Reading the Pattern
Every cause this companion has covered leaves a slightly different fingerprint. A mismatch that grows with distance behaves differently from one that stays constant everywhere, and both behave differently again from one that only shows up as a rotated bearing. Learning to read that pattern turns a vague discrepancy into a short list of specific, checkable causes.
The way a discrepancy behaves, whether it grows with distance, stays constant regardless of distance, or shows up only in direction rather than length, points toward a specific cause among elevation factor, grid scale factor, a units mismatch, or convergence, well before any deeper investigation begins.
Grows With Distance: Scale Factor at Work
A discrepancy that is barely noticeable over a short baseline and steadily worsens the longer the measured distance gets is the signature of a proportional effect, exactly what both elevation factor and grid scale factor produce. A combined scale factor problem behaves exactly this way, negligible over a hundred meters and inches over a half mile, as the earlier chapters on this companion's own corridor example demonstrated directly.
Any discrepancy that scales up cleanly with distance is behaving like a percentage, not like a fixed offset. Combined scale factor is the first, most likely explanation to check whenever that pattern appears.
Constant Everywhere: A Units or Offset Problem
A discrepancy that stays roughly the same size regardless of how long or short the measured distance is behaves completely differently, and points somewhere else entirely. A foot-definition mismatch on a large coordinate value, or a fixed reference frame offset, both produce this kind of flat, distance-independent gap, since neither one scales with the length of what is actually being measured.
A constant-size discrepancy is not a smaller version of a scale factor problem. Scale factor error is proportional by nature and shrinks toward zero on short distances. A discrepancy that stays the same size on a short line and a long one points toward a units mismatch or a fixed offset instead.
Direction, Not Length: Convergence at Work
A discrepancy that shows up as a consistent angular difference, distances agreeing reasonably well while bearings do not, points toward convergence rather than anything covered so far that affects distance. This pattern is easiest to spot on a long, roughly east-west alignment, exactly the situation where convergence grows largest.
A survey reports distances that closely match an older record, but bearings on the same lines are consistently rotated by a small, similar angle throughout. What does that pattern suggest? Convergence, not scale factor or a units mismatch, since it is affecting direction consistently while leaving distance largely unaffected.
A discrepancy's own behavior is a diagnostic clue. Growing with distance points to scale factor. Staying constant regardless of distance points to a units mismatch or a fixed offset. Showing up in direction rather than length points to convergence. Reading that pattern narrows an investigation before a single additional check is run.
Recognizing a pattern is the first step. Turning that recognition into an actual, repeatable procedure is the last thing this companion has left to build.