The Lewis County Web Map shows elevations referenced to NAVD88 (North American Vertical Datum of 1988). This will make the elevations shown compatible with new flood studies, recent lidar-based digital elevation models, and the newly resurveyd Chehalis Basin river gauges. (See here for more about the changes to the gauges.)

However, these elevations are not directly compatible with older maps that used NGVD29 (National Geodetic Vertical Datum of 1929). Most significantly, the official FEMA Flood Insurance Rate Maps (FIRMs) still show Base Flood Elevations (BFEs) using NGVD29 because most of those maps in Lewis County have not changed since the early 1980s.

NAVD88 elevations can be converted to NGVD29 but the conversion factor differs from place to place. In Lewis County NAVD88 values are always higher then their equivalent NGVD29 values for a given location. Around our populated areas the conversion factor varies from about 3.3 feet in Vader to about 3.7 feet near Ashford. For instance, a ground elevation of 180 feet NAVD88 is equivalent to about 176.6 feet NGVD29 around Chehalis.

Software titled VERTCON and CORPSCON can convert between elevation datums and are available from NOAA and USACE. A simple to use web-based version of VERTCON for converting one value at a time is available here.

Note: The Official FEMA FIRM maps should always be referenced before making any decisions about flood insurance. Our maps are not official FEMA maps -- ours are just a representation of them for general use and estimation. Furthermore, our maps, data, and applications do not meet legal, engineering, or survey standards. Please practice due diligence and consult with licensed experts before making decisions.

A list of the GIS map layers that show elevations and their descriptions:

2007 HWMs (NAVD88):
Chehalis Basin 2007 flood event High Water Marks (HWMs) from field observations. Labeled in our program using NAVD88 datums, just as the name implies. It also lists the elev in NGVD29 by clicking on the point. Not all of these HWM points have elevations determined -- some are just locations where evidence of high water was observed.
2007 Water Elevation (NAVD88):
Chehalis Basin 2007 flood event water elevations as determined by consultant models. These lines show approximate water elevations in NAVD88.

The Base Flood Elevation locations and elevations of the 100-year flood taken from FEMA's FIRM maps. The labels for these in our Lewis County GIS Web Map are referenced to NAVD88.
These elevations were converted from the FEMA BFEs NGVD29 data using averaged conversion factors for each river. The averaged conversion factors used are those listed in the FEMA 2010 Preliminary Flood Insurance Study for Lewis County. The original NGVD29 values are available in the program by clicking on the BFE line.
FEMA Panels:
These do not explicitly show elevations on our Lewis County GIS Web Map, but the Official FEMA maps themselves will show elevations listed per NGVD29 datum. You can see the Official FEMA maps by clicking inside a panel area and then clicking on the link to the scanned map.

The ground contours are derived from lidar surveys collected from 2005 to 2012 and show elevations referenced to NAVD88.

Elevation Certificates:
These do not explicitly show elevations on our program, but the certificates themselves will show elevations of various features. These are usually listed in NGVD29 datum as a requirement by FEMA. Some may also list NAVD88 values in addition the NGVD29 values if they were originally surveyed as NAVD88.
USGS Topo basemap:
These backgound images show scanned USGS Topographic maps with 20 foot or 40 foot contour intervals depending on the location. The elevations shown on this basemap are refernced to NGVD29 datum. But the maps are typically much more generalized than our other data and these elevations are only expected to be accurate to within half of the countour interval: +/- 10ft or +/-20ft. Therfore the NAD88 to NGVD29 conversion difference is negligable compared to the accuracy of these elevations.