@Mac Dude 

I realize that y = -sqrt(-x^2+1) is a solution mathmatically but this is obviously a negative solution in the real domain (the one which I am trying to work in), the assumption inserted that y>0 should then exclude this result (if it worked how I would like). This is obviously not the case and I guess that's my question, why doesn't it work and how can I circumvent this problem.

Unfortunately my question with the units is purely a maple one. I have tried define the variables in terms of themselves to give itself units (ie y=y[m]) but this prove fruitless in this example.