16.59. refr_index¶

The refr_index program computes the effective refraction index of water for a specific satellite sensor band and given temperature and salinity. This has applications for shallow water bathymetry (Section 8.31).

16.59.1. Examples¶

Single wavelength refraction index:

refr_index --salinity 35 --temperature 20 \
    --wavelength 532

This uses the Quan and Fry method (Section 16.59.2.3).

When a spectral response file is provided, two methods are available. The invocation:

refr_index --salinity 35 --temperature 20  \
  --wavelength-method weighted_mean        \
  --spectral-response WV03_Green.csv

returns the refraction index corresponding to the response-weighted average of the CSV wavelengths and is the default.

Alternatively:

refr_index --salinity 35 --temperature 20  \
  --wavelength-method peak_response        \
  --spectral-response WV03_Green.csv

returns the refraction index corresponding to the wavelength at which the spectral response is largest (the peak of the response curve).

The result is printed to standard output.

16.59.2. Methodology¶

16.59.2.1. Spectral response¶

It is assumed that a satellite band, such as Green for WorldView-3, records light in a narrow range of wavelengths. The spectral response of the band contains the sensitivity of the sensor to each wavelength in that band. An effective wavelength is computed from the response curve and used as input to the refraction index models (Section 16.59.2.3). Two reduction methods are available, controlled by --wavelength-method:

weighted_mean (default) - the response-weighted average of the CSV wavelengths.
peak_response - the wavelength at which the spectral response is largest.

The spectral response CSV file must have two columns, with the wavelength (in nanometers) in the first column, the relative response for that wavelength in the second column. Use commas, spaces, or tabs as separators. The first line must be a header and will be ignored.

Wavelengths outside the range 300-1100 nm will be ignored (skipped). Wavelengths in the range 300-1100 nm but outside 400-700 nm will be used but a warning will be printed, as they are outside the validated range for the models (Section 16.59.2.3).

Example spectral response table:

wavelength response
0.78240309
0.78540934
0.79221241
0.79904049
0.80684987

16.59.2.2. Salinity and temperature¶

The salinity is measured in parts per thousand (ppt). Typical seawater has a salinity of 35 ppt and freshwater has a salinity of 0 ppt.

The temperature is measured in degrees Celsius. The model expects values between 0°C and 30°C.

16.59.2.3. Modeling¶

Two methods are available for computing the refraction index.

Quan and Fry ([QF95], source code) - directly handles salinity without interpolation. This is the default.
Parrish (2020) - uses linear interpolation between freshwater (S=0) and seawater (S=35) coefficients.

Use the --mode switch to choose between these two.

16.59.3. Command-line options¶

--salinity <float>: Salinity in parts per thousand (ppt). A good value is 35 ppt. Required.
--temperature <float>: Temperature in degrees Celsius. Must be between 0 and 30. Required.
--mode <string (default: Quan-Fry)>: Refraction index equation to use: Quan-Fry (default) or Parrish. See Section 16.59.2.3 for details.
--wavelength-method <string (default: weighted_mean)>: How to reduce a spectral response CSV to a single effective wavelength. weighted_mean (default) uses the response-weighted average of the CSV wavelengths. peak_response uses the wavelength at which the response is largest. Ignored when --wavelength is used.
--spectral-response <string (default: “”)>: CSV file containing the spectral response of the sensor band. See an example in Section 16.59.2.1. Mutually exclusive with --wavelength.
--wavelength <float>: Calculate refraction index for single wavelength (nm). Validated range is 400-700 nm. A warning will be printed for wavelengths between 300-1100 nm and outside the narrower range. There will be an error outside the 300-1100 nm range. Mutually exclusive with --spectral-response.
-v, --version: Display the version of software.
-h, --help: Display this help message.