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Open-File Report 2007–1194

Documentation of the U.S. Geological Survey Oceanographic Time-Series Measurement Database

Network Common Data Format (netCDF) Storage

Variable Attributes

Each variable in the file has its own attributes to describe and quantify the contents. The descriptors in the left column of table 7 are found in most variables; the column on the right contains sample values. If the parameter has more than one dimension, the minimum and maximum may be vector quantities instead of scalars. The minimum and maximum of the data are of the same units as the data—in the example in table 7, because the transducer temperature is “degrees C,” the maximum and minimum are as well. The sensor depth (water depth minus sensor height) is always meters. _FillValue is the number that represents erroneous or missing data in a time series. Sometimes the software may display _FillValue as 1.00000004091848e+035, but it is truly 1.0e+35. The valid_range attribute, if present, specifies the potential range of acceptable data.

Table 7: Attributes associated with each variable.

Attribute Example Value
name "Tx"
long_name "ADCP Transducer Temp."
generic_name "temp"
units "degrees C"
epic_code 1211
sensor_type "RD Instruments ADCP"
sensor_depth 30.558629989624
serial_number 138
minimum 4.65000009536743
maximum 12.0299997329712
valid_range [-5 40]
_FillValue 1.00e+035

There may also be a “note” or “comment” added to document a sensor failure, such as “optical path obscured on Jan. 10, 2005; subsequent data replaced with fill value.”

Variable Names

Variable names in our files are formed from the EPIC key (ftp://ftp.epic.noaa.gov/epic/epic.key) by combining the abbreviation code and the number. The EPIC key lists code 1 as a pressure, abbreviated as “P,” in decibars, so the variable name that contains pressure data (in decibars) is “P_1.” Similarly, ocean temperature data has the abbreviation T for temperature and EPIC code 28 indicates it is from sea water of ITS–1990 standard, so the variable name is “T_28.” In the CF version of a file, the T_28 variable would have a “standard_name” attribute of “seawater_temperature” from the CF vocabulary, but is otherwise the same. When more than one sensor of a type is logged by the same instrument, a digit is added so the name becomes “Sed1_981” or “Sed2_981” for sediment concentration data. The most often used variable names are listed in table 8. When variable names are not present in the EPIC key, the CF standard name or another appropriate name is used.

Table 8 provides a subset of data variable names, the equivalent CF standard_name, and units for variables often found in the database, by type of measurement. Each file will contain only the variables appropriate to the kind of data the instrument collected. Alternate names for variables used in the older, pre-EPIC files are also listed.

Table 8: Data variable names, by type of data.
[cm/s, centimeter per second; m, meter; dbar, decibar; Pa, pascal; mbar, millibar; °C, degree Celsius; ITS–1990, International Temperature Scale of 1990; ADCP, acoustic Doppler current profiler; S/m, siemens per meter; PSU, practical salinity unit; microEinstein/m2/s, microEinstein per square meter per second; kg/m3, kilogram per cubic meter; %, percent; m/s, meter per second; NTU, nephelometric turbidity unit; W/m2, watt per square meter; --, not applicable]

Variable name Alternate variable name CF standard name attribute Contents (units)

Currents

u_1205 East eastward_seawater_velocity eastward velocity (cm/s)
v_1206 North westward_seawater_velocity northward velocity (cm/s)
w_1204 -- upward_seawater_velocity vertical velocity (cm/s)
CD_300 vdir_1 seawater_direction current direction (degrees)
CS_310 vspd_1 seawater_speed current speed (cm/s)
Werr_1201 -- -- error velocity (cm/s)
AGC_1202 -- -- average echo intensity (percent)
PGd_1203 -- -- percent good pings (percent)
hght_18 -- -- height of sea surface (m)

Hydrographic properties

P_1 pressure sea_water_pressure pressure (dbar)
P_1ac pressure_corrected sea_water_pressure_due_to_sea_water pressure (dbar)
P_4 pressure sea_water_pressure pressure (pascals)
P_4023 -- sea_water_pressure average burst pressure (mb)
T_28 temperature sea_water_temperature temperature (°C) (ITS–1990 standard)
Tx_1211 -- sea_water_temperature ADCP transducer temperature (°C)
C_51 conductivity sea_water_ electrical_conductivity conductivity (S/m)
S_40 salinity sea_water_salinity salinity (parts per thousand)
S_41 salinity sea_water_salinity salinity (PSU)
STH_71 sigma_theta -- density (kg/m³)
Ph_159 -- sea_water_ph_reported_on_total_scale pH
PAR_905 -- downwelling_photosynthetic_radiance_flux_in_sea_water Photosynthetically Active Radiation (microEinsteins/m2/s)
Fch_906 -- mass_concentration_of_chlorophyll_in_sea_water Chlorophyll-a (fluorometric) (kg/m3)
OST_62 -- volume_fraction_of_oxygen_in_sea_water Dissolved Oxygen saturation (%)

Turbidity

tran_4010 -- -- transmission (volts)
ATTN1_55 -- volume_beam_attenuation_coefficient_of_radiative_flux_in_sea_water beam attenuation (m-1)
NEP1_56 -- -- backscatter intensity (volts)
Turb -- sea_water_turbidity turbidity (NTU)

Meteorology

T-21 -- air_temperature air temperature (°C)
WD_410 -- wind_from_direction wind direction (degrees)
WS_401 -- wind_speed wind direction (m/s)
WG_402 -- -- wind gust speed (m/s)
BP-915 -- air_pressure barometric pressure (Pa)
RH_910 -- relative_humidity relative humidity (%)
Qs_133 -- net_downward_shortwave_flux_in_air shortwave radiation W/m2
PAR_905 -- Surface_downwelling_photosynthetic photosynthetically Active Radiation (microEinsteins/m2/s)
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