You can measure how far away a mountain is without walking to it. Stand at two ends of a known baseline, measure the angle to the peak from each, and the two sight-lines cross at exactly one point — its distance falls out of the triangle. It’s how surveyors mapped continents and how your phone camera guesses depth. Slide the target and watch the fix.
Forward intersection (triangulation): from two stations a known baseline b apart, measure the angles A and B that the sight-lines to a distant point make with the baseline. The third angle is C = π − A − B, and the law of sines gives the range from station 1: r = b·sin(B)/sin(C). Two angles plus one measured length fix the point exactly — the whole of surveying, and the geometric core of stereo depth. A fail-loud self-check throws unless a symmetric 45°/45° sighting across a 100 m baseline returns 70.7 m. ◆ real geodesy, node-verified.
A planar two-angle intersection (the exact law-of-sines fix); real geodesy adds earth curvature and angle error that grows the fix uncertainty — the two-angles-plus-baseline determination is exact.