When It Costs You Something: The Corridor Stakeout
A road corridor gets staked from end to end, from a plan built on GNSS-derived, State Plane coordinates. Every individual stake lands within a centimeter of its intended grid position. And by the far end of the corridor, the physical distance between consecutive stakes no longer matches the plan's stated dimensions.
A combined scale factor too small to notice over a hundred meters becomes impossible to ignore over eight hundred meters, because the same proportional gap is simply being multiplied by a much larger distance.
Scaling Up the Same Small Number
Take the same combined scale factor from the last chapter, roughly 0.99982, and apply it to an eight-hundred-meter corridor instead of a hundred. The same proportional gap that produced eighteen millimeters over a hundred meters now produces roughly fourteen centimeters over the full corridor. That is not a rounding error anyone can shrug off. That is a dimension a contractor will notice, and a discrepancy an inspector will ask about.
Combined scale factor error does not grow at a constant rate by coincidence. It is a proportional effect, so doubling the length of a project simply doubles the size of the discrepancy, arithmetic that catches plenty of experienced crews off guard the first time a project is longer than what they are used to.
Why the Stakes Themselves Look Perfect
This is precisely what makes the failure so disorienting in the field. Every individual GNSS-set stake sits exactly where its grid coordinate says it should, centimeter for centimeter. The receiver never reported an error, never dropped from FIX, never gave any indication anything was wrong. The discrepancy only appears once someone measures between two stakes with a tape or a total station and compares that measurement against the plan's stated ground distance, at which point the mismatch has already been built into eight hundred meters of physical infrastructure.
A perfectly staked point, sitting exactly on its GNSS-derived grid coordinate, does not guarantee the physical distance between it and the next stake matches the plan's ground dimension. Both can be true at once: every stake correct on the grid, and the ground distances between them still off by a real, measurable amount.
The Fix Is Cheap, If Caught in Time
None of this requires abandoning GNSS or State Plane. It requires deciding, explicitly, before staking begins, whether the plan's dimensions are meant to be held as grid distances or as ground distances, and applying the combined scale factor in the correct direction to convert between them if the two do not match. That decision costs a few minutes at the start of a project. Discovering the mismatch after eight hundred meters of corridor is already staked costs considerably more.
A road corridor is staked entirely from GNSS-derived grid coordinates, every point centimeter-accurate on its own. A contractor later reports the plan dimensions do not match the ground. Does that point to a GNSS error? Not necessarily, and probably not. It is a classic signature of an unapplied combined scale factor, growing exactly as expected over a project this long.
Combined scale factor error grows proportionally with distance, turning a millimeter-scale effect over a short baseline into a centimeter-scale discrepancy over an eight-hundred-meter corridor or longer. Every individual GNSS point can be perfectly correct on the grid while the physical distances between them still disagree with a plan's ground dimensions, and catching this before staking begins is far cheaper than discovering it afterward.
Distance is not the only thing a projection distorts. The next chapter turns to direction, and a mismatch just as easy to miss as this one.