144
combination of autonomy and stability. Airborne imaging spectroscopy operators
are usually national institutions involved in aircraft research. In particular, the
European Facility for Airborne Research (EUFAR, http://www.eufar.net/) is a
European Commission project to integrate 24 of these operators and provide
researchers with easy and open access to airborne research facilities.
The first whiskbroom airborne imaging spectrometers developed around 1980
(Green et al. 1998) were manufactured with a full width half maximum (FWHM) of
nearly 10 nm in the visible and near infra-red (VNIR) and short wave infra-red
(SWIR) wavelength regions. Pushbroom systems offer advantages in robustness,
integration time, speed, and spectral/spatial resolution (Schlapfer et al. 2007).
Moreover, the pushbroom system increases the levels of signal to noise ratios (SNR)
to around 1000:1.
Near-ground flight conditions impose some radiometric and geometric constraints on the hyperspectral imagery: (1) the sensor requires a large (between 40º
and 90º) field of view (FOV) to cover as much ground area as possible in each flight
line. Consequently, off-nadir pixels tend to increase in pixel size and are subject to
radiometric gradients if the scanning plane is not perpendicular to the Sun’s principal plane. In addition, rugged terrain enhances these changes of incident sun angle
variations and pixel size; (2) to cover a typical local scale study area of 20 × 20 km,
several flight lines, which may take 2–3 hours, are needed, to also ensure that Sun
angle variations between flight lines are reduced; sun angles variation between flight
lines; (3) atmospheric components with more relevance in the radiative transfer
optic response are concentrated in the very first kilometers near-ground, thus airborne imagery is also affected by the atmosphere. Likewise, the platform stability is
influenced by high-frequency velocity and attitude variations. All these aspects
must be corrected and normalized between flight lines to obtain a better georeferenced ground reflectance mosaic of the study area. Algorithms for geometric correction are becoming accurate and can be implemented in a fully automatic way
(Biesemans et al. 2007).
A wide variety of studies have been conducted on species-level mapping in different vegetation types, including grasslands (Miao et al. 2006; Möckel et al. 2014),
shrublands (Roberts et al. 1998), mangroves (Ustin et al. 2004), marshlands
(Silvestri et al. 2003), and forest (Kalacska et al. 2007; Asner et al. 2008).
Field Spectroscopy
Field spectroscopy is the measurement of high-resolution spectral radiance or irradiance in the field to derive the reflectance or emissivity spectral signatures of targets at the Earth’s surface under natural environmental conditions. In comparison
with airborne or spaceborne imaging spectroscopy, the sensing instrument in the
field can remain fixed over the subject of interest for much longer, and the path
length between the instrument and the object being measured is thereby reduced
(Milton et al. 2009).
M. Jiménez and R. Díaz-Delgado
combination of autonomy and stability. Airborne imaging spectroscopy operators
are usually national institutions involved in aircraft research. In particular, the
European Facility for Airborne Research (EUFAR, http://www.eufar.net/) is a
European Commission project to integrate 24 of these operators and provide
researchers with easy and open access to airborne research facilities.
The first whiskbroom airborne imaging spectrometers developed around 1980
(Green et al. 1998) were manufactured with a full width half maximum (FWHM) of
nearly 10 nm in the visible and near infra-red (VNIR) and short wave infra-red
(SWIR) wavelength regions. Pushbroom systems offer advantages in robustness,
integration time, speed, and spectral/spatial resolution (Schlapfer et al. 2007).
Moreover, the pushbroom system increases the levels of signal to noise ratios (SNR)
to around 1000:1.
Near-ground flight conditions impose some radiometric and geometric constraints on the hyperspectral imagery: (1) the sensor requires a large (between 40º
and 90º) field of view (FOV) to cover as much ground area as possible in each flight
line. Consequently, off-nadir pixels tend to increase in pixel size and are subject to
radiometric gradients if the scanning plane is not perpendicular to the Sun’s principal plane. In addition, rugged terrain enhances these changes of incident sun angle
variations and pixel size; (2) to cover a typical local scale study area of 20 × 20 km,
several flight lines, which may take 2–3 hours, are needed, to also ensure that Sun
angle variations between flight lines are reduced; sun angles variation between flight
lines; (3) atmospheric components with more relevance in the radiative transfer
optic response are concentrated in the very first kilometers near-ground, thus airborne imagery is also affected by the atmosphere. Likewise, the platform stability is
influenced by high-frequency velocity and attitude variations. All these aspects
must be corrected and normalized between flight lines to obtain a better georeferenced ground reflectance mosaic of the study area. Algorithms for geometric correction are becoming accurate and can be implemented in a fully automatic way
(Biesemans et al. 2007).
A wide variety of studies have been conducted on species-level mapping in different vegetation types, including grasslands (Miao et al. 2006; Möckel et al. 2014),
shrublands (Roberts et al. 1998), mangroves (Ustin et al. 2004), marshlands
(Silvestri et al. 2003), and forest (Kalacska et al. 2007; Asner et al. 2008).
Field Spectroscopy
Field spectroscopy is the measurement of high-resolution spectral radiance or irradiance in the field to derive the reflectance or emissivity spectral signatures of targets at the Earth’s surface under natural environmental conditions. In comparison
with airborne or spaceborne imaging spectroscopy, the sensing instrument in the
field can remain fixed over the subject of interest for much longer, and the path
length between the instrument and the object being measured is thereby reduced
(Milton et al. 2009).
M. Jiménez and R. Díaz-Delgado