A map grid is a flat picture of a curved Earth, so distances on the grid differ slightly from distances on the ground. Over a building nobody notices; over a 20 km railway it adds up to decimetres or metres. The scale factor turns one into the other.
A building is small enough that the Earth is flat for its purposes. A railway or a pipeline isn't. Map grids flatten the curved Earth onto a plane, and that flattening stretches distances slightly in some places and shrinks them in others. On a long corridor, the difference between a distance on the grid and the same distance on the ground becomes a design problem.
National grids are designed to keep the scale factor close to 1: typically within a few hundred parts per million (ppm) across the country. A factor of 0.99985 is −150 ppm: every kilometre on the grid is 1,000.150 m on the ground. Over a 300 m building that's 45 mm, often ignored. Over a 20 km route it's 3 m.
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