lat_0, lon_0 ignored

まず Basemap の投影設定と座標変換のエントリーポイントを確認し、次に、lat_0 と lon_0 を指定した場合と指定しない場合で、提供された Cylindrical Equal Area の例を実行します。m(0, 0, inverse=True) が左下の地図境界を返すのではなく、指定された原点を考慮するかどうかを検証します。動作がドキュメント化されたパラメーターと一致するか、またはその制限が明確に確認できれば完了です。

I am trying to set up Basemap to use a Cylindrical Equal Area projection and to place the origin at a specific lat/lon (this is to match some input x/y data that uses a false easting/northing), but it seems that setting the lat_0 and lon_0 parameters does nothing.

from mpl_toolkits.basemap import Basemap

# Oklahoma bounds 
# ('o' = origin at corner of Kingfisher/Logan/Canadian/Oklahoma counties)
lats = {'n':  37.1, 's':  33.6,  'c':  35.5, 'o':  35.726}
lons = {'e': -94.4, 'w': -103.0, 'c': -97.5, 'o': -97.674}

buf = 0.05 # plot buffer size (in degrees)

res = 'c' # map shapes resolution

# projection parameters
proj = 'cea' # cylindrical equal area
lat_ts = lats['c'] # true scale at geographic center
lon_0 = lons['o']  # x=0 at counties corner
lat_0 = lats['o']  # y=0 at counties corner

m = Basemap(projection = proj, lat_ts = lat_ts, resolution = res,
            lon_0 = lon_0, lat_0 = lat_0,
            urcrnrlon = lons['e'] + buf, urcrnrlat = lats['n'] + buf,
            llcrnrlon = lons['w'] - buf, llcrnrlat = lats['s'] - buf*4)

Then checking the origin gives:

In [15]: m(0, 0, inverse=True)
Out[15]: (-103.05000000000305, 33.399999999963455)

...which is the lower-left corner of the map bounds, not the coordinates I gave for lat_0 and lon_0. I get the same output if I leave out lat_0 and lon_0 entirely.

At first, I thought maybe the CEA projection doesn't support these parameters and that I should just shift my data manually, but the docs claim that lat_0 and lon_0 are used by all projections to set the origin.

As a workaround, I can set an offset to get the intended results:

offset = m(lon_0, lat_0)
df['x'], df['y'] = (df.x + offset[0], df.y + offset[1]) # where df is a Pandas dataframe with my data

Note: Originally posted to StackOverflow, where I was encouraged to move the issue here. http://stackoverflow.com/questions/31302427/lat-0-lon-0-ignored-by-basemap