The curvature of the earth's surface, is taken into account only if the extent of survey is more than

ANS:D - 260 sq km.

Yes, it is right that error starts coming with area 195.5 km2, but in that case, only small error is there that is 1 sec, so we neglect it, so we consider greater than 250km2, that would be more so we can't neglect it. If less than 250 km^2 then plane service consider. More than 250 Km^2 then geodetic service considered, where the curvature of earth surface taken (consider). I think 260 km^2 is the right answer 199.5 area of well condition triangle.