Bmm Ngvd-navd

NGVD Æ NAVD?

Regulatory floodplains are defined by the elevation of the base flood in relation to the elevation
of the ground. Base flood elevations are used to determine the required elevation of new
buildings in the floodplain. Floodplain management will not succeed without accurate
measurements of flood elevations, ground elevations and building elevations. Needless to say, if
flood elevations are based on one system and ground or building elevations are based an another,
things won’t work.
NGVD 29 stands for National Geodetic Vertical Datum of 1929.
It is a system that has been used by surveyors and engineers for
most of the 20th Century. It has been the basis for relating ground
and flood elevations, but it has been replaced by the moreaccurate North American Vertical Datum of 1988 (NAVD 88).
Because it has such an impact on floodplain management, it is
important for local officials to understand what’s happening.
First, what is a “datum?” If we say that a flood will rise to 100
feet, one must ask “100 feet above what?” The starting point for
measuring elevations is our datum. We need a consistent starting
point so we can compare flood and ground elevations. In most
cases, we mean “above sea level.” But, some inland communities’
elevation records were developed in relation to some other
starting point. For example, Chicago City Datum started from the
level of Lake Michigan.

Effective floodplain
management depends on
accurate surveying

The National Geodetic Survey, the government people responsible for mapping, needed a
common, consistent national datum to map the whole country. During the 1920s, NGS
established a network of 26 tidal gauges in the United States and Canada. Maps were prepared
with elevations based on “mean Sea Level Datum of 1929.” In the 1970’s, the name was changed
to the National Geodetic Vertical Datum (NGVD) of 1929.
One of the reasons for the name change was that it was found that the sea is actually not level.
There are local variations caused by currents, wind, barometric pressures, temperature,
topography of the sea bed, and salinity differences. NGS ran more surveys around the country
and had trouble making the numbers fit because mean sea level at one location was higher or
lower than mean sea level elsewhere. This leveling work also found that ground elevations had
risen or fallen, due to earthquakes, subsidence, and rebounding of the earth that has continued
since the glaciers left. New satellite technology has discovered distortions in surveyed elevations
caused by gravity.
Because of these shortcomings, the NGS has established a new system to base elevation
measurements. The North American Vertical Datum of 1988 corrects many of the problems with
NGVD 29. It is also based on satellite systems that account for differences in gravitational forces
in different areas.

March 30, 2007

One can readily convert elevations in one datum to those based on
another. For example, zero in the Chicago City Datum is 579.48
feet above zero (“mean sea level”) in NGVD 29. If one tries to
compare a ground elevation in CCD to a flood elevation in
NGVD 29, the 579 foot difference will make it readily apparent
that something is off. A simple formula can convert elevations
from one datum to the other.
It’s not so easy converting to NAVD 88, though. The North
American Vertical Datum is the product of thousands of correcAccurate elevation surveying
tions in elevation data. In the Rocky Mountains (where gravitastarts from a dependable
tional forces caused a lot of distortion to traditional surveys) the
elevation reference mark.
difference can be three feet or more. In other areas, the difference
may be inches. It takes a computer program called VERTCON to relate the two systems at any
one point. However, it must be noted that VERTCON 2.0 is not to be considered reliable beyond
the boundaries of the lower 48 United States.
Up until recently, most FEMA Flood Insurance Rate Maps used NGVD 29. However, FEMA’s
new maps are now using NAVD 88 as the basis for published flood elevations. If local surveyors
or your community have not made the switch, errors will arise unless elevations in NGVD 29 or
a local datum are converted to NAVD 88.
What is most important is that the same datum be used consistently. Since the base flood
elevations used by the NFIP are on the FIRM, the FIRM datum must be used for the FEMA
Elevation Certificate, LOMAs, LOMRs and other insurance-related purposes.
A community and the surveyors in the community may
normally use NAVD 88 for most purposes, but if the
community’s FIRM uses NGVD 29, then NGVD 29 must
be used for all flood, ground, and building elevations on
Elevation Certificates and other NFIP applications.
It is basically the responsibility of the professional
surveyor, engineer or architect to use the appropriate
datum on FEMA documents. However, the community
must be aware of the potential for errors if the datums are
mixed. You don’t need to know the conversion factor
between two datums, but you do need to ensure that the
same datum is used for all elevations on the same
document. In time, that datum will be NAVD 88 for just
about every community. Meanwhile, local officials
should review their bench marks and other elevation
reference marks to ensure that they state which datum
they reference and that they are consistent with any code
requirements.
For more information on datums and their use in FEMA
mapping, see www.fema.gov/pdf/fhm/frm_gsab.pdf

Elevation Certificates must have
flood, ground, and building elevations
based on the same datum

March 30, 2007