Why an Old Deed and a New GPS Survey Don't Agree (and What That Doesn't Mean)
It is worth restating plainly before this chapter goes any further: this companion is not legal guidance, and nothing in this chapter substitutes for the judgment of a licensed Professional Land Surveyor. Boundary determination is a legal act, performed under jurisdiction-specific rules and evidence standards this companion has no authority over. What follows explains a technical reason two numbers can disagree. It does not tell anyone how to resolve a boundary.
A distance recorded in a decades-old deed or plat and a distance derived from a modern GPS survey were very often never going to match exactly, for entirely technical, explainable reasons that have nothing to do with either measurement being wrong.
What an Old Record Actually Measured
A distance recorded by a chain or a tape decades ago describes a real, physical, ground measurement, taken at the elevation and location the crew actually stood at, using whatever unit definition, U.S. survey foot most commonly, was standard for that era and jurisdiction. It was never a grid distance, and never claimed to be one. It is, in the terms this companion has used throughout, a ground distance, recorded long before GNSS, State Plane, or combined scale factor were part of the conversation at all.
An old chain-and-tape distance and a modern GNSS-derived grid distance are answering the two different questions this entire companion has been organized around, separated additionally by however many decades sit between the two surveys.
Every Cause in This Companion, Stacked at Once
A modern GPS survey, by contrast, produces a grid coordinate by default, in whatever reference frame and projection the current software assumes, converted through whatever units definition that software applies. Every distortion covered so far, elevation factor, grid scale factor, convergence, and a possible foot definition mismatch, can all be present in a single comparison between an old record and a new survey, on top of whatever ordinary measurement uncertainty the original chain-and-tape work carried in the first place.
A gap between an old deed distance and a new GNSS-derived distance is not, by itself, evidence that either the old survey or the new one was performed incorrectly. It is frequently the accumulated, explainable result of the exact technical distinctions this companion has covered, stacked across a real span of decades.
What This Chapter Is Not Saying
None of this is a claim about which number should control a boundary, which record takes legal precedence, or how a discrepancy like this should be resolved in a given jurisdiction. Those are legal questions, answered by principles, statutes, and case law entirely outside this companion's scope, and by the professional judgment of a licensed surveyor weighing evidence the way their license and jurisdiction require. This chapter's job ends at explaining, technically, why the numbers might not match in the first place.
A distance recorded in a fifty-year-old deed disagrees with a distance derived from a same-day GPS survey of the same line. Does that disagreement, on its own, indicate an error in either survey? Not necessarily, and this companion takes no position on how such a disagreement should ultimately be resolved. It only explains the technical reasons a gap like this is common and often fully explainable.
An old chain-and-tape distance and a modern GNSS-derived grid distance were frequently never going to match exactly, for stacked, explainable technical reasons already covered in this companion. Understanding those reasons is a technical exercise. Deciding what a discrepancy means for an actual boundary is a legal one, outside this companion's scope and squarely inside a licensed surveyor's.
Understanding why a mismatch can happen is one thing. Recognizing which specific cause is actually responsible for a given mismatch is the next, more practical skill this companion builds toward.