Gnomonic vs Mercator — which straight line is shortest?
Straightness belongs to a projection, not a route
On Mercator, a rhumb line is straight: it crosses every meridian at a constant angle. On Gnomonic, a great circle is straight: the shortest route over a sphere. Neither map is lying; each answers a different question.
Why does Gnomonic straighten great circles?
Points are projected from Earth's center onto a tangent plane. Every great circle also lies in a plane through Earth's center, and that plane meets the map in a straight line.
Which is larger in reality: Africa or Greenland?
The price is severe. Points 90° from the center run to infinity, so our tool automatically centers the view on the selected route's midpoint.
Try Warsaw–New York
Open the route lab. The great circle is straight on Gnomonic. Switch to Mercator: it bends north while the straight rhumb line becomes the longer route.
How navigators combined both maps
A shortest route could be planned on a gnomonic chart, divided into waypoints, and transferred to Mercator. Great-circle geometry then met the practical convenience of compass bearings.
Sources: D3's projection geometry documentation and the USGS manual Map Projections — A Working Manual.
Turn the idea into a click
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