Spectroadiometric Measurements – Surface Calibration Targets
Site selection and layout
Calibration targets should be large (ideally, calibration targets should cover an area of at least 3 x 3 pixels of the airborne mission), homogeneous, spectrally featureless in the spectral zone to be investigated, Lambertian and encompass a range of albedo levels (bright to dark). Calibration targets can either be natural ‘psuedo-invariant’ features at the site or artificial targets specifically placed into the flight lines. Pseudo–invariant calibration target examples include: asphalt, concrete, salt, sand, gravel, limed and painted surfaces, tin or painted tin. Ideally, vegetation and water should not be included (although managed putting greens can be successfully used if mown daily). Deep water can also be used if dark NIR/SWIR signals are required (and providing a nadir measurement can be made from it). 8 m x 8 m standard canvas calibration targets are available to support TERN AusCover field campaigns. The site chosen to place these targets should preferably in the centre of one of the flight lines (i.e. at the nadir view), flat and open. If it is not possible to find a flat site, surface slope and direction of slope using an inclinometer/compass should be estimated. Target site selection should be done in advance of the airborne campaign regardless of the target type.
Basic data required
The following is essential data required for data management purposes.
General name for the site at which the measurements are being made.
Geographic coordinates (easting, northing, zone) at each corner point of each target, measured using GPS
Operators: name those who collected the data and their affiliations
Date: consistent format (dd/mm/yyyy)
Time of measurement: in consistent format (hh:mm)
Target name: name of specific target
Summary of general overhead conditions during the survey.
Description of the background surface over which the targets were laid
Multiple digital photographs of the targets in situ showing general situation and layout.
Spectralon panel number
Field equipment checklist
Spectroradiometer
Spectralon reference panel (+ mounting tripod)
Pistol grip
Laptop
Extra batteries for spectroradiometer and laptop
GPS (preferably DGPS)
Inclinometers
Calibration targets
Dark clothing for operators
Data Collection Process
Setup calibration targets
Artificial Targets
There are four targets (2 black, 1 white and 1 grey). One of the black targets acts as an underlay for the white target to prevent any background signal from influencing the white target reflectance. Unpack the targets from their carry bags and lay them out well in advance of the flights taking place. To ensure the targets are laid out flat and cleanly, use four people, one at each corner. Measure the black underlay prior to laying out the white target over the top. Secure corners with tent pegs.
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Measurements to be performed outside under full, clear sun illumination (alternatively, could use artificial light source and perform the measurements indoors).
Set up intercalibration Spectralon panel.
Run optimization (each team uses its own Spectralon panel to do this).
Run a White Reference procedure measuring over the intercalibration panel and save the result (the WR signal should ideally be flat with a value of 1.0 – any difference indicate calibration differences). Repeat 10 times.
Take radiance measurement of the irradiance spectralon panel and save the result. Repeat 10 times.
Transfer all cross-calibration measurements to one computer for cross-reference reflectance calibration. Compare radiance results of all ASDs.
Data Recording and Submission
Ancillary data
If data was recorded in the field using the ODK ‘Calibration’ form, then simply check the field form for completeness and accuracy and submit. If paper records were used, then the details will need to be transcribed into the ODK form (compatible with any android mobile device) and submitted. Details on using the ODK forms can be found here Field Data Collection Forms
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