Measurement errors as documented in metadata for
source coverages; in addition, there may exist a datum shift
among the three bathymetric coverages. In other words,
for each data set, the measured depths had to be
standardized to some benchmark elevation. For example,
a depth can be recorded at -3 ft., but we need to know the
answer to the question, ?Three feet below what?? We do
not have information on how the historical data sets were
referenced, but we suspect that the reference points were
probably not the same. Komar (1992) suggested that data
on global sea level and local elevation changes indicated
that, between 1931 and 1988, sea level has risen relative
to the Tillamook area at a rate of 1 to 2 mm/yr. Assuming
this rate has held since the first bathymetric survey, the rise
in sea level in Tillamook Bay between 1867 and 1995
would be about 13 to 26 cm (0.4 to 0.8 ft). The official
vertical datum shift of mean sea level was 4 ft between
1867 and 1957 and 3 ft between 1957 and 1995. Both of
these vertical shifts in sea level were shifted upwards
(Bruce Follansby, TBNEP, pers. comm.).
The inferred changes from 1957 to 1995- many more areas
were inferred to have become deeper than to have
become shallower. No datum shift is assumed because we
do not know what shift to apply to standardize these data
sets.
see source metadata