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Atmospheric Correction
This page is designed to give information on atmospheric correction of ARSF hyperspectral data and also what needs to be done before and during data collection.
Information on collection of field spectra
To validate the atmospheric correction it is important to collect field spectra during the flight campaign. These, ideally, should be from large homogeneous targets such that when comparing to remotely sensed data they cover the entire pixel. In practice it is better to cover much more than 1 pixel due to spectral bleed and stray light in the airborne hyperspectral instrument. Targets should also be precisely located using GPS so that they can be identified accurately in the remotely sensed image.
Useful guides to the collection of field spectra for validation purposes:
Please read the above guides as they cover the subject in much more detail than we can here. The following should give a brief summary of general "do's and don'ts", some of which may be obvious but are easy to forget when out in the field.
Prior to the survey
- Test the instruments - both to check that they are working and to give you practice in using them. Ideally attend training courses such as that offered by the Field Spectroscopy Facility (FSF), this is a requirement if you borrow instruments from FSF.
- Plan the survey - identify suitable targets before the survey day, either by visiting the site in person or using online or offline map data. Save as much time as possible so that when the survey day arrives you can spend your time collecting measurements. Only consider targets within the area of the remote sensing survey.
- Remember the spare batteries - don't get out to the field only to find you have no power.
- Remember to take lunch / refreshments with you - it will be a long day!
At the survey
- Do not stand between the sun and the survey point - you will cast a shadow over the area. Ideally stand as far back as possible so as not to create erroneous reflections.
- Measure level, homogeneous targets of greater size than the resolution of the remote sensing data. In most cases this is not possible (even a square metre of concrete is not perfectly homogeneous) but try and find suitable targets. For example an expanse of car park may be suitable, a field of grass, a body of deep water. Surfaces that are not level will suffer greater from erroneous reflections (see bi-directional reflectance distribution fuinction (BRDF)).
- Make sure you collect more than one observation over the target to improve the measurement statistics (get as many as possible).
- Try to measure near the centre of the targets rather than the edge where possible. For example, if you survey a car park do not observe near the boundary fence or hedge. If observing a water body remember the depth of the bottom will also affect the reflectance (e.g. the shallow boundaries will be different to the deep centre).
- Always get an accurate measurement of position (use GNSS / GPS connected to the measurement instrument). You may need to measure the same target for many seconds to get a more accurate position measurement.
- Take photographs - they will act as a reminder to the conditions of the day and the targets surveyed.
- Measure a mixture of targets - each target will have different spectral responses and you don't want to collect only targets with response in the VNIR (visible near infra-red) range if you are also interested in the SWIR (short wave infra-red).
- Use calibrated targets - if you have these available and they are suitable for positioning over the airborne survey site (and suitable for the airborne instruments) then use them.
Available software
There are a handful of software packages that offer atmospheric correction but few that specialise for airborne / low to mid-atmosphere cases. There are papers written which show comparisons between various packages used over the same scene. The below software are used by ARSF-DAN.
ATCOR4
Requires a licence and runs in an IDL environment. Specially designed for airborne remote sensing data. ARSF-DAN own a licence for this software and offer an atmospheric correction service to ARSF data users.
6S and Py6S
Open source software. Not specifically designed for airborne remote sensing data. ARSF-DAN use this for data quality checks of hyperspectral data.