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I think this is primarily a GIS based question as far as for example:

var crs = L.CRS.proj4js('EPSG:2154',
                    '+proj=lcc +lat_1=49 +lat_2=44 +lat_0=46.5 +lon_0=3 +x_0=700000 +y_0=6600000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs',

So if I read in a wms tile layer through the leaflet api to create an overlay on a wfs base map how do I decide how to adjust those lat/lon values? And what is ellps, towgs, and units?

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ellps is ellipsoid, units=m are meters; towgs84 is conversion into wgs84 datum proj.maptools.org/gen_parms.html –  nadya Nov 27 '12 at 17:26
Ok, do you know how to determine what I should set those values to render a wms overlay on a wfs base map? –  Carl Carlson Nov 27 '12 at 17:46
you can look then up at spatialreference.org –  iant Nov 28 '12 at 8:18

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