Calculate salinity from what is actually measured by a CTD, i.e. conductivity, in-situ temperature and pressure. Often this is done by the CTD processing software, but sometimes it is helpful to do this directly, e.g. when there is a concern about mismatches in sensor response times.
swSCTp( conductivity, temperature = NULL, pressure = NULL, conductivityUnit, eos = getOption("oceEOS", default = "gsw") )
a measure of conductivity (see also
oce object holding hydrographic information. In the second case,
all the other arguments to
swSCTp are ignored.
in-situ temperature (\(^\circ\)C), defined
on the ITS-90 scale; see “Temperature units” in the documentation for
string indicating the unit used for conductivity.
This may be
"" (meaning conductivity ratio),
"S/m". Note that the ratio mode assumes that
measured conductivity has been divided by the standard conductivity
of 4.2914 S/m. In dealing with unfamiliar data for which the measurement
unit has not been recorded, it can be sensible to try all three possibilities
conductivityUnit, to see which yields the most sensible salinities.
equation of state, either
Two variants are provided. First, if
"unesco", then salinity is calculated using
the UNESCO algorithm described by Fofonoff and Millard (1983) as in
reference 1. Second, if
"gsw", then the
Gibbs-SeaWater formulation is used, via
in the gsw package. The latter starts with the same formula
as the former, but if this yields a Practical Salinity less than 2,
then the result is instead calculated using
formulae provided by Hill et al. (1986; reference 2), modified to match the
"unesco" value at Practical salinity equal to 2 (reference 3).
Fofonoff, P. and R. C. Millard Jr, 1983. Algorithms for computation of fundamental properties of seawater. Unesco Technical Papers in Marine Science, 44, 53 pp.
K. Hill, T. Dauphinee, and D. Woods. “The Extension of the Practical Salinity Scale 1978 to Low Salinities.” IEEE Journal of Oceanic Engineering 11, no. 1 (January 1986): 109-12. doi:10.1109/JOE.1986.1145154
gsw_from_SP online documentation, available at
Other functions that calculate seawater properties:
# 1. Demonstrate agreement with test value in UNESCO documents swSCTp(1, T90fromT68(15), 0, eos="unesco") # expect 35 #>  35 # 2. Demonstrate agreement of gsw and unesco, S>2 case swSCTp(1, T90fromT68(15), 0, eos="gsw") # again, expect 35 #>  35 # 3. Demonstrate close values even in very brackish water swSCTp(0.02, 10, 100, eos="gsw") # 0.6013981 #>  0.6013981 swSCTp(0.02, 10, 100, eos="unesco") # 0.6011721 #>  0.6011721