A small marker is read as pointing along its sharpest corner, so a deeply notched arrowhead points backwards

Fact

At map and HUD sizes the eye has only corners to go on, and it reads a pointer as pointing along its most acute one. A notched arrowhead has three acute corners, the tip and the two wings. Once the wings are sharper than the tip, it reads as a chevron flying tail-first.

With the tip at (L, 0), the wings at (-a, ±w) and the notch at (-n, 0):

tip  = 2 * atan(w / (L + a))
wing = atan(w / (a - n)) - atan(w / (L + a))

L = 1.25, a = 0.85, w = 0.95, n = 0.35 looks like an ordinary arrow when drawn large, but its tip is 48.7° and each wing 37.9°.

Why it matters

Nothing is wrong with the geometry or the rotation: a pixel probe ahead of the marker finds its tip exactly where it should be. The fault only shows as a person, often its author, reading the direction backwards at a glance, so it gets past the tests that check the maths.

How to apply

• Make the tip the single sharpest corner by a clear margin: keep the notch shallow or drop it. An unnotched isosceles triangle's base angles are each (180° - tip) / 2, so its tip must stay under 60°.
• A teardrop has one corner and cannot be misread. A tip two radii out leaves the circle at ±60°: moveTo(2 * r, 0); arc(0, 0, r, Math.PI / 3, -Math.PI / 3); closePath();
• Judge the marker rendered at its real size and at an off-axis angle, not as a large mock-up.