Return true if the map projection matches that used by VEarth, Google, OSM, etc.
    
        Is currently necessary for zoom-level shorthand for scale-denominator.

def is_merc_projection(srs):
    """ Return true if the map projection matches that used by VEarth, Google, OSM, etc.
    
        Is currently necessary for zoom-level shorthand for scale-denominator.
    """
    if srs.lower() == '+init=epsg:900913':
        return True

    # observed
    srs = dict([p.split('=') for p in srs.split() if '=' in p])
    
    # expected
    # note, common optional modifiers like +no_defs, +over, and +wkt
    # are not pairs and should not prevent matching
    gym = '+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null'
    gym = dict([p.split('=') for p in gym.split() if '=' in p])
        
    for p in gym:
        if srs.get(p, None) != gym.get(p, None):
            return False

    return True