How to derive projection parameters from sample data pairs (planar <-> geographic)?

Hello,

I have about 5000 planar points in the (x,y)-coordinate system that I derived from a planar geographical map. I guess that the equidistant conic (eqdconicstd) projection has been used to project the original geographic (lat,long) data to the planar map (x,y) - but I'm not sure.

However, I would like to convert all the (x,y) data points I have back to latitude and longitude. My question is: Is there a way, using the mapping toolbox, to find out the projection parameters that have been originally used? I have a number of correlated pairs (x,y) - (lat,long) that I could use.

I would appreciate all sort of ideas from guys who know more about the mapping toolbox and geographic projections than I do. :)

Kind regards, Sven