A Repeatable Reconciliation Method
Everything this companion has covered can now be gathered into a short, practical sequence, the same way earlier companions in this series turned their own subject matter into a repeatable checklist rather than a pile of separate facts.
Reconciling two disagreeing coordinate sets or distances is most efficient when worked through in a fixed order: confirm reference frame and epoch first, then units, then scale factor, then convergence, rather than guessing at a cause before ruling out the others.
Step One: Rule Out Frame and Epoch
Before anything in this companion applies at all, confirm both coordinate sets share the same reference frame and epoch, the subject of earlier companions in this series. A frame or epoch mismatch produces a discrepancy that looks similar to some of what follows, and ruling it out first prevents chasing a scale factor or units problem that was never actually there.
A frame or epoch mismatch and a units or scale factor mismatch can look superficially alike from a distance. Confirming frame and epoch first keeps this companion's own causes from being blamed for someone else's problem.
Step Two: Confirm Units
Next, confirm which foot definition, if either dataset uses feet at all, each side actually assumes, particularly for any large State Plane coordinate value. This single check, quick to perform and easy to skip, catches one of the more common and most confusing causes this companion has covered.
Step Three: Check the Distance Pattern
With frame, epoch, and units ruled out, apply the previous chapter's diagnostic: does the discrepancy grow with distance, or stay roughly constant regardless of it. A growing discrepancy points toward scale factor, elevation, or both, and is worth checking against a computed combined scale factor for the actual project location before assuming anything more exotic is at play.
Skipping straight to computing a combined scale factor, without first confirming frame, epoch, and units, risks applying a correct correction to the wrong problem. The order of this checklist exists specifically to prevent that.
Step Four: Check the Bearing Pattern
Separately, compare bearings if any are involved. A consistent angular offset, largely independent of the distance pattern just checked, points toward convergence, and is worth confirming against the project's distance from its zone's central meridian before looking elsewhere.
A newly computed distance disagrees with an older record, and a quick check confirms both datasets share the same reference frame and epoch. What is the next, cheapest check to run before assuming a scale factor problem? Units. Confirming which foot definition each side assumes takes moments and rules out one of the most common, most overlooked causes in this companion.
A disagreement between two coordinate sets or distances is best worked through in order: frame and epoch first, units second, the distance pattern third, the bearing pattern fourth. Each step is cheap to check and rules out an entire category of cause before moving to the next, turning a vague mismatch into a specific, explainable one.
One short chapter remains, to bring this companion's central question back to where it started.