ASP.NET - Accessing a local system path using a Webservice

I  have hosted a web service in  IIS server. and that webservice has a method that takes a param to access the local path of the iis server . i passed the path from client as ~\proposal\independent\Devtesting to the webservice method and i am trying to access the local path using server.mappath (~\proposal\independent\Devtesting) but i get an error
'F:\pweb\proposal\independent\Devtesting' is not a valid virtual path. any help on this?