What it is
The FMB, or Field Measurement Book, sketch is a scaled drawing maintained by the Tamil Nadu Survey and Land Records Department, showing a survey number's boundary lines, dimensions, and how it has been split into sub-divisions where the original number is now held by more than one person. It is the technical, dimensioned counterpart to the descriptive patta-chitta record, drawn from the department's own field measurements rather than from ownership paperwork.
Why it matters when you buy
An FMB sketch lets you check that the boundaries and shape described on paper match what you actually see walking the site, and that the sub-division you're buying is correctly marked off from the plots next to it. A fence in the wrong place, or an access path that crosses into a neighbouring sub-division, is worth raising with a surveyor or the seller before you commit — correcting a boundary after purchase is far harder than confirming one beforehand.
Where you find it
An FMB sketch for a given survey number can be viewed and downloaded through eservices.tn.gov.in, Tamil Nadu's e-services portal, usually alongside the patta-chitta record for the same number. Properties inside town-survey areas use the equivalent TSLR sketch instead, referenced against a town survey number rather than a rural survey number.
What it does not prove
An FMB sketch is a survey reference showing recorded measurements and boundaries. It does not by itself establish legal title, possession, current occupation or planning permission.