Differences between LMSL and MLLW

[Harvey Greenberg]

Keven, with at least one of our experts on vacation, I will attempt a
partial answer.  In Puget Sound (See
http://students.washington.edu/~dfinlays/pugetsound/presentation/newsound.htm),
we dealt with the difference between MLLW and MSL.  Those values are
substantially different, and the difference varies substantially through the
Sound.   David Finlayson gathered MLLW values for every tide station, and
interpolated the values.  This difference "surface" was used to adjust bathymetric
data to the same vertical datum as the NAVD29 topographic data.

We also attributed the shoreline with the value of MLLW and used it with the
bathymetric points in building a surface.  We were particularly dissatisfied with
the interpolation between the lowest terrestrial contour and the shoreline in
the creation of the USGS DEMs (not that we are happy with the upland
interpolations), and looked into replacing part of the USGS DEM with our
own interpolation.  However, we have not come up with a technique that works
well on all natural and altered shorelines.

If you can find the difference between MSL and NAVD29, use it, but I don't
expect it to be substantial value.  As for geoids, stick to the one that mathed
your horizontal datum.

   ~harvey

>     1.  What is the difference between LMSL (local mean sea level) and
> MLLW in Willapa Bay in meters, on average, within the bay?
>     2.  I have a paper that explains the evolution and technical issues
> concerning  controlling the NAVD88 datum, and in it, it explains that
> there may be local differences between MSL as represented in NAVD88 and
> LMSL for some locations within North America.  If I find LMSL is
> different in Willapa Bay from MSL for NAVD88 for that area, does this
> mean that I should correct the elevation of the DEM for Willapa bay (I
> have a decimeter sheet given to me by Harvey Greenberg, so maybe he
> knows), or has this already been done?
>     3.  If there are updates made to NAVD88, or NAVD29, do I need to
> grab new geoid info for ArcView and/or Arc/INFO so that corrections
> don't need to be made?
>     Thanks
>
>     Keven Bennett

--
hgreen at u.washington.edu  162 Johnson Hall  206-685-7981
http://duff.geology.washington.edu/

        Harvey Greenberg
        University of Washington
        Department of Earth and Space Sciences
        63 Johnson Hall (Box 351310)
        Seattle WA 98195-1310