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Frequently Asked Questions:

1) What if my data cross the dateline?

The answer depends on what you're trying to do with your data. If your data don't cross the Prime Meridian (0 degrees longitude), then you can re-normalize longitudes from [-180,180] to [0,360]. If it crosses both the Prime Meridian and the dateline, you may be able to duplicate data at the edges, so that search routines will still work at the edges (you'll need to know ahead of time how far your searches will expect to go and still find neighbors).

The general solution to this problem is in the within_mapx function in the mapx library.

2) Why are the poles such a problem?

The Earth is round and the poles are where all that roundness comes into focus. The so-called “polar problem” is actually due to an equatorial bias. The poles are not a special case, the poles are the general case.  If you work with a flat Earth the poles will be a problem. If you work with a round Earth all those problems disappear.

For more about working with the round Earth, take a look at the spheres package.

3) What’s the best interpolation method for my data?
Drop-in-the-bucket, weighted averaging, cubic convolution, inverse-distance squared, etc... A lot depends on the data. For more information on interpolation techniques check the Interpolating Data section of this site.
4) Wouldn’t it be nice if the Earth were flat?
Indeed, that would be nice. The geometry would certainly be a lot easier. But then people might fall off the edge. So maybe it’s better the way it is.
5) What’s the difference between a map and an image?
An image is just a picture. A map also has associated direction and distance. There is a known, well-defined mathematical relationship between points on a map and locations on the Earth. For example, if you take an aerial photograph, it's just an image until you establish the relationship between points in the image and points on the ground.
6) Did they build Greenwich there on purpose?
Yes they did. It’s a very nice location for an observatory. If you're wondering whether they built the observatory on the Prime Meridian, well, no, the observatory was there first, and later they defined the Prime Meridian to be where the observatory was. For more information, do a google search on "History of the Royal Observatory, Greenwich".

7) How can I avoid all this trigonometry?
You could get some software that correctly handles the trigonometry for you, then you wouldn’t have to deal with it. For dealing with the trigonometry associated with map projections and coordinate transforms you may be able to use the mapx library. For dealing with the the geometry of the sphere, and the associated messy spherical trigonometry, you may be able to use the spheres package.
8) What’s up with the “unprojected” projection?
We don’t know, and we wish people would stop using that name. There are so many other names that could be used instead. Anything in the list below is preferable.
9) Why are orbits such a weird shape?
Orbits aren’t a weird shape. If you think orbits have a weird shape you’re probably looking at a picture of an orbit on a flat Earth. The Earth is round. On a round Earth orbits have a terribly ordinary, even boring, shape. The circular orbits of satellites gathering Earth Science data are especially boring, and consequently quite easy to work with. <shameless plug>The Backtrack Orbit Search Algorithm takes advantage of this to perform spatial searches of orbital data faster, better, and cheaper than other methods.</shameless plug>

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