Verification of plot with unclear borders, need land sketches from survey department

I am considering buying a plot that was bought a few years ago by the seller. During the previous transaction, the registration document has a mention of a 20 ft road adjoining the plot. The problem however is that the owner of that piece of land is unlikely to enough land parcel to allow for a road 20ft wide. I am not sure how sub-registrar office would have allowed such registration without a clear sketch of the land/layout. It appears that the adjoining land piece is kharab (b) land and is not for that land owner to give away. The present owner is claiming that since the adjoining portion is kharab (b) land, the previous land owner was justified in including it in the 20 ft wide road. What I wish to understand is how do I proceed with the verification. Most of the lawyers seem to be only interested in doing a document verification whereas what I really need here is a thorough verification of the land sketch, kharab (b) land extension and titles. I am unsure if any lawyers in Bangalore are able to obtain this information from the relavant departments (survey, land records, etc) as the current owners do not possess them.