[Proj] Re: "Double ellipsoid" case?
dagnew at ucsd.edu
Tue Dec 2 12:38:47 EST 2008
I've been following the debate about Google Earth (GE) and Google Maps,
wanted to contribute that I've seen the same kind of misfit between
geodetic-grade GPS coordinates and those from GE (for points I've
so can locate on the photos): a few meters, up to about 10 m--and higher
elevations do tend to have larger errors. I'd agree that for a small
of this error can be removed by a local x-y shift.
But I am baffled by the discussion about what kind of projection GE
and especially the aspersions cast by several participants. There seem
three different issues:
1. What *coordinates* does GE produce? The answer would seem to be,
coordinates in a global datum, presumably something close to WGS-84 (in
various incarnations) or ITRF (in its). That is, what you get from GE
same as what you will get from a GPS receiver. Viewing GE as a device
producing pairs of numbers from where you put the cursor, the ellipsoid
irrelevant: there is just some kind of algorithm from pixels to
which is probably done by local interpolation. Registration of the
much more important than anything else. Whether or not the WGS84
"fits" the projection used to make a rendering is beside the point when
comes to lat and long, since there is no projection. Whether that
"fits" the Earth (or a part thereof) is even more beside the point, for
2. What shape is used for making the oblique views? Clearly a sphere,
and orthographic, is fine for GE's purely visual presentation.
3. What algorithm is used to determine distance when using the ruler
That is, how is distance found from two pairs of coordinates? I don't
again would say that using a sphere would be fine for the accuracy that
should expect from a viewing tool: for casual applications, an error
of 1 in 300 isn't going to be an issue. Note that the projection used
the map on the screen could be Mercator on the sphere, while the tools
use an ellipsoidal algorithm. Does anyone know what is used?
I think the same ideas apply to OSM; any inadequacies of their rendering
engine (which, for a Web application, seem minor) need not affect the
quality of the underlying data.
Finally, the Mercator on the sphere is exactly conformal, as is the
on an ellipsoid, as defined mathematically. This result, coming from
differential geometry, is independent of the eccentricity.
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