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We must first distinguish between flatness and level. 
In mathematics, the flatness of a surface is the degree to which it approximates a mathematical plane. We call this plane level if it is on average orthogonal (at right angles) to gravity. (Sometimes called “in water”) So we only talk about something being level if it is also very flat. 

To measure flatness we require a reference plane. To measure level we require a reference to gravity. Typically we measure a matrix of points or angles and interpolate between these discrete measurements.


Spezification and mechanical Adaption

In engineering applications we measure and sometimes correct the flatness of machine components. In flatness and straightness measurements the mechanical adaptation has a dramatic effect on what we measure. It is important to consider the flatness specification in detail in order to choose an appropriate method.

A flatness can state that all points on the surface shall lie between two parallel planes separated by "X" distance, where "X" is the flatness tolerance. It can specify a tolerable wave over a specific area. Alternatively it can specify the variance of the deviation for a best fit plane. Often the specification will define explicitly or inexplicitly how the measurement must be performed.


The best method for flatness and level depends on the following:

  • What exactly are we measuring? (Dimensions, accessibility, environment)
  • What is the required spec? (tilt, roll, line deviation, waviness etc )
  • What provisions (adjustment screws) are available to correct the deviations?
  • Who is doing the work and how much time does he/she have?
  • What kind of a report is required? (For whom?)
  • What’s your budget?