That's the work of a valuer to find value using various base, he just don't only have to rely on circle rate on 01.04.2001. he can take value of sale of land in that area during that time or any other thing as base. It's his decision, he is a professional if he can prepare a proper report and convert value of 2006 into value of 2001 then that also can be accepted.
But as far as income tax is concerned if your land was purchased before 2001 then you need to take value in 2001 as base value.
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