You can put vector-effect="non-scaling-stroke" on the line but beware, it won't work on IE.

java,math,2d,map-projections,mercator

As comments have pointed oit correctly, in order to precisely convert between geographic coordinates and map position, you have to know the method of projection used for the map, and a sufficient number of parameters so that tuning the remaining parameters using a suitable set of reference points becomes feasible....