Elevation certificate
A FEMA form completed by a licensed surveyor that establishes how your building sits relative to the base flood elevation. It is usually about insurance money.
All of it tied to a published vertical datum, not an assumed benchmark.
This is the part worth knowing about, because it is where the real money sometimes is.
Flood maps are drawn at a scale that cannot see individual lots. A property sitting on higher ground inside a broadly mapped zone can be included simply because the mapping was not fine enough to exclude it.
If a survey demonstrates the structure sits above the base flood elevation, you can apply to FEMA for a Letter of Map Amendment. A successful LOMA removes the mandatory insurance requirement, and on a property carrying a large premium the survey pays for itself quickly.
Worth checking before you assume. If you are paying meaningful flood premiums and your house sits noticeably higher than the surrounding ground, an elevation certificate is a cheap test of whether you should be paying at all.
FEMA revises flood mapping periodically. A property outside a zone ten years ago may be inside one now, and the reverse happens too. An elevation certificate reflects the map in effect when it was prepared, so an old certificate may no longer describe your situation.
Send the address and your current premium. An elevation certificate is often the cheapest way to find out whether it is justified.