Reference documents

Data held in the ODMS are usually archived in the units specified by the International System of Units, commonly known by the French abbreviation “SI” (Système International d'unités). However, instruments frequently do not measure data in these same units, making conversion necessary. In addition, many variables have traditionally been recorded in non-SI units and their use remains common in the scientific community. For the sake of simplicity, these traditional units are archived (water column pressure in decibars is a good example).

Below we give information on the most common variables measured, the units they are reported in, and unit conversions (including some units that were formerly used but that are now obsolete). The list is not exhaustive; if additional information is needed, there are many on-line sites available that are dedicated to unit conversion (e.g., http://physics.nist.gov/cuu/index.html or http://www.digitaldutch.com/unitconverter).

We also include methods for converting latitude and longitude to decimal degrees and local time to GMT. 

Position (archived in decimal degrees)

By convention, latitude north and longitude east are positive.

• degrees + (minutes.mm / 60) = decimal degrees
  example: 45°45.0' W = –45.750°
• degrees + (minutes + (seconds.ss/60))/60 = decimal degrees
  example: 45°30'30.0'' N = 45.50833°
• to convert decimal degrees to degrees minutes.mmm, multiply the decimal portion by 60 to get decimal minutes
  example: 45.2500°N = 45+(0.25*60) = 45°15.0'

Time (archived in UTC)
UTC (Universal Time Coordinate) or Z time (Z for zero meridian or Zulu time). Before 1972, universal time was known as Greenwich Mean Time (GMT), but this term is no longer in use. Convert local time to UTC by adding the offset in the western hemisphere and subtracting it in the eastern hemisphere. Don't forget that changing times around midnight might also result in a change of the date.

Standard Time
Eastern Atlantic Newfoundland	EST AST NST	+5 h +4 h +3.5 h	=GMT =GMT =GMT
Daylight Time
Eastern Atlantic Newfoundland	EDT ADT NDT	+4 h +3 h +2.5 h	=GMT =GMT =GMT

examples :	0800 EST + 5 hours = 1300 GMT
	2000 16 May EDT + 4 hours = 2000 17 May GMT

To indicate local time, specify a positive offset west of 0° longitude and a negative offset east of 0° longitude. For example, to indicate Eastern Standard Time, one would specify –0500.

Air temperature (archived in °C)
° C = (°F-32)*(5/9)
example: (50°F-32)*(5/9) = 10°C

Atmospheric pressure (archived in hectopascals, hPa)
millibars (mb) hPa
1 atm=101325 Pa = 760 mm Hg = 1013.25 hPa = 1013.25 mb = 29.921261 in Hg
in of mercury * 33.86388 = mb = hPa

Cloud cover (World Meteorological Organization [WMO] code table 2700 for recording cloud amount)
To use this table, the sky is divided either into eighths (“okta”) or tenths and the cloud cover is estimated.

Code		Cloud cover
0		Zero		Zero
1		1 okta or less, but not zero		1/10 or less, but not zero
2		2 oktas		2/10 to 3/10
3		3 oktas		4/10
4		4 oktas		5/10
5		5 oktas		6/10
6		6 oktas		7/10 to 8/10
7		7 oktas or less, but not 8 oktas		9/10 or more, but not 10/10
8		8 oktas		10/10
9		Sky obscured, or cloud amount cannot be estimated

Sea state (WMO code table 3700 for recording sea state)

Code		Description		Wave height (m)
0		Calm-glassy		0
1		Calm-rippled		0-0.1
2		Smooth-wavelet		0.1-0.5
3		Slight		0.5-1.25
4		Moderate		1.25-2.5
5		Rough		2.5-4
6		Very rough		4-6
7		High		6-9
8		Very high		9-14
9		Phenomenal		>14

Secchi disk depth (m)
A measure of water clarity

Wind speed estimation: the Beaufort Wind Scale (marine)
The Beaufort Scale was originally developed in 1805 by Sir Francis Beaufort as a system for estimating wind strengths without the use of instruments. It is still in use for this same purpose as well as to tie together various components of weather (wind strength, sea state, observable effects) into a unified picture.

Even though the Beaufort Scale is not used in the ODMS archive, we include it here to dispel any confusion between it and the WMO code table 3700, which we use for describing sea state.

Force		Wind speed				Marine Conditions
		Knots		mph
0		<1		<1		Calm, sea like a mirror
1		1-3		1-3		Light air, ripples only
2		4-6		4-7		Light breeze, small wavelets (0.2m), crests have a glassy appearance
3		7-10		8-12		Gentle breeze, large wavelets (0.6m), crests begin to break
4		11-16		13-18		Moderate breeze, small waves (1m), some white caps
5		17-21		19-24		Fresh breeze, moderate waves (1.8m), many white caps
6		22-27		25-31		Strong breeze, large waves (3m), probably some spray
7		28-33		32-38		Near gale, mounting sea (4m) with foam blown in streaks downwind
8		34-40		39-46		Gale, moderately high waves (5.5m), crests break into spindrift
9		41-47		47-54		Strong gale, high waves (7m), dense foam, visibility affected
10		48-55		55-63		Storm, very high waves (9m), heavy sea roll, visibility impaired. Surface generally white
11		56-63		64-73		Violent storm, exceptionally high waves (11m), visibility poor
12		64+		74+		Hurricane, 14m waves, air filled with foam and spray, visibility bad

Wind speed (archived in m/s)
knots * 0.514 = m/s

Wind direction (degrees)
The wind direction is by convention the direction from which the wind blows and is measured as degrees from true north.
0: wind from the north; 90: from the east; 180: from the south; 270: from the west

The most common physical variables held in the ODMS archive include:

Altimeter (m)
Distance of the instrument off-bottom

Conductivity (archived in S/m)
Siemens were formerly called “mho”; mho are still commonly used. Siemens are the reciprocal of ohms: 1 ohm = 1/S.

Current components (m/s)
The north-south (V) component is positive toward the geographic north.
The east-west (U) component is positive toward the east.

Current direction (degrees)
By convention, current direction is the direction toward which the current flows, measured in degrees from true or magnetic north.
0: current flowing toward the north; 90: flowing toward the east; 180: flowing toward the south; 270: flowing toward the west

Current direction; magnetic declination (degrees)
The angle between the magnetic and geographical meridians at any place, expressed in degrees east or west to indicate the direction of magnetic north from true north

Density (archived in kg/m³)
Density of seawater

Density, sigma-t (archived in kg/m³)
Sigma-t = the density of seawater – 1000 kg/m³

Depth (m)

Descent rate (m/s) of the profiler (e.g., a ctd)

Fluorescence (archived in mg/m³)
An index of the amount of chlorophyll in the water

Oxygen (archived in mL/L)
The amount of oxygen dissolved in the seawater. It can be expressed in many different units (mg/L, mL/L, µmol kg, % saturation; see the next section for unit conversions). The unit used most frequently is mL/L. The percent saturation corresponds to the measured value divided by the value for 100% saturation of oxygen in seawater (calculated from water temperature and salinity) multiplied by 100.

Oxygen sensor current (archived in µA)
Older oxygen sensors used sensor current and sensor temperature to calculate dissolved oxygen

Oxygen sensor temperature (archived in °C)
Older oxygen sensors used sensor current and sensor temperature to calculate dissolved oxygen

PAR radiation (Photosynthetically Active Radiation; archived in µE/s/m²)
Units currently used include moles, einsteins (E), photons, and quanta:
1 µmol/s/m² 1 µeinstein/s/m² 6.02*10¹⁷ photons/s/m² = 6.02*10¹⁷ quanta/s/m²

Pressure (db, decibars)

Salinity (unitless)
The calculation of salinity is a ratio so there are no units. One will frequently see salinity recorded in units of “PSU” to indicate “practical salinity unit,” making reference to the Practical Salinity Scale 1978 (PSS78).

Temperature (°C)
The calculation of temperature is based on the International Temperature Scale of 1990 (ITS-90) or the International Practical Temperature Scale of 1968 (ITPS-68)
° C (ITS-90) = °C (ITPS-68)/1.00024

Transmissometer (%)
Measurement of light transmission (index of the concentration of suspended matter)

Data from bottle samples. The SI unit for the amount of a substance is mole, abbreviated as "mol."

Dissolved oxygen (archived in mL/L; SI unit is mmol/m³)
mL/L * 44.66 = mmol/m³
mL/L = mg/L * (1 mL/1.42903 mg)
mg/L = mL/L * 1.42903 mg/mL
mg/L mg/kg
mg/L ppm

Nutrients (archived in mmol/m³)
The most common nutrients measured are nitrite, nitrate + nitrite, phosphate, silicate
µmol/L = mmol/m³
M gram molecular weight per liter (or molar concentration)
µg-atoms/L mg-atoms/m³ µM µmol/L
µg/L = mg/m³
ppm mg/L
ppb µg/L
µg/L / atomic mass = µg-atoms/L
µg/L / molecular mass = µM = µmol/L

• unspecified or complex compounds (including chlorophyll) (precise molecular mass unknown):
  mg/m³ = µg/L
  mg/L = g/m³
• POC (particulate organic carbon): (µg L)/12.01 = mmol/m³ (archived in mmol/m³)
• PON (particulate organic nitrogen): (µg L)/14.01 = mmol/m³ (archived in mmol/m³)

0: No quality control (QC) has been performed on the value
1: QC performed, value appears correct
2: QC performed, value appears inconsistent with other values
3: QC performed, value appears doubtful
4: QC performed, value appears erroneous
5: Value has been changed as a result of QC
6-8: Reserved for future use
9: Value of the parameter is missing

1 degree of latitude = 60 nautical miles
1 minute of latitude = 1 nautical mile = 1852 m
1 nautical mile = 1.85 km = 1.15 statute miles
1 fathom = 6 feet = 1.83 m
1 knot = 1 nautical mile per hour