Choice of reference frame decides what happens after a turn

What it is

Someone turns ninety degrees. The panel that sat in front of the belly may still sit in front of the belly, or it may keep the room’s old heading and become a point at the side or back. Choice of reference frame decides what happens after a turn: whichever coordinate frame stores the input target’s yaw already writes, after the turn, where the hand must reach.

This is not translation. Walking away with the tools still on the body was the previous leaf. This one asks only whether, after a rotation about the vertical, the same arrival still names that target.

Why it happens

Yaw is commonly stored in three frames, and a turn then yields three arrivals. Stored in the room: the person turns, the target’s world heading does not, belly becomes back, the old belt-reach hits air, and the person must look around or turn back. Stored on the torso: the target follows hip yaw, belly stays belly, the belt-reach still hits, but the target is no longer on the old wall. Stored on the head: the target follows the nose, a head turn puts it in view, and the place the hand must reach runs with the line of sight — arrival is no longer a stable body path.

Before the turn, “reach the belly” was already a compiled movement. After the turn, whether that movement still has an object depends entirely on where yaw lives. The failure mode is not “turning feels bad.” It is a first reach into an emptied location, then a visual search to find the target again. Input is broken for that moment: the hand walks the old map, the target lives on another.

Studying it

Let people become fluent with an entry, then make one in-place turn (ninety or one hundred eighty degrees) and immediately reach for it, without looking first.

Independent variables: target yaw stored in the room / on the torso / on the head, turn angle, whether a last glance is allowed before the turn. Dependent variables: landing of the first reach (old room location / new belly / current line of sight), number of corrections, verbal “where I thought it still was.”

The first reach is the gold standard. It exposes the map people internalised, not a strategy after they adapt. Five seconds of viewing after the turn measures visual search, not reference frame. Compare the three writings on the same entry and the same people; do not paste numbers across apps.

Where it stops holding

When the content itself has a heading in the room (a window, a stage, another person’s face), walking the entry off with the torso splits it from the shared object; room coordinates are the right ones. In continuous spinning (dance, checking all around), a head-stored target fills the view and a torso-stored one leaves the view on every head turn; both can be wrong. What is needed is the target sitting unambiguously in one frame through the turn, not a blend. Torso yaw and head yaw often travel together in daily life (a turn of the feet usually takes the head). Experiments must separate “head only, feet planted” from “feet turn, head follows,” or the three writings collapse under body coupling. A person on a swivel chair yaws with the chair; if the hip sensor lags, torso coordinates are briefly wrong.

Applying it

• Write down, for each class of entry, where yaw lives: tools that serve only this person on the torso, objects that belong to the room in the room. Do not let one entry change frame during a single turn.
• The first arrival after a turn must still point at the location that entry declared. A reach into empty space is not the user turning wrong; it is a declaration that does not match storage.
• Do not fake torso coordinates by “taking the menu with a head turn” — that is head yaw, and the arrival path will run with the gaze.
• How to check: after fluency, hide the entry visually, turn ninety degrees, reach. A reach to the new belly means torso; to the old wall, room; to in front of the nose, head. If the landing is not the frame you thought you wrote, the turn will startle in the product.

Related

• Same group: N2.07.1 The body supplies a location reference that does not need vision · N2.07.2 Body-anchored interfaces travel with the person · N2.07.4 The head is the worst body site for an anchor · N2.07.5 A waist anchor is more stable than a shoulder anchor · N2.07.6 A wrist anchor brings the interface into view at the cost of raising the arm · N2.07.7 Anchor distance must scale with body size, not a fixed length
• Nearby: N3.01 Locking Modes · N3.08 World-locked, Body-locked, and Head-locked
• Search terms: reference frame after rotation · yaw storage · egocentric vs allocentric