Выражение встречается в следующем контексте:

Руководство по монтажу металлоконструкций

Подраздел "Deviations from the centre and square lines"

These deviations can be checked with piano wire and plumb bobs or with transit.

(A) Match the piano wires with the datum bench marks
Match the piano wires with the bench marks, hang the plumb bobs on the positions to be measured and measure the deviations from the machine center line mark by means of a steel rule.

(B) Match the piano wires with the center flanks.
Match the piano wires with the center flanks of the machines to be installed (translated bench marks), hang the plumb bobs on the positions to be measured and measure the deviations from the reference surface with a steel rule.
The center line could be checked with total station which can measure directly the angle and distance. Even if the total station can measure the elevation it is strongly suggested to avoid this operation due to low accuracy.

Заранее спасибо