Research Opportunities
Research Database Documentation
ctd
Introduction to the Database Document series
Fisheries Oceanography Database
Data Structures
Table Summaries
ctd Tables
References
Appendix A
Appendix B
Table 1

Appendix B Data Accuracy

Accuracy of data on the database varies greatly and depends on the measurement methods used.

Table 2 shows the accuracy of measurement of depth, temperature and salinity using different gear methods. The gear methods are listed down the side of the table and the physical quantities are listed along the top. Some quantities may be measured using different techniques for the same gear type (e.g. In a messenger bottle cast depth may be measured in three different ways). The body of the table contains accuracy of measurement and any variations in accuracy due to the measurement techniques used.

Note that according to Intergovernmental Oceanographic Commission Standards salinity is defined against a single reference point (S = 35), which has the same electrical conductivity as a reference potassium chloride solution with a temperature of 15 oC at atmospheric pressure, and is no longer measured in parts per thousand (from Joint Panel on Oceanographic Tables and Standards 1991).



Method

Physical Quantity

Depth

Temperature(oC)

Salinity

Bottle Casts
  • Metre block counter (MBC) ±10% (10m or more at deptha.
  • Cosine of wire angle ±5% (10m or more at depth)b.

  • Protected and unprotected thermometers (PUT)
    ±5m down to 1000m,
    ±5% at depths >1000mc.
  • Usually ±0.02oC, can be as good as ±0.01oC
  • Autolab salinometer ±0.010d
  • Guildline Autosal 8400 salinometer ±0.002e
CTD Probe ±0.15% (<2m at 2000m)e ±0.005oCe ±0.005 after calibration against water samplese, ±0.005 otherwisef

MOCNESS CTD

±5me

0.1e

Not yet tested.

MBTg

±1%

±0.5


XBTe

Mechanical recording

Electronic recording


±2% (a few metres)

±2% (a few metres)


±0.2

±0.1






Table 3: Accuracy of depth, temperature, and salinity data acquired using various methods.



a Estimate only. If the wire angle is less than 5o the length of wire can be used as a measure of depth.

b Estimate based on deviation from correct depth at maximum wire angle of 20o, taking no account of variations in

subsurface currents.

c 0.01oC given in Sverdrup et al (1942) but thermometer performance may change with time.

d Anthony Cole, NIWA, pers. Comm.

e Manufacturer's Manual

 f This is the largest difference between CTD and rosette salinities that has been found from calibration against bottle samples at Greta Point.

g From a set of locally-produced instructions on calibrating the MBT.

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Updated : 16 November 2007