Flight Line Spacing from Sidelap

The across-track twin of the air base. Whatever fraction of the frame's across-track width has to overlap the neighbouring strip, the remainder is how far apart the lines go: $S = W_c(1 - p_{side})$. Same arithmetic, different edge, different overlap figure, and a different set of reasons for choosing it.

Sidelap is not endlap. Endlap is between consecutive frames on one line and sets the base; sidelap is between adjacent lines and sets the spacing. They are usually different — 75% endlap with 65% sidelap is an ordinary drone setting — and a planner that offers a single "overlap" box is hiding a decision from you. The width $W_c$ in this equation must be the frame edge lying ACROSS the flight direction, which on most missions is the LONG sensor edge, since flying the long edge across track widens the swath, cuts the number of lines, and cuts the number of turns. Turns are where a battery goes: a multirotor at the end of a line decelerates, rotates, accelerates, and photographs nothing useful for several seconds each time.

Historically 30% sidelap was the aerial-survey standard, and it existed for a purely practical reason — to guarantee that no gap opened between strips when an aircraft drifted off its line in a crosswind. It was never intended to feed an automatic matcher. Structure-from-motion wants considerably more, because a point seen only in the narrow band where two strips just touch is observed from a poor range of angles and contributes little to the solution. Sixty to seventy per cent is where drone practice has settled, and blocks flown much below that tend to show a visible seam of weakness along every line junction.

Three practical additions the equation does not contain, and all three are worth building into a habit. Extend every line past the block boundary by at least one air base at each end, so the frames covering your actual area of interest are not themselves the outermost ones. Add a line outside each edge of the area of interest, for the same reason: the outermost frames of a block are seen from one side only, they carry the fewest tie points, and the edge of a photogrammetric block is always its weakest part. And fly a crossed pass at right angles when the site has tall structures or when the camera is being calibrated on the fly. Crossed flight lines are the cheapest known fix for the systematic bowl-shaped deformation that a single-direction block develops when the focal length and lens distortion are being solved from the imagery itself — the geometry is nearly degenerate in one direction and adding a perpendicular set breaks the degeneracy.

Corridor work — a pipeline, a road, a transmission line — deserves its own note, because it is where these rules are most often broken and most costly. A single line of images along a corridor is close to the worst possible photogrammetric geometry: the block is one image wide, there is no cross-strip redundancy, and errors accumulate along its length like a traverse that never closes. The standard answer is at least three parallel lines, plus periodic cross-lines, plus ground control at both ends and at intervals along the way. It costs more than it looks like it should, and skipping it produces a model that drifts.