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### Re^6: Perl and Solving Trig/Converting GPS to x,y Cords

by BrowserUk (Pope)
 on Jan 09, 2013 at 14:25 UTC ( #1012498=note: print w/replies, xml ) Need Help??

I was unaware of the GPS difficulties,

Even without GPS inaccuracies at those latitudes; extraordinary measures need to be taken in the polar regions anyway.

The same Law of Cosines Spherical Geometry that work just fine for larger distances over most of the Earth's surface are entirely inadequate in polar regions.

For more details, see the GIS FAQ.

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
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Re^7: Perl and Solving Trig/Converting GPS to x,y Cords
by roboticus (Chancellor) on Jan 09, 2013 at 14:41 UTC

Correct, and that's in line with what I'm saying: The law of cosines isn't involved in the conversion from 3D to 2D in all projections--it's inadequate to the task. Further in the FAQ it talks about the Haversine formula which suffers when the points are extremely far apart, rather than being close to the poles. All spherical to planar mappings suffer in one way or other. Without knowing what type of map the OP is using, we can't be certain in exactly which ways it will suck. 8^)

Even when the law of cosines is used, the Z-axis isn't always the the same as either the geographic N/S axis or the magnetic N/S axis. It all depends on the feature(s) the map is trying to portray.

When your only tool is a hammer, all problems look like your thumb.

Given that the area between the +-70° latitudes covers 95% of the Earth's surface and 99.8% of the world's population; and the 0.2% inaccuracy represents a 0.00016 mm on a standard large scale (1::25,000) map -- or, you'd need a map 800 meters wide in order to be able to detect the inaccuracy between the actual and calculated position of a point -- the "general case" is not required for the vast majority of cases.

Hence why I passed along the information that the general case is not usually required, without in any way impugning the accuracy of your post.

I was simply pointing out that unless you run an airline -- and one hopes they know what they are doing -- the simple 2D trig approach to mapping GPS coordinates to map coordinates is usually good enough.

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

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