Calculation of LTCG on sale of property or land.

Sir,

I would like to know what would be cost of acquisition of a land or property in case it is gifted from my father due to inheritance.

In this case the real purchaser of the land is grand grand father and the purchase value and documents are not available as of now. In such case what i should do to calculate the capital gain from the sales proceeds ? Is there any options such as choosing the muncipal value of that year and calculate such and taking it as the cost of acquisition.

I hope that my question is clear.

Please do kindly make replies one who knows the provisions.