The Elevation Certificate and the Approved Plan That Is 1.24 Feet Off
Your firm is completing a FEMA Elevation Certificate for a new single-family house in unincorporated Chatham County. The site is in Zone AE with a base flood elevation (BFE) of 812.0 ft NAVD88, and the county floodplain ordinance requires the lowest floor to be at least 1.0 ft above the BFE. The builder's approved site plan states a finished floor elevation (FFE) of 813.84 ft referenced to NGVD29. A static GNSS/RTK shot on the slab returns 812.44 ft NAVD88 with a 95% vertical uncertainty of 0.09 ft. A closed level run from a published NAVD88 benchmark returns 812.60 ft. The published local offset between the datums is NGVD29 minus 1.24 ft equals NAVD88. The builder asks you to 'just use 813.84, since it is already on the approved plans and the county has signed off.'
1. Convert the plan elevation and determine whether the house complies with the freeboard requirement. Show the arithmetic.
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Convert the plan value to the certificate datum:
813.84 (NGVD29) − 1.24 = 812.60 ft NAVD88
That matches the differential level run (812.60 ft) exactly, which strongly corroborates both the conversion and the leveling. The RTK shot of 812.44 ft differs from the level run by 0.16 ft, which is larger than the stated 95 percent uncertainty of 0.09 ft, so the RTK value is not what you certify.
Compliance check: required lowest floor = BFE + 1.0 = 812.0 + 1.0 = 813.0 ft NAVD88. The as-built floor is 812.60 ft. It is 0.40 ft short of the local freeboard requirement, though it is 0.60 ft above the BFE and therefore meets the minimum NFIP standard.
The plan was never 'wrong'; it was expressed in a different datum. The error would arise only from treating 813.84 as a NAVD88 number, which would overstate the floor by 1.24 ft and make a non-compliant floor look like it had 1.84 ft of freeboard.
2. Which elevation goes on the Elevation Certificate, and why? Address GNSS versus differential leveling and the reporting precision.
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812.6 ft NAVD88 goes on the certificate.
Reasons:
- The certificate is a certified statement of fact, and the surveyor signs it. The number must be the one the evidence supports, not the one on a plan.
- Differential leveling from a published NAVD88 benchmark is the appropriate method here: it is directly tied to the datum, it is checkable against a second mark, and it produced a result identical to the converted plan value. A GNSS-derived orthometric height depends on a geoid model and on the stated uncertainty; when two independent methods disagree by more than the stated uncertainty, the surveyor resolves the difference rather than picking the convenient number.
- FEMA forms expect elevations reported in the datum of the certificate to the precision the form requests - typically tenths of a foot - with the datum stated and the benchmark identified in the comments.
Before signing, run a second published mark as a check if one is available, and record the benchmark used, the loop data, the adjustment, and the RTK result with its uncertainty. If the two-level check disagrees beyond tolerance, resolve it before certifying; do not average your way to a number.
3. How do you handle the builder's request and the county's prior approval? Describe your communications and your professional obligations.
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Decline, in writing, and explain the consequence rather than just the refusal. Tell the builder: the plan elevation of 813.84 is an NGVD29 value that equals 812.60 NAVD88; the certificate must state 812.6; and at 812.6 the lowest floor is 0.40 ft below the county's required 813.0. Using 813.84 on the certificate would misstate the elevation by 1.24 ft in the direction that makes a deficient floor look compliant, which is a false certification and would also expose the owner to flood loss and to a retroactive enforcement problem.
Notify the owner and the local floodplain administrator that the as-built floor does not meet the freeboard requirement. The county's earlier approval was based on a plan expressed in NGVD29 without conversion; the administrator needs the fact to decide whether to require a variance, a raised floor, or another remedy. Do not let the certificate become the document that hides a known discrepancy from the agency.
Do not sign, and do not alter the form to match the plan. If pressed, withdraw from the certificate rather than issue it. Document every conversation, keep the field data, and be prepared to explain the datum conversion to the county in plain terms. The professional obligation runs to the accuracy of the certified statement, not to the schedule of the closing.