The factor that converts distances between a map grid and the ground: grid scale factor times elevation factor. Small per metre, large over a long corridor.
The combined scale factor converts a distance measured on a map grid into the distance on the ground, and back. It combines the grid scale factor (how much the map projection stretches or shrinks distances at that place) with the elevation factor (the effect of the site's height above the reference ellipsoid). Close to 1, it matters little on a building and a great deal on a long corridor.
The project surveyor states the coordinate reference system and the combined scale factor, often by zone along a route. Teams then agree whether design coordinates are grid (in the projected system) or ground (at true size), and the factor is applied consistently: for setting out, for ordering lengths of pipe or rail, and when placing ground-scale models in a grid-based federation. Many long projects define a low-distortion project grid whose factor stays within a few ppm along the whole route, so grid and ground almost coincide.
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