Hardy Severe Watches & Warnings NOAA Weather Radio

Public Information Statement
Statement as of 12:14 PM EDT on April 1, 2015

... Elevation adjustment for Washington mean low water begins
Monday April 6th...

On Monday April 6th... the base elevation of Washington mean low
water /wmlw/ will be slightly adjusted... affecting the Potomac
River at Wisconsin Avenue/Georgetown... site ID gtnd2. This gauge
reports the water height above Washington mean low water... and
will continue to do so.

As part of a study conducted by the US Army corps of engineers...
the elevations of benchmarks from the National geodetic survey
listed in the North American vertical datum of 1988 /navd88/ were
compared to the elevations of those same benchmarks above
Washington mean low water as denoted in the corps Potomac Park
manual. It has been determined through this study that the
difference between navd88 and wmlw is 2.26 feet.

This comparison is deemed more accurate because the previous
difference of 2.18 feet was based on a computer-based conversion
from yet another datum... the National geodetic vertical datum of
1929 /ngvd29/.

The difference is so small /about one inch/ that it has no impact
on flood stage or flood categories at the site. Data will continue
to be reported as height above Washington mean low water just as
it has been. This message is simply to notify users who reference
navd88 that the conversion to that elevation will change.

Users who have any questions should email
senior service hydrologist Jason Elliott at: