According to WELMEC's 
Gravity Zone Concept for Weighing Instruments under Legal Control manufacturers of g-dependent weighing instruments may define acceptable places of use in the form of a 3 dimensional zone. Thus, national regulations based on administrative boundaries loose their meaning, nevertheless they remain valid.
The zone margins are declared first by a northern and southern latitude and second by a lower and upper height above the sea level. (
see example: Paris). The longitude of the designated place of use has no meaning, since the associated mathematical model, the Normal Gravity Field, is rotationally symmetric. The maximum acceptable extension of the margins is fixed by a third of the maximum permissible error.
The following program should ease the determination of gravity zones concerning a specific weighing instrument type in consideration of the aforementioned criterium. As the characteristics of the individual instrument, first, the number of verification scale intervals
(n = Max/e) and second, the factor of the maximum permissible error (mpe/e) have to be set. For the designated place of use approximations of the latitude and the height are sufficient. By pressing " >> " the margins will be computed in that way, that the occuring gravity differences between the latitudes and heights are equally distributed referring to the reference location. The single margins can be changed with the " < " " > " buttons again in consideration of the error criterium. (e.g. extented flat zones in lowlands)
A detailed description of the zone concept inclusive some examples can be found in 
The New Gravity Zone Concept in Europe for Weighing Instruments under Legal Control
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Contact
Head of Department
Dr. Dorothea Knopf
Phone: +49 531 592-1100
Email:
dorothea.knopf(at)ptb.de
Secretariat
Heinke Harms
Phone: +49 531 592-1101
Email:
heinke.harms(at)ptb.de
Address
Physikalisch-Technische Bundesanstalt
Bundesallee 100
38116 Braunschweig