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I have two points defined: $A$ and $B$

For both I know $x,y$, longitude, and latitude (gps coordinates). How do I calculate $x,y$ of a third point $C$ when I know its longitude and latitude?

I know this is very basic but I cannot get my head around it at the moment.

Edit: I want a basic math proportion calculation, without taking into consideration Earth's surface and how coordinates is calculated.

Edit: I think I figured it out. (Can't answer my own question yet) for $x$: $$\frac{|lon_A-lon_B|}{|lon_C-lon_A|} = \frac{|x_A-x_B|}{|x_C-x_A|}$$

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    I think the question as stated doesn't make sense. Somewhere behind the question there's a function from the Earth's surface to a Cartesian co-ordinate system, but if all we know about that function is two of its values, there's no way to calculate a third value. It's a little like asking, find the third number in the sequence 4, 8, x. Unless you know something about the rule behind the scenes, there's not much you can say.2011-06-22
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    I am new here, also considerably new to latex. Can anybody fix the equation in the question? I think it validates in latex but does not show here.2011-06-22
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    @rok: I hope that's what you wanted. You need to use only a `$` at the beginning of a formula and at the very end of it, so e.g. `$\frac{x_1}{y_1} = 2^3$` gives $\displaystyle\frac{x_1}{y_1} = 2^3$2011-06-22
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    @rok, if all you want is a proportion calculation, then you've done it correctly. And I think you can see how to do it for $y$ as well.2011-06-22
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    This is valid in a small region. Particularly if the latitude covers a wide range, this will not be accurate as the circumference of the earth varies with latitude. Wide range of longitude is not a problem.2011-06-22
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    Just curious: any suggestions about how this should be tagged? equations doesn't seem to capture the problem at hand...I've seen a number of lat/lon problems, etc, I'm just now sure what the "niche/tag" for such questions...finding coordinate? Geometry?2011-06-22
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    @Ross, it's perfectly valid, given that rok is only interested in doing an interpolation and doesn't care about whether it corresponds to anything in real life (which is the only way I can interpret rok's first edit).2011-06-23
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    @Ross, @gerry-myerson: Thanks for the help, I was only asking for interpolation. Plus, I am only doing this in town scale so results are close to being accurate. After all this is not intended for a science project :D Thanks a lot2011-06-24
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    Well, GeoCaching is becoming more and more popular as portable GPS devices are reasonably priced. That may have something to do with the rise in popularity. How could we pass up an opportunity to teach all and sundry a little bit spherical geometry/trig!2011-07-05

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