The spectroradiometric measurements should be normalised in terms of solar angle to suit the satellite products. Ideally, field spectroradiometric measurements should be made at the same solar angle as the satellite overpass. However, for standardising the BRDF correction process in Australia, surface reflectance measurements of ground features are unlikely to change in the short term (within a month). Since NBAR process also normalizes the solar angle, it should not be the problem as long as you normalize the spectroradiometer measurements to the same solar angle.
Scale is important. Careful consideration needs to be given to field sampling schemes for scaling up ground-based measurements to satellite pixel scales. As a rule, measurements on the ground should range over several pixels (e.g. uniform targets of 3 x 3 Landsat pixels would be ideal for integration with Landsat data). The sites should be of a uniform target, but as homogeneous sites are difficult to find as pixels sizes become larger, airborne image data can be used for up-scaling. For validation and statistic analysis, it would be ideal to identify as many sites as possible.
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 ground target site, preferably measured using a differential 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 and description: name of specific target and its general description.
Summary of general overhead conditions during the survey.
Digital photographs of the area within which ground measurements are performed.
Data Collection Process
Field equipment checklist
Spectralon reference panel (+ mounting tripod)
Laptop and extra batteries
Extra batteries for spectroradiometer
GPS (preferably DGPS)
Ground Target selection:
In the selection of suitable locations for ground-based measurements, consideration needs to be given to the scale over which measurements are required (see discussion regarding scaling in site selection section above). It is useful to perform a general selection of ground measurement sites over the whole area prior to arrival at the field site (e.g. identify areas at a broad scale using Google maps).
Areas measured should be as flat as possible (if not flat, estimate surface slope and direction of slope using an inclinometer/compass).
Ground surface measurement sites should be homogeneous over a number of pixels (ideally 3 x 3 pixels) of the satellite images of interest (e.g. Landsat, MODIS).
Before beginning spectral measurements determine how best to move through the area without significantly disturbing the vegetation or ground surface.
Think carefully about the fore-optic field of view and height of the fore-optic over the surface area to achieve the desired footprint size (see table of field of view diameters below).
All staff involved in specroradiometric measurements should wear dark clothing (ie. Black t-shirt, black jeans).
Preparation – ASD Instrumentation
Connect computer to the ASD.
Turn the ASD on, at least 30 minutes before operation.
Turn the computer on, check if date and time of PC is correct.
Launch the instrument software.
Install a fore optic to your requirement. See Table in Appendix 2 [need to agree fore-optic to be used].
Check fiber optics safety and connection to the pistol grip.
On the computer, create a proper file directory for the mission.
Prepare documentation for recording the measurement process, to include headings such as sample number, measurement numbers, GPS coordination, any comments on both the measurement and the targets (see Appendix 3 for suitable draft form, a better version can be downloaded from the TERN AusCover wiki or from http://fsf.nerc.ac.uk/resources/logsheets/log_pdfs/asd_log.pdf).
Set up instrument internal averaging as follows (for outdoor use):
DC (Dark Current) 25
WR (White Reference) 40
Select the proper fore optic in the software.
Select the agreed mode for the mission (reflectance, radiance).
Mount Spectralon horizontally on tripod (no fingerprints, dust, etc. allowed on Spectralon) close to the target area.
Initial instrument calibration check prior to measurements
Optimize the spectrometer against Spectralon panel: Point the optic head to the center of the Spectralon and make sure there are no adjacency interferences.
Measure a standard ‘sample’ to assure the ASD is ready for measurement (e.g. vegetation sample).
Measurement of ground surface sites
Depending on the purpose of the measurements, measure as many spectra as possible from the ground surface over the area (scale) of interest. .
Make sure all your measurements will be at the same position relative to the sun (recommended same position of the airborne sensor to the sun if measurements are coinciding with this).
Take measurements in NADIR at the height previously determined to achieve the field of view desired (see 1e above).
Photo the area using a digital camera, to include general area shots and close up of the site vegetated and soil surfaces (use an object for scale).
During the measurements, monitor for changes in overhead conditions / irradiance. If changes are occurring, re-measure the Spectralon reference standard (if in doubt, measure the Spectralon panel after each target measurement). As a matter of routine measure the Spectralon again upon completion of the target measurements.
Run measurements in reflectance and radiance (radiance only in calibration targets) configurations (automatic in the new version of the ASD software).
Measure the Spectralon every 5 minutes, if the spectrum is not a straight line – optimize the spectrometer and run WR. If spectrum is a straight line only run WR and repeat your measurement from 4d.
If possible, also periodically measure overhead optical conditions using a sun photometer.
Record GPS position of the centre of each ground site measured. Also record the terrain height from the instrument.
Process the data to absolute reflectance using calibration data for the specific Spectralon reference panel used (i.e. to establish NIST or similar traceability).
Graphically evaluate each spectrum to determine any issues with each measurement. Define any apparent outliers (attempt to explain them, e.g. variations in overhead irradiance, in which case delete, or real variations in surface properties, in which case keep). If the area has been defined homogeneous omit outliers with high standard deviation.
Subsequent analysis will depend on the specific purpose of the ground surface spectral measurements.
Cross-calibration of different spectroradiometers used in the same campaign
Measurements to be performed outside under full, clear sun illumination (alternatively, could use artificial light source and perform the measurements indoors).
Set up an 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 Storage
File and Folder naming conventions
Each folder name should be composed of the following (separated by underlines):
Spectral Team (A, B, C) or Spectroradiometer Unit (A, B, C)
Date of the measurement - in the format DDMMYY
Separated by underlines
Target name should be the first three or four letters (only first letter capital) of the specific target (e.g. Vegetation: Veg, Asphalt: Asph). Names should be understandable. Targets which are not common should be written in their full name. White reference measurements should be saved and named WR. Reflectance measurements should be saved without additional labels. Radiance measurements should be saved with additional Rad label: A_12012011_1_Black_Rad
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 https://ternaus.atlassian.net/wiki/spaces/TERNSup/pages/2222686255
Instrument data, field photos, paper forms
All instrument files, field photographs (note: only field photos not captured with an android device as a component of an ODK form need to be uploaded) and scanned paper forms should be sent to email@example.com
Provide your feedback about the experience with Knowledge base