295
When you use Google Earth to get an overhead view of your house, Google
had to get those images from somewhere, but there are a number of steps that
occur before that image of your house can be acquired and finally turned into
a picture you can view on the computer screen. In Chapter 9, we discussed
how what’s really being captured via remote sensing is reflected light—this
is a form of electromagnetic energy and a sensor is measuring the amount of
energy that reflects off a target. In addition, in remote sensing the device being used to obtain this information about energy reflectance will be on some
type of airborne or spacecraft platform that will be a considerable distance
away from a target on the ground below. Some remote sensing devices measure the energy emitted from objects on the ground (for instance, as a means
of measuring the heat or thermal properties of an item); however, this chapter focuses on measuring the reflection of energy by a sensor.
If remote sensing is measuring the reflectance of energy, there needs
to be a source for that energy. Luckily, we have one nearby—our solar system’s Sun provides the source of the electromagnetic energy that is being
sensed. The Sun is almost 93 million miles away from Earth, and sunlight
takes about 8.3 minutes to reach Earth from the Sun. This energy from the
Sun radiates through space at the speed of light (almost 300 million meters per second) to reach Earth. This energy passes through and interacts
with Earth’s atmosphere and then reaches objects on Earth’s surface. The
energy then interacts with those objects, and some of the energy is reflected
back into space. That reflected energy again passes through the atmosphere
a second time on its way back off Earth and finally makes it to a sensor
either on an airborne or spaceborne platform, where the energy is recorded
( Figure 10.1, page 296).
These measurements made at the sensor are being captured as remotely
sensed data. Thus, the remotely sensed information you see on things like
Google Earth is actually the reflection of energy off targets on the ground
How Remote Sensing Works
Electromagnetic Energy, the Remote Sensing Process, Spectral
Reflectance, NDVI, Digital Imagery, and Color Composites
10
remote sensing the
process of collecting
information related to
the reflected or emitted
electromagnetic energy
from a target by a
device a considerable
distance away from
the target onboard an
airborne or spacecraft
platform.
When you use Google Earth to get an overhead view of your house, Google
had to get those images from somewhere, but there are a number of steps that
occur before that image of your house can be acquired and finally turned into
a picture you can view on the computer screen. In Chapter 9, we discussed
how what’s really being captured via remote sensing is reflected light—this
is a form of electromagnetic energy and a sensor is measuring the amount of
energy that reflects off a target. In addition, in remote sensing the device being used to obtain this information about energy reflectance will be on some
type of airborne or spacecraft platform that will be a considerable distance
away from a target on the ground below. Some remote sensing devices measure the energy emitted from objects on the ground (for instance, as a means
of measuring the heat or thermal properties of an item); however, this chapter focuses on measuring the reflection of energy by a sensor.
If remote sensing is measuring the reflectance of energy, there needs
to be a source for that energy. Luckily, we have one nearby—our solar system’s Sun provides the source of the electromagnetic energy that is being
sensed. The Sun is almost 93 million miles away from Earth, and sunlight
takes about 8.3 minutes to reach Earth from the Sun. This energy from the
Sun radiates through space at the speed of light (almost 300 million meters per second) to reach Earth. This energy passes through and interacts
with Earth’s atmosphere and then reaches objects on Earth’s surface. The
energy then interacts with those objects, and some of the energy is reflected
back into space. That reflected energy again passes through the atmosphere
a second time on its way back off Earth and finally makes it to a sensor
either on an airborne or spaceborne platform, where the energy is recorded
( Figure 10.1, page 296).
These measurements made at the sensor are being captured as remotely
sensed data. Thus, the remotely sensed information you see on things like
Google Earth is actually the reflection of energy off targets on the ground
How Remote Sensing Works
Electromagnetic Energy, the Remote Sensing Process, Spectral
Reflectance, NDVI, Digital Imagery, and Color Composites
10
remote sensing the
process of collecting
information related to
the reflected or emitted
electromagnetic energy
from a target by a
device a considerable
distance away from
the target onboard an
airborne or spacecraft
platform.