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I'm currently trying to parse a shapefile (http://www.esri.com/library/whitepapers/pdfs/shapefile.pdf) with c++. While I have no problems parsing all the data I need, there is a problem with the coordinates. I got a description file for the data, which states that the used projection is "ETRS89 (Ellipsoid: GRS80), UTM (Zone 33)".

I need all points to be in lon/lat format. I spent the whole day googling around to find a solution for this problem, but there doesn't seem to be one.

data example:

33354891.477852, 6060028.67074774
        ^               ^
       X (E)           Y (N)
  • Download QGIS 2.0 load the shapefile and set the CRS (projection) to WGS84 (latlng) and export will give you point shapefile with latitude and longitude. qgis.org/en/site/forusers/download.html – Mapperz Oct 4 '13 at 16:11
  • there must be a programmatical way. i don't want to load each shapefile manually... – Mr.Manhattan Oct 4 '13 at 16:13
  • you still need to reproject the data. C++ reproject shapefiles stackoverflow.com/questions/2139335/… – Mapperz Oct 4 '13 at 16:20
  • @Mapperz that link is for reading shapefiles, not reprojecting data. – Russell at ISC Oct 4 '13 at 16:56
  • also, i don't want to reproject the whole shapefile. it would be enough to get a formula to calculate the lon/lat from these coordinates – Mr.Manhattan Oct 4 '13 at 17:14
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C++ projection library proj

https://trac.osgeo.org/proj/

We've used DLLImport to write calls into it from C# code for MapDotNet and MapsJS, works great!

You'll need to go to http://spatialreference.org/ to get proj4 strings for your source and destination projections (WGS 84 is a good choice for lat/lon)

Call pj_init_plus(initParameters) with each of your strings (returns an int*)

Then call

pj_transform(projFrom, projTo, arraySize, 1, xArray, yArray, null);

where arrays hold your coordinates (the null is for a z array which I assume you don't need).

Degrees will come out in radians, you'll need to convert to 360 degree format. Returns 0 on success, or a proj error code.

  • hmm...isn't there just a formula to convert a point? i don't really want to use big libraries... – Mr.Manhattan Oct 4 '13 at 17:08
  • Yes, but it is different for every family of projections. – Russell at ISC Oct 4 '13 at 17:19
  • Here's the Wikipedia for UTM to degrees en.wikipedia.org/wiki/… It is not a simple formula. – Russell at ISC Oct 4 '13 at 17:21
  • Hmmm...some code i could steal, without using a whole library? i only need that one conversion... – Mr.Manhattan Oct 4 '13 at 17:30
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    It's an open-source library. You can dig through the source. – Russell at ISC Oct 4 '13 at 17:31
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If you've got X/Y values in ETRS89/UTM 33 but you want ETRS89 lat/longs instead, then you will need to use the Transverse Mercator formula. However, the ellipsoid formula is ... rather complicated.

tm_projections

If you've got a little geodetic knowledge, then it is entirely possible to calculate lat/longs from the X/Ys yourself, but I would strongly suggest reading the EPSG Guidance on Conversions and Transformations (p.45 starts the Transverse Mercator) for a full description of all the terms involved.

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