[Proj] implementing inverse equirectangular projection

[John Cartwright]

Hello All,

I'm trying to implement the inverse projection for the equirectangular 
(Plate Carree) projection.

The context for this is a map viewer that needs to convert from pixels 
to geographic decimal degrees w/ each mouse move.  I have the 
equirectangular coordinates  in meters for the map image.   The formula 
as described in USGS Bulletin 1532 or MathWorld 
(http://mathworld.wolfram.com/CylindricalEquidistantProjection.html) 
seems very simple, but I don't seem to be implementing it correctly.

It looks like it should be:

#lon,lat in radians; central meridian in decimal degrees
#R=radius of earth in meters; x,y= projected coordinates in meters
lon = math.radians(centralMeridian) + x
lat = y / R

However, the calculated longitude values are incorrect.  GDAL/OSR and 
proj4 (naturally) produce correct results. Here's the Well Known Text 
I'm using to define the projection:

'PROJCS["World_Plate_Carree", GEOGCS["WGS 84", DATUM["WGS_1984", 
SPHEROID["WGS_1984", 6378137.0, 298.257223563]], PRIMEM["Greenwich", 
0.0], UNIT["degree",0.017453292519943295]], 
PROJECTION["Equirectangular"], PARAMETER["false_easting",  0.0], 
PARAMETER["false_northing", 0.0], PARAMETER["Central_Meridian", -180.0], 
UNIT["Meter",1.0]]'

and this is the proj4 command I'm using to test:

cs2cs +proj=eqc +lon_0=-180 +datum=WGS84 +to  +proj=longlat +datum=WGS84


Can anyone help me out here?

Thanks so much!

-- john