Vertical positions — such as a flood elevation certificate's figures — are referenced to:
Correct Answer
A) A vertical datum defining the elevation reference
A vertical datum provides the reference for reported heights or elevations. A benchmark realizes a known height relative to a specified reference; contour interval describes spacing between contour elevations. Different datums cannot be directly compared without the appropriate transformation.
Why This Is the Correct Answer
Option A is correct because vertical measurements refer to a known point of vertical height known as a datum. A vertical datum defines the reference for reported elevations, such as those on a flood elevation certificate. Elevations on different datums cannot be compared directly without the appropriate transformation.
Why the Other Options Are Wrong
Option B: A zoning setback line on the plat
Option B is wrong because a zoning setback line on the plat is a horizontal distance requirement. It does not define a reference for heights.
Option C: A privately selected mark with no stated height reference
Option C is wrong because a privately selected mark with no stated height reference has no defined relationship to a recognized elevation system. Elevation figures need a stated vertical datum to be meaningful.
Option D: A contour interval on a topographic map
Option D is wrong because a contour interval on a topographic map is the vertical spacing between contour lines. It describes spacing, not the reference that elevations are measured from.
Datum is ground zero
An elevation is a height above some zero line, and that zero line is the datum.
How to use: Use this when a question asks what elevation figures are referenced to.
Exam Tip
Separate the reference for heights (datum) from spacing (contour interval) and horizontal lines (setbacks).
Common Mistakes to Avoid
- -Confusing a contour interval with a vertical datum
- -Comparing elevations from different datums without conversion
- -Treating a setback line as a height reference
Concept Deep Dive
Analysis
This question tests what elevations are measured from. Vertical measurements refer to a known reference of vertical height called a datum. Flood elevation certificate figures are meaningful only when tied to a stated vertical datum.
Background Knowledge
A vertical datum is the reference surface from which elevations are measured. Benchmarks are physical marks with known heights relative to a datum. Flood maps and elevation certificates must use the same datum for their figures to be compared.
Real-World Application
An appraiser compares a home's elevation certificate with the BFE on the flood map and notices the two use different datums. The appraiser does not compare them directly and relies on a properly converted figure from a qualified source.
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