is always taken on a point of known elevation or can be computed

is added to the known level to obtain the instrument height

taken on an inverted staff is treated as negative

all the above.

100 sq. km of area

150 sq. km of area

200 sq. km of area

none of these.

carelessness

faulty instrument

inattention

none of these.

estimation

graphical means

computation

all of these.

it consists of two equal piano convex lenses

the curved surfaces of plano-convex lenses face each other

the two lenses are separated by a distance equal to 2/3 of the focal length of either lens.

All of these