Vertical Datum for Flood Maps

 


Vertical Datum for Flood Maps 

 

Snohomish County has been part of the National Flood Insurance Program since the early 1980’s.  As part of this program, the County is required to regulate development within flood-prone areas.  These flood-prone areas are depicted on Flood Insurance Rate Maps (FIRMS), most of which were generated in the early 1980’s for Snohomish County.  Even 30 years ago, the gathering of data for these maps was tedious and difficult.  Remember, we are talking pre-GPS, pre-satellite photos, pre-electronic surveying equipment, pre-computer, and even pre-cell phone.  (How did we ever survive?)  Since the inception of these maps, inconsistencies have shown up and mapping technology has improved.  So, with help from the Federal Emergency Management Agency (FEMA), Snohomish County has been in the process of updating its flood maps and is almost ready to roll out digital versions of the updated FIRM’s, known as DFIRM’s.  These DFIRM’s have incorporated more accurate mapping and data using the latest mapping technology. 

There will be one major change in the maps which has caused some confusion among people not versed in vertical data. The old FIRM’s had a vertical datum officially known as the National Geodetic Vertical Datum of 1929 (NGVD29).  Without getting too technical, this datum is based upon the different sea levels measured at various points along the coasts of Canada and the United States.  The new DFIRM’s use the National American Vertical Datum of 1988 (NAVD88) which is based upon one tidal bench mark in Canada.  The difference between the two data varies across the country depending upon location and elevation.

Here in the lowlands of Snohomish County, the difference between the two data is approximately 3.6 feet, with the NAVD88 value being higher.  As an example, let’s assume that the elevation of the main floor in your house in Monroe is 50.0 feet using the NGVD29 vertical datum.  The new elevation of the main floor of your house is now 53.6 feet using the NAVD88 vertical datum.  Your house did not move up or down or sideways.  We are now simply using a new datum to define its elevation.  This is the same principle with the new DFIRM’s.  In some areas, the flood levels may not change, but the elevation of that flood level will be 3.6 feet higher than the old flood level because of the change in the datum.  So, when the new maps come out, keep this in mind as you compare the old FIRM with the new DFIRM.  

 

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