Public Information Statement
Issued by NWS Chicago, IL

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NOUS43 KLOT 162014
PNSLOT

SERVICE CHANGE NOTICE
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE, IL
315 PM CDT TUE APR 16 2019

To:   Family of Services (FOS) subscribers, NOAA Weather Wire
      Service (NWWS) subscribers, Emergency Managers Weather
      Information Network (EMWIN) subscribers, NOAAPort
      subscribers, National Weather Service (NWS) partners
      and other users, and NWS employees.

From: W. Scott Lincoln
      Senior Service Hydrologist
      NWS Chicago

Subject: Change to the datum, or gage zero, for the Rock River at
Latham Park, Illinois

Effective immediately, the datum, also known as gauge zero, will
change at the U.S. Geological Survey (USGS) stream gauge on the Rock
River at Latham Park, Illinois. The datum is the elevation used as a
reference point for measurement of river stage.

Prior to today, the datum was indicated as 700.00ft NGVD29, or
699.81ft NAVD88. Based upon a recent survey by the USGS, it was
determined that the previously-used datum was incorrect and was
subsequently corrected today to 698.66ft NAVD88.

For most users, this change will have no impact. Stage values will
remain relatively the same for the time period of automated
observations. For example, a stage value of 10.0ft today will mean
the same as 10.0ft yesterday. The corrected datum will now provide a
more accurate means of calculating elevation from stage. Most
historical crests will remain the same except for crests from high
water marks collected prior to installation of the automated gauge.
A few flood impact statements have also been updated.

No changes are being made to forecast and warning information which
is available from the NOAA/NWS Chicago web page at
weather.gov/chicago as well as the NOAA/NWS Advanced Hydrologic
Prediction Service (AHPS) web page at water.weather.gov.

If you have any questions regarding this change, please contact:

W. Scott Lincoln
scott.lincoln@noaa.gov
815-834-0600 x493


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