416
functional traits or species identity of individuals. The desired pixel size (within the
limits of instrument and platform) determines flight altitude and swath width.
Airplanes need to fly lower and slower to acquire images with smaller pixels resulting in narrower flight lines. Thus, there is a trade-off between pixel size and the area
covered with a single flight line or during one flight. Generally, flight lines should
overlap 15–25% to ensure full area coverage. Flight lines can be stitched together in
a process called mosaicking, but spectra from the same pixels from different flight
lines vary, because of changing sun angles and atmospheric conditions over the
course of the day. In part, these differences can be accounted for during atmospheric
correction, but not perfectly. Given inevitable constraints, covering a research area
in fewer flight lines or a single line and working with larger pixels could be an
option, depending on the phenomenon of interest.
At midlatitudes, flights approximately ±2 h from solar noon are often ideal; solar
noon times can be looked up, for example, on the NOAA website (https://www.esrl.
noaa.gov/gmd/grad/solcalc/). When images are acquired around solar noon, flight
lines are often oriented in a north-south direction to align the flight direction with
the principle plane of the sun and to prevent the plane from casting a shadow on the
image. However, in areas such as the Tropics, it might be better to fly in the morning
or afternoon because of cloud formation during the middle of the day. In the morning, flight lines oriented southeast-northwest are a good option; in the afternoon,
flight lines oriented southwest-northeast may be preferable. The sun azimuth angle
(i.e., the angle between the sun’s position and north along the horizon: north equals
0°, east equals 90°, etc.) can be used as flight-line bearings. Additionally, it is seldom possible for flight crews to commit to specific dates and exact times, so the sun
azimuth angle for the approximate time of overpass is generally fine as a bearing.
Drawing separate sets of flight lines for different times of day is also an option.
Again, information on sun azimuth angles for specific dates, times, and locations
can be found online. During the time window of the light, it is often a judgment call
if sky conditions are “good enough” for image acquisition; the costs of having the
plane, air, and ground crews wait for a delay are important factors to consider.
Generally, although cumulus clouds obscure parts of the image, they are preferable
over cirrus or stratus clouds, which keep changing illumination conditions resulting
in overall low-quality image data.
15.3.3.2 Reference Data Collection for Image Processing
In summary, the most important steps in image processing are:
• Radiometric correction: Sensors record electromagnetic radiation in digital
numbers (DNs). During radiometric correction, DNs are converted to at-sensor
radiance using sensor- and pixel-specific radiation sensitivity coefficients.
Information about the sun’s geometry, including Earth-sun distance and solar
angle, is used to convert at-sensor radiance to top of the atmosphere (TOA)
reflectance.
A. K. Schweiger
functional traits or species identity of individuals. The desired pixel size (within the
limits of instrument and platform) determines flight altitude and swath width.
Airplanes need to fly lower and slower to acquire images with smaller pixels resulting in narrower flight lines. Thus, there is a trade-off between pixel size and the area
covered with a single flight line or during one flight. Generally, flight lines should
overlap 15–25% to ensure full area coverage. Flight lines can be stitched together in
a process called mosaicking, but spectra from the same pixels from different flight
lines vary, because of changing sun angles and atmospheric conditions over the
course of the day. In part, these differences can be accounted for during atmospheric
correction, but not perfectly. Given inevitable constraints, covering a research area
in fewer flight lines or a single line and working with larger pixels could be an
option, depending on the phenomenon of interest.
At midlatitudes, flights approximately ±2 h from solar noon are often ideal; solar
noon times can be looked up, for example, on the NOAA website (https://www.esrl.
noaa.gov/gmd/grad/solcalc/). When images are acquired around solar noon, flight
lines are often oriented in a north-south direction to align the flight direction with
the principle plane of the sun and to prevent the plane from casting a shadow on the
image. However, in areas such as the Tropics, it might be better to fly in the morning
or afternoon because of cloud formation during the middle of the day. In the morning, flight lines oriented southeast-northwest are a good option; in the afternoon,
flight lines oriented southwest-northeast may be preferable. The sun azimuth angle
(i.e., the angle between the sun’s position and north along the horizon: north equals
0°, east equals 90°, etc.) can be used as flight-line bearings. Additionally, it is seldom possible for flight crews to commit to specific dates and exact times, so the sun
azimuth angle for the approximate time of overpass is generally fine as a bearing.
Drawing separate sets of flight lines for different times of day is also an option.
Again, information on sun azimuth angles for specific dates, times, and locations
can be found online. During the time window of the light, it is often a judgment call
if sky conditions are “good enough” for image acquisition; the costs of having the
plane, air, and ground crews wait for a delay are important factors to consider.
Generally, although cumulus clouds obscure parts of the image, they are preferable
over cirrus or stratus clouds, which keep changing illumination conditions resulting
in overall low-quality image data.
15.3.3.2 Reference Data Collection for Image Processing
In summary, the most important steps in image processing are:
• Radiometric correction: Sensors record electromagnetic radiation in digital
numbers (DNs). During radiometric correction, DNs are converted to at-sensor
radiance using sensor- and pixel-specific radiation sensitivity coefficients.
Information about the sun’s geometry, including Earth-sun distance and solar
angle, is used to convert at-sensor radiance to top of the atmosphere (TOA)
reflectance.
A. K. Schweiger