Lofting coordinates

The system derives from the one that was used in the shipbuilding lofting process, with longitudinal axis labeled as "stations" (usually fuselage stations, frame stations, FS), transverse axis as "buttocks lines" (or butt lines, BL), and vertical axis as "waterlines" (WL).

Fuselage stations are traditionally nonnegative, thus the origin is located at the nose of the plane or, sometimes, ahead of it.

Two typical alignments for the base line are the tip of the nose (negative WL are possible) or the "nominal ground plane" (measurements will be nonnegative).

[4] Butt line ticks increase to the right of the pilot with the origin at the centerline.

When compared to the (right-handed) aeromechanics coordinate systems, the direction of the butt line is opposite to the y-axis.

Two points on a fuselage at waterline 100/fuselage station 93 and waterline 101/fuselage station 276
Fuselage stations on a scheme of a fuselage (atypically FS 0 is amidship, usually origin is at the nose, and FS values are nonnegative)
Water lines on a scheme of a fuselage. Baseline is at the ground line, all WL values are nonnegative
Butt lines on a scheme of a stabilizer (BL ticks are symmetrical here, usually ticks on the left are negative)
Other coordinate references on the aircraft body