Given a specific state plane coordinate system (whether NAD27 or NAD83), are there any resources to convert to latitude-longitude?
If you know the relevant EPSG codes or proj.4 parameters, both
gdaltransform and proj.4's
cs2cs (man page) are a good route, as Reed mentioned.
However, if you find yourself without this information, try spatialreference.org: it provides the parameters, and a web map for doing a lat/long to coordinate system transformation (example with IL state plane). As many of the state plane projections lack an EPSG code, use sr.org to find the proj.4 string, which
gdaltransform understands. Here's an example converting from California State Plane Zone V to WGS84 lat/long pairs:
gdaltransform -s_srs '+proj=lcc +lat_1=34.03333333333333 \ +lat_2=35.46666666666667 +lat_0=33.5 +lon_0=-118 +x_0=2000000 +y_0=500000 \ +ellps=GRS80 +units=m +no_defs' -t_srs epsg:4326
After which, typing
1815241.25377291 557301.336190851 should return
Client or server? Language specific? One-off or bulk?
0) Import all the appropriate libs
1) create your point
var lonlat = new OpenLayers.LonLat( ....)
2) convert it
lonlat.transform( new OpenLayers.Projection("epsg:XXXX"), new OpenLayers.Projection("epsg:4326"));
now lonlat.lon and lonlat.lat are in WGS84 lon/lat values.
One free and extremely easy program is CorpsCon which will convert single points, batches of points in text files, and has a DLL that can be used from within programing environments like python.
2014-07-09: Updated link is http://www.agc.army.mil/Missions/Corpscon.aspx Clicking "Download CorpsCon" takes you to a page that looks a little odd and temporary but does work.
If you want to build it yourself, here's a document from the National Geodetic Survey that describes the transformations:
You'll still need to know the specific parameters for the projection of interest.
Another option (but just for NAD83) is SPCS83 at http://www.ngs.noaa.gov/PC_PROD/SPCS83 . It runs in a command line window on a PC and converts either interactively or from a text file input. It includes source code if you really just want the algorithm. Source is Fortran, which may be a pain to use but is easy enough to decipher to get the algorithms out. Also see www.metzgerwillard.us/spcge/spcge.html which is a front end for SPCS83 in Google Earth.
Check out the free translation service. Upload your data and then specify the desired output coordinate system. By selecting the same format for output as the input you are effectively doing a reprojection.
The service is found at http://fmeserver.com/userweb/sharper/Portal/EasyTranslator/index.html
and is based on the FME product. There is also a free 14 day trial of this as well. www.safe.com
This site may help if your just looking to convert a single point. It didn't work for me but seems to be working for most areas.