Programmable Electronic Levels and Leveling
This article refers to electronic level systems that comprise of two level sensing heads (henceforth referred to as “level heads”) and a common gaging amplifier.

Normally, electronic levels measure the angular deviation between a horizontal surface and the earth's gravitational field. The programmability of the amplifier makes the electronic level a flexible measurement system, and this is one of its advantages over precision spirit levels. Another advantage is accuracy: Precision Spirit levels can have a resolution of 0.00024" per foot or 4.13 Arc Seconds. Electronic levels commonly have a resolution of 0.000006” per foot, or 0.1 Arc Second.
Absolute Level
All the measurements are relative to the earth's gravitational field. This gravitational field is commonly called "absolute level”. When the level head is setup to be parallel to the gravitational field, then the level head is also at absolute level.
Comparative Level
With machine tools, the axes must travel precisely flat, parallel, and perpendicular to one another and the spindle. Normally, absolute level is not required with machine tools, if it were, machine tools would not work on ships. When absolute level is not used as the reference, it is called “comparative level”. Comparative level means the out-of-parallel angle to the earth's gravitational field is used as the reference angle, the level heads can be set to read 0 (zero) at that angle.
Dynamic Leveling
There are a number of different ways to use the level heads in pairs. One method involves the use of software called DynaLevel that reads both heads setup perpendicular to one another on a machine's axis. The axis is then moved to predetermined positions that reading are collected on. The end result is a graph that shows how flat and parallel the guideways are that the axis slides upon. This is called “dynamic leveling” because the weight of the axis has been included in the results.

The picture to the right comes from DynaLevel's help file. It is a top down view of a large CMM (Coordinate Measuring Machine). It shows where to mount the level heads, the level head orientation, and the inputs the software requires to calculate.
Differential Leveling
Another two level head method is known as differential leveling. With differential leveling, the level heads compare the angle of separate surfaces to one another. For example, one level head is mounted to a machine tool spindle and the other level head is placed on the table. If the angle of the level head on the spindle changes exactly the same as the one on the table when the table is moved to a different position then, the relationship stays the same and the leveling system would show no error. This same method can be used to compare two guideways to each other to see if they are parallel to one another vertically.

How differential leveling works is, the two level head are both connected to the amplifier and arranged to respond opposite to each other. If both are tipped 1 arc second in the same direction, and one level head reads +1 arc second and the other reads -1 arc second. The amplifier adds the two values and displays zero, +1 + -1 = 0. If, on the other hand, the two level heads tip in opposite directions from each other 1 arch second where one level head reads -1 and the other also reads -1, the amplifier adds those two values and displays -2 arc seconds.