Google’s Maps JavaScript API documentation describes world, pixel and tile coordinates derived from latitude and longitude through Mercator. At each zoom level the map is divided into tiles. The browser can request the pieces needed for the current view instead of receiving a new image of the entire planet.

The tiles make panning and zooming feel like moving over one continuous canvas. This is convenient for interactive maps, but it does not make the projection equal-area. You still need to account for its scale when comparing country polygons.

来源: Google Maps · Map and tile coordinates

The pole would be infinitely far away on the page

In spherical Mercator, the projected vertical coordinate grows without a finite bound as latitude approaches 90°. A usable square world therefore needs a cutoff before the poles. The familiar square tile scheme reaches approximately 85.05° north and south rather than ±90°.

The projection forces the display to omit a polar strip even though land can extend beyond that cutoff. To see the pole in a finite image, use another projection or a globe. A larger browser window cannot solve an infinite-coordinate problem.

来源: PROJ · Mercator equations · Google Maps · Map and tile coordinates

Zoom fixes readability, not global area distortion

Zooming in gives roads, labels and small shapes more screen space. It does not change Mercator into an equal-area projection. At a fixed location the local scale changes with zoom; the latitude-dependent differences across a large world view remain part of the projection.

A close-up street map can still be useful. To compare areas across the world, though, you need to check how the display scales places at different latitudes. A readable map may still be unsuitable for that comparison.

Why does the world repeat when you pan sideways?

The left and right edges of the square world tiles meet at the same longitude. When the world repeats during sideways panning, the map is displaying another copy of the connected tile grid. It is the same land drawn again to continue a curved world across a rectangular screen.

The map cannot continue toward the poles in the same way. Longitude wraps around the globe, while Mercator’s vertical coordinate grows without limit at a pole. That is why the left and right edges can join, but the top and bottom edges stop before reaching 90 degrees.

来源: Google Maps · Map and tile coordinates · PROJ · Mercator equations

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