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I recently asked a question about a coordinate system in a file, and found out that it used SWEREF99TM, but I can't figure out how to convert those coordinates to the "geographic coordinate system". I've found a bit about it on lantmateriet.se. It says that:

It is an advantage for many different applications at the national level if the whole Sweden can be mapped seamlessly, i.e. on one projection with position given in one co-ordinate system. A rigid application of, for example, UTM in Sweden would require a division into three zones.

The definition of a dedicated projection based on a meridian 15° E of Greenwich, with a scale factor of 0.9996, with the Equator as the origin for N co-ordinates and with E co-ordinates measured from the central meridian and increasing eastwards (the central meridian is assigned a value of 500 000 m to avoid negative values) will give consistency with UTM in the major part of the country.

I still can't really figure out how to convert it, but I would be glad if someone could help me with it. I got a suggestion to use the java library proj4j (because I'm going to convert it using java, and then use the coordinates in an android app).
But I can't figure out how to use that either, and I also want to do it myself. I don't really like libraries. I want to do stuff myself. Don't know why. It just is like that :)

What does the scale factor mean, and what unit is the coordinates measured with, and so on.

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I admire your gusto for thinking about converting your XYs to lat/longs yourself, but most libraries exist for a reason. :-)

If you are undeterred, then I strongly encourage you to download the excellent OGP Guidance Note 7-2 "Coordinate Conversions and Transformations" as it contains the definitions to mapping terms, and Transverse Mercator (the projection of SWEREF99 TM) starts on page 45. The formula for converting to lat/longs begins on page 49 and carries on for several pages.

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  • Thanks! I'll defenitely read the book! I couldn't find any which looked good, but I'll try this one. – Daniel Kvist Jan 30 '15 at 23:22
  • So SWEREF99 TM uses Transverse Mercator? It those two equivalent? My bad. I found it on wikipedia :) (They are the same) – Daniel Kvist Jan 30 '15 at 23:26
  • After a bit of reading and trying, I think I'll use the Java library JMapProjLib anyway (instead of doing it on my own). :) – Daniel Kvist Jan 31 '15 at 23:46

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