An Elevation Certificate is used to document your building's elevation, so that it can be compared to the estimated height of projected flood waters to help determine "your flood risk and the cost of your flood insurance."
Per FEMA, "If your home or business is in a high-risk area, your insurance agent will likely need an elevation Certificate (EC) to determine your flood insurance premium." These high risk areas, or Special Flood Hazard Areas (SFHA) are described by FEMA as "the area that will be inundated by the flood event having a 1-percent chance of being equaled or exceeded in any given year." The "1-percent" flood event is also commonly referred to as the 100-year storm.
FEMA has put together a very informative guide to help property owners determine if they need an Elevation Certificate:
Letter of Map Amendment (LOMA)
A Letter of Map Amendment (LOMA) is typically issued "because a property has been inadvertently mapped as being in the floodplain, but is actually on natural high ground above the base flood elevation."
A LOMA is an "official amendment, by letter, to an effective National Flood Insurance Program (NFIP) map." Much like how the Elevation Certificate helps to establish the building's elevation as compared to the Special Flood Hazard Area (SFHA), as outlined above, the LOMA, establishes a property's elevation in relation to the SFHA.