By Frank Fortino
Developers of new buildings and those conducting major alterations are hearing more from FEMA than they’re used to.
Since Hurricane Sandy, FEMA has taken an active role in understanding, measuring and upgrading the City’s floodplains and elevations. Base flood elevations are used to determine the required elevation of new buildings in a floodplain. Properly managing the process requires precise, accurate measurements of flood elevations, ground elevations and building elevations.
It goes without saying that if flood elevations are based on one system and ground or building elevations are based an another, things probably won’t work out as planned. Problem is, that’s often the case.
Surveyors and engineers need a consistent starting point to measure elevations, which is known as the datum. The leading system for measuring elevations for much of the 20th century was called National Geodetic Vertical Datum – or commonly NGVD 29. Based on insights and information gathered over the next fifty years, a new system for basing elevations was established, known as North American Vertical Datum, or
Until recently, FEMA followed NGVD 29, yet new FEMA maps, post Sandy, are referencing NAVD 88.
Starting at the beginning of 2014 the DOB switched in step with FEMA, requiring that any submitted land surveys, plot plans and site plans cite to NAVD 88 data. Any data within the survey which referenced NGVD 29 must be converted to the new standard. The submission may keep the earlier datum within the documents, so long as a legend is presented which highlights which datum is which.
These requirements apply to all new projects where initial construction documents were submitted after January 6, 2014. Projects that were previously submitted with NGVD 29 data and were accepted, can stick to the same datum for future submissions.
Converting from NGVD 29 to NAVD 88 is a pretty straightforward calculation. Over time, it’s safe to say that more cities and municipalities will follow FEMA and convert to NAVD 88. Like all cases when the future seems apparent – it’s a good bet to get started sooner rather than later.