Why touch tolerates less distortion than sight
Sighted readers correct for schematisation automatically. London's Tube map compresses some distances and inflates others by a factor of several times, but a sighted commuter reconciles it with what they see outside the window. Touch offers no such correction channel. A tactile reader builds a spatial model from the map itself — line angles, relative distances, the gap between two intersections — and then walks that model into the world. If the map says two streets meet at a right angle when they actually meet at sixty degrees, the navigator arrives at an unfamiliar junction with a confident but wrong mental picture.
This is the core argument: topological distortion that costs a sighted reader a moment of adjustment costs a tactile reader the whole route.
A correctly produced tactile map is drawn from real survey data — the same coordinate-referenced geometry that underlies a printed street map — and then adapted for fingertip resolution rather than invented from scratch. Adaptation means simplifying fine detail, merging features too close together for the finger to distinguish, and labelling in braille. It does not mean straightening curves for convenience or relocating landmarks to clear space. Every geometric change creates a debt the reader must pay at street level.
The practical requirements follow directly. The underlying data must carry accurate coordinates. Line angles must be preserved within the tolerance of the production medium, whether thermoform plastic, swell paper, or microcapsule sheet. Landmark positions must be georeferenced, not estimated. Where two features genuinely cannot be shown without collision at the available scale, the rule is to omit one and note the omission — not to nudge either sideways.
Scale also matters in a way it does not for schematic diagrams. A tactile reader has no peripheral overview; the finger reads one small patch at a time, and scale determines whether those patches correspond to walkable reality. A map that is topologically accurate but metrically wrong by a large factor still misdirects.
The minimum viable tactile map, in other words, is a geometrically accurate one. Simplification is necessary; distortion is not.