To find the true difference of level between two points, the level should be kept

Which one of the following mistakes/errors may be cumulative positive or negative?

Axis of altitude level is

Pick up the odd instrument

Tacheometry is best suited

A level surface to which elevation of different points are referred to, is known as:

In case of a double line river, contours are

In chain surveying tie lines are primarily provided

The method of surveying in which field work and plotting work are done simultaneously, is called