does WW use an ellipsoid?

[kcchesnut]

i'm using Proj.4 to reproject tiles onto WW.
they match up perfect for longitude, but latitude is a bit off (the further you get from the equator).
north of the equator, the tiles become a little too high,
and south of the equator, the tiles become a little too low.

when i call, Proj.4 i'm telling it to project to ellips=WGS84.
so i'm wondering if WW uses a similar ellipsoid, or if its mesh is a pure sphere?
else, is there a different ellipsoid parameter i should be passing to Proj.4?

Thanks,
casey

[mdsumner]

It's a sphere:

http://forum.worldwindcentral.com/index....28&hl=ellipsoid

(Not sure it's going to matter, but . . .) You can use PROJ.4 to project to a sphere using ellps=sphere and specify the radius and eccentricity (squared):

E.g. (better check for the right radius)

+proj=longlat +ellps=sphere +a=6370997.0 +es=0.0

Please let me know how you go, I'm just starting to get my head around using PROJ.4.

See the PROJ.4 documentation for more info: OF90-284.pdf

http://proj.maptools.org/

[kcchesnut]

Thanks a ton, that worked perfectly.
After the projection, everything lines up [img]style_emoticons/<#EMO_DIR#>/smile.gif[/img]

Many people cannot understand how and why the earth is a sphere. You will never fall off the edge of the world. I want to provide some explanations to it and hopefully that my one will help.

Just think in this way. If the whole earth is really flat then it must stand on something beneath it, there must be an edge and there must be something beyong the edge of earth. But as you know that you can always travel in one direction without coming to an end, just like the concept of infinity, therefore you can only conclude that the earth is a sphere as there is no such thing as an end on the surface of a sphere if you travel on it. No other shapes can exist. Then many of you would think that the earth might be a boundless flat land, but it wouldn't be a boundless flat land as you know from science that all matters besides space or vacuum take up shapes and volumes and so the concept of boundless doesn't exist on earth nor any other planet. Therefore the earth must have a shape and volume and the only conclusion is that it must be a sphere.

This will make you even harder to imagine. Just as you think that when each of the two person travel in two opposite directions they will get further and further away and will never meet each other again but on a sphere but it does as been proved. I'm not a real cosmologist and the only thing that I can think of is that the surface of anything can only be concave or convex if not 180 degrees flat. For example, when you look onto the computer screen you see it as a flat surface but as you look closer you actually see a bit of concavity rather than 180 degrees flat. Unlike the surface of the table which is manufactured to be 180 degrees.

Just say that you have travelled a 3700km from Sydney to Bali on earth. You actually have travelled an arc already if you see from the globe model. but if you travel by Train from Sydney to Darwin and then catch ship from Darwin to Bali you apparently feel that you travelled on a flat surface. This difference suggests that what people feel is not exactly right. The concept of flat surface only refers to 180 degrees between two different points on a surface according to expert and no surface can exactly be 180 degrees unless measured, therefore it must be concave or convex as mentioned before. If the surface on earth is not 180 degrees then it must be concaved in some way, and if it is concaved then it must form a arc which eventually form a circle which is the sphere.