Helsinki University of Technology
Department of Surveying
The subject of the thesis is atmospheric correction for imaging spectrometer measurements. Measurements of inland and coastal waters have been made with AISA instrument (Airborne Imaging Spectrometer for Applications) by the Finnish Environment Institute. Atmospheric attenuation is caused by absorption and scattering. The wavelength range of AISA is at the visible and near infrared region of the electromagnetic spectrum. In order to eliminate the effects of atmosphere a correction method based on modelling the radiative transfer in the atmosphere with a modelling code MODTRAN has been applied. AISA measurements are corrected according to the correction coefficients calculated from MODTRAN simulation results. In order to enable fast and flexible calculation of atmospherical parameters, a correction procedure has been designed and implemented at the Finnish Environment Institute. Atmospherical parameters for the correction have been obtained from a numerical weather prediction model by the Finnish Meteorological Institute.
The method for atmospheric correction has been applied for water measurement campaigns during years 1996-1998. In applying data jointly from several flight campaigns the atmospheric correction was useful. For separate flight lines the correction was not equally useful. The atmospheric correction reduced the effects of scattering. The correction of absorption would require more precise calibration of the instrument or wider channels for wavelengths where the absorption occurs. The shape of the corrected measurement was not similar with spectrometer measurements made at the proximity of the water in all measured wavelength regions. In analysing water quality parameters like turbidity and suspended matter the results were better with atmospherically corrected values. The results for chlorophyll-a were not improved by the atmospheric correction.
Keywords: Atmospheric correction, scattering, absorption, spectrometer, water quality
Language: English, Finnish abstract