Using minvtran to convert mercator projection to geographic lat lon coordinates

Hi,

I am trying to use minvtran (<http://www.mathworks.co.uk/help/map/ref/minvtran.html>) to convert mercator projection to lat lon coordinates. Typical mercator points include:

[x y]

ans =

-3228800 6890893

-3228512 6861000

However when I use minvtran, the geographic lat lon coordinates I get out are teh same for each x and each y coordinate. The code I use is as follows...

mstruct = defaultm('mercator');

mstruct.origin = [38.89 -77.04 0];

mstruct = defaultm(mstruct);

[lat,lon] = minvtran(mstruct,lat,lon);

[lat,lon]

ans =

-51.1100  -77.0400
-51.1100  -77.0400

Please help in allowing me to understand why I get the same lat coordinate of -51.1100 and the same lon coordinate of -77.04 when the input mercator points are different.

Thank you

Danny