Is a straight line on google earth or google maps geodesic?

Question

Is a straight line on google earth or google maps geodesic? That is, when I view google earth or google maps data on my 2d computer screen, is the image I am seeing "adjusted" correctly that a straight line on this "adjusted" image is actually geodesic?

If it is not, are there any options that I can choose within these applications which will let me "adjust" the image so that a straight line on this "adjusted" image will be geodesic?

Are there any other free applications out there that do display "adjusted" maps where straight lines on the screen are actually geodesic?

Are there any free applications out there that will provide me with a geodesic path between two points on earth?

Thank you in advance.

Jobbery

Answer 1

First of all it's clear that the lines are not ellipsoidal geodesics. For example, the line between (lat,lon) 0,0 and 0,179.99 in Google Earth is shown along the equator instead of the shorter path going near the pole.

The KML documentation says that the sides of a path are "great circles" and that the sides of a polygon are "lines of constant bearing". I believe that "great circles" become great ellipses (a path along an ellipse whose center is the center of the earth). An ordinary understanding of "lines of constant bearing" is that they are rhumb lines, i.e., that they correspond to straight lines on a Mercator projection. However, the documentation lies. Instead there are straight lines on a plate carree projection.

I have asked Google to use geodesics for both sorts of object, and they are considering this.

I have written a C++ library GeographicLib which solves the geodesic problem accurately. If you want to see how to generate a sequence of points along a geodesic, see the example in the documentation for the GeodesicLine class. If you just want to test the solutions, look at the online geodesic calculator.

Jandy

Thanks CFFK for a detailed reply. – Jobbery

Hi @cffk, you say "... it's clear that the lines are not ellipsoidal geodesics. For example, the line between (lat,lon) 0,0 and 0,179.99 in Google Earth is shown along the equator instead of the shorter path going near the pole." This doesn't follow-- a geodesic need not be a shortest path; it need only be a "locally shortest path"; see en.wikipedia.org/wiki/Geodesic#Introduction. In fact, the path you describe does happen to be a geodesic on the ellipsoid. You're right, though, that "straight lines on a plate carree projection" is the only correct description you've mentioned. – Tapir

Fair enough. Perhaps I need to check what Google Earth thinks of the line between (lat,lon) 0.0001,0 and 0.0001,179.99. – Jandy