Dimension Origin Symbol — Where the Measurement Starts

A small circle replacing one arrowhead of a dimension: the dimension originates from the surface the circle sits on — fixing where the measurement starts without creating a datum feature.

Definition

The symbol is a small circle drawn on the dimension line at the end where measurement begins, replacing what would be an arrowhead. The dimension runs FROM the indicated feature TO the other end. For a surface origin, the high points of the indicated surface define the plane from which the dimension is measured — the same simulator logic a surface plate applies.

The ambiguity it removes

A step block with faces at heights 30 and 50 has two plausible reading orders: measure each step from the base, or stack one step off the other. Both are legal dimensioning; they produce different parts. Placing the origin symbol on the base face states the part is inspected from the base — every other height runs from it. Without the symbol, the drawing reads as either interpretation until the first dispute.

Origin vs. datum feature

• A datum feature symbol names a surface and invites geometric controls to reference it — a heavier, part-wide statement
• The origin symbol only fixes the start of the one dimension it touches — a per-dimension statement
• Both use the indicated surface's high points (for plane surfaces) as the practical measuring reference
• A face can carry a datum symbol and an origin symbol independently; neither implies the other

Measurement

Setup — fixture or rest the part on the origin face; every dimension called from it is read directly from that seating, the way the drawing intends.

CMM — align the primary axis from the origin surface, then report each dimensioned feature from that alignment; the origin symbol is effectively a one-dimension alignment instruction.