What is the surface that is normal to the direction of gravity at all points?

Horizontal surface

Horizontal line

Level surface

Level line

Vertical line

Datum line

Horizontal line

+/- 3 mm

+/- 5 mm

+/- 6 mm

+/- 8 mm

Natural error

Personal error

Artificial error

Instrumental error

+/- 2 mm

+/- 4 mm

+/- 6 mm

+/- 8 mm