Determining important information including seafloor depth, instrument depth and height above sensor from IMOS Moorings velocity data.
The vertical reference and vertical information provided in the dataset collections from Acoustic Doppler Current Profilers (ADCP) are the same as the ones provided in the original instrument file and it is crucial for users to have a good understanding of them in order to analyse this data confidently and successfully. Key metadata is found in the corresponding netCDF either in the global attributes or variable attributes.
Looking at the current direction measured by the ADCP, found in the CDIR variable:
float CDIR(TIME, HEIGHT_ABOVE_SENSOR) ;
CDIR:coordinates = “TIME LATITUDE LONGITUDE HEIGHT_ABOVE_SENSOR” ;
CDIR:compass_correction_applied = 13. ;
CDIR:comment = “A compass correction of 13degrees has been applied to the data by a technician using RDI’s software (usually to account for magnetic declination).” ;
CDIR:standard_name = “direction_of_sea_water_velocity” ;
CDIR:long_name = “direction_of_sea_water_velocity” ;
CDIR:units = “degree” ;
CDIR:reference_datum = “true north” ;
CDIR:positive = “clockwise” ;
CDIR:valid_min = 0.f ;
CDIR:valid_max = 360.f ;
CDIR:_FillValue = 999999.f ;
CDIR:ancillary_variables = “CDIR_quality_control” ;
CDIR:magnetic_declination = 12.6333333333333 ;
The variable attribute ‘coordinates’ lists the spatial and temporal coordinate variables included in the file that are relevant to the variable CDIR. Each current direction data has been collected at a time and location described in the following variables TIME, LATITUDE, LONGITUDE and HEIGHT_ABOVE_SENSOR.
This ADCP uses a magnetic compass so a compass correction of 13 degrees has been applied to transform the directional reference from Magnetic North to True North, which is consistent with the magnetic declination (difference between True North and Magnetic North) calculated at this location and time (‘magnetic_declination’ = 12.63).
The variable CDIR is associated to an ancillary variable CDIR_quality_control which provides quality flags for each measurement found in CDIR.
Because the ADCP has been collecting several measurements over time and across the water column but always at the same geographic location CDIR is only a function of TIME and HEIGHT_ABOVE_SENSOR. Metadata about these variables are as follows:
double TIME(TIME) ;
TIME:comment = “Time stamp corresponds to the start of the measurement which lasts 299.7 seconds. timeOffsetPP: TIME values and time_coverage_start/end global attributes have been applied the following offset : -10 hours.” ;
TIME:seconds_to_middle_of_measurement = 149.85 ;
TIME:standard_name = “time” ;
TIME:long_name = “time” ;
TIME:units = “days since 1950-01-01 00:00:00 UTC” ;
TIME:calendar = “gregorian” ;
TIME:axis = “T” ;
TIME:valid_min = 0. ;
TIME:valid_max = 90000. ;
Note that the timestamp provided is the start of a given time period over which the measurement has been averaged.
float HEIGHT_ABOVE_SENSOR(HEIGHT_ABOVE_SENSOR) ;
HEIGHT_ABOVE_SENSOR:comment = “Data has been vertically bin-mapped using tilt information so that the cells have consistant heights above sensor in time.” ;
HEIGHT_ABOVE_SENSOR:standard_name = “height” ;
HEIGHT_ABOVE_SENSOR:long_name = “height_above_sensor” ;
HEIGHT_ABOVE_SENSOR:units = “m” ;
HEIGHT_ABOVE_SENSOR:axis = “Z” ;
HEIGHT_ABOVE_SENSOR:positive = “up” ;
HEIGHT_ABOVE_SENSOR:reference_datum = “sensor” ;
HEIGHT_ABOVE_SENSOR:valid_min = -12000.f ;
HEIGHT_ABOVE_SENSOR:valid_max = 12000.f ;
- Actual depth of the instrument can be found in the “DEPTH” variable. It is derived from the pressure measured on the instrument or on nearby instruments on the mooring line when the instrument is not equipped with a pressure sensor.
- Nominal depth of the instrument can be found in the NetCDF file, both in the global attribute ‘instrument_nominal_depth’ and in the variable ‘NOMINAL_DEPTH’. Nominal depth is where the instrument “should” be located, based on the depth at the location and geometry of the mooring.
- Seafloor depth is always provided, listed as either ‘site_nominal_depth’ or ‘site_depth_at_deployment’ in the global attributes.
- Facing position is determined by simply looking at the value sign of the ‘HEIGHT_ABOVE_SENSOR’ variable. Positive indicates upwards and negative indicates downwards facing.
It is important to note that ‘HEIGHT_ABOVE_SENSOR’ is not a depth, it is the vertical distance positive up between the transducers of the ADCP and the centre of each cell in the water column where velocity is measured.
The sampled water column may be less than the maximum HEIGHT ABOVE SENSOR value. Depending on where and how an ADCP is deployed, its range may extend beyond the sea surface or sea floor, at which point the collected data is unreliable. This may be a conscious design decision to ensure data is always collected up to the surface or floor, as an instrument’s depth on a mooring may vary over time, as an ADCP can tilt and may not always be collecting data along the vertical.
So, to obtain the depths of the velocity cells, one needs to subtract HEIGHT_ABOVE_SENSOR from DEPTH.
Some of the most frequently downloaded dataset collections from the AODN Portal are Moored Instruments, such as AODN Waverider buoys, IMOS National reference stations and IMOS Australian National Mooring Network Data. The data available is often quality controlled, and it is important to take into account the QC flags for every variable used. In these files all the data is provided, even bad, but they are flagged accordingly.
For any queries or more detailed information on this data or any other dataset collections please contact info(at)aodn.org.au