explained how calypso calculate Perpendicularity

[[K...]]

I defined a plain 50 x50 mm as datum A, datum B is constructed from cylinder / 3-d line by using 3 circles. cylinder / 3-d line length is 17 mm.
When I called perpendicularity of 3-d line w.r.t datum A is 0.101mm, if I called perpendicularity of plain (datum A) w.r.t. cylinder (3-d line) line is higher (0.181mm) then perpendicularity of 3-d line w.r.t datum A.
I will appreciate if anyone can explain its theoretically justification.

Thank you

[[Ja...]]

See attachments

Perp_datum_feature.pdfPerpendicularity.pdf

[[Da...]]

Yep, the feature size is the difference. To keep it simple, if feature 1 is 100mm long, while feature 2 is 50mm long, for the same measured part, if you create 2 measures, the first using feature #1 as the datum, and the second measure using feature #2 as the datum, the second measure will be twice as bad because the feature is longer.

[[Me...]]

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That’s not entirely correct. If the considered feature is planar, then the form error (flatness) of the plane will be included in the perpendicularity result.