306
Chapter 10 How Remote Sensing Works
How Do You Display a Digital Remotely
Sensed Image?
Once this digital data is collected, the next step is to transform it into something that can be viewed on a computer screen, especially since many of these
bands of energy are only visible to the sensor and not to the human eye. Sensors
can only “see” a certain area of the ground at a time and will record measurements of that particular section. The size of this area on the ground is the sensor’s spatial resolution, representing the smallest amount of area the sensor
can collect information about. For example, if a sensor’s spatial resolution was
30 meters, each measurement would consist of a 30 meter ϫ 30 meter section
of the ground, and the sensor would measure thousands of these 30 meter ϫ
30 meter sections at once. No matter how many sections are being measured,
the smallest area the sensor has information for is a block that is 30 meters by
30 meters. Keep in mind, the spatial resolution of the image is going to constrain the kind of visual information you can obtain from it. For instance, in an
image with 4 meter resolution, you may be able to discern that an object is a
large vehicle, but no details about it, whereas an image with 0.5 meter resolution may allow you to determine specific features about the vehicle in question.
We’ll discuss more about spatial resolution in Chapter 11, but for right
now, each one of these blocks of the ground being sensed represents the size
of the area on the ground having its energy reflection being measured by
the sensor. Due to the effects of the atmosphere, there’s not a straight oneto-one relationship between the reflectance of energy and the radiance being
measured at the satellite’s sensor. The energy measurements at the sensor are
converted to a series of pixels, with each pixel receiving a brightness value,
or BV (also referred to as a Digital Number, or DN), for the amount of radiance
being measured at the sensor for that section of the ground. This occurs for
each wavelength band being sensed.
spatial resolution the
size of the area on
the ground being
represented by one
pixel’s worth of energy
measurement.
NASA (the National Aeronautics and Space Administration) has an online tool called ICE (Image Composite Explorer) that allows users to analyze different applications of remotely sensed data, and one
component features an interactive NDVI tool. To use
ICE’s NDVI exercise, open your Web browser and go to
http://earthobservatory.nasa.gov/Experiments/
ICE/panama/panama_ex2.php. This section of ICE
is part of a larger exercise examining the health of
tropical rainforests. The tools will allow you to build
NDVI images using the near-infrared and red bands of
a Landsat satellite image. To properly see the NDVI
imagery, select Channel 4 (this is the near-infrared
band) in the red Channel 1 and then select Channel 3
(this is the red band) in the green Channel 2. Click
Build and you can view the NDVI image of the area.
Change the color table and zoom around the imagery. How can NDVI be used to measure the health
of the rainforests?
Hands-on Application 10.2
Examining NDVI with NASA ICE
brightness values the
energy measured at a
single pixel according
to a pre-determined
scale. Also referred
to as Digital Numbers
(DNs).
Chapter 10 How Remote Sensing Works
How Do You Display a Digital Remotely
Sensed Image?
Once this digital data is collected, the next step is to transform it into something that can be viewed on a computer screen, especially since many of these
bands of energy are only visible to the sensor and not to the human eye. Sensors
can only “see” a certain area of the ground at a time and will record measurements of that particular section. The size of this area on the ground is the sensor’s spatial resolution, representing the smallest amount of area the sensor
can collect information about. For example, if a sensor’s spatial resolution was
30 meters, each measurement would consist of a 30 meter ϫ 30 meter section
of the ground, and the sensor would measure thousands of these 30 meter ϫ
30 meter sections at once. No matter how many sections are being measured,
the smallest area the sensor has information for is a block that is 30 meters by
30 meters. Keep in mind, the spatial resolution of the image is going to constrain the kind of visual information you can obtain from it. For instance, in an
image with 4 meter resolution, you may be able to discern that an object is a
large vehicle, but no details about it, whereas an image with 0.5 meter resolution may allow you to determine specific features about the vehicle in question.
We’ll discuss more about spatial resolution in Chapter 11, but for right
now, each one of these blocks of the ground being sensed represents the size
of the area on the ground having its energy reflection being measured by
the sensor. Due to the effects of the atmosphere, there’s not a straight oneto-one relationship between the reflectance of energy and the radiance being
measured at the satellite’s sensor. The energy measurements at the sensor are
converted to a series of pixels, with each pixel receiving a brightness value,
or BV (also referred to as a Digital Number, or DN), for the amount of radiance
being measured at the sensor for that section of the ground. This occurs for
each wavelength band being sensed.
spatial resolution the
size of the area on
the ground being
represented by one
pixel’s worth of energy
measurement.
NASA (the National Aeronautics and Space Administration) has an online tool called ICE (Image Composite Explorer) that allows users to analyze different applications of remotely sensed data, and one
component features an interactive NDVI tool. To use
ICE’s NDVI exercise, open your Web browser and go to
http://earthobservatory.nasa.gov/Experiments/
ICE/panama/panama_ex2.php. This section of ICE
is part of a larger exercise examining the health of
tropical rainforests. The tools will allow you to build
NDVI images using the near-infrared and red bands of
a Landsat satellite image. To properly see the NDVI
imagery, select Channel 4 (this is the near-infrared
band) in the red Channel 1 and then select Channel 3
(this is the red band) in the green Channel 2. Click
Build and you can view the NDVI image of the area.
Change the color table and zoom around the imagery. How can NDVI be used to measure the health
of the rainforests?
Hands-on Application 10.2
Examining NDVI with NASA ICE
brightness values the
energy measured at a
single pixel according
to a pre-determined
scale. Also referred
to as Digital Numbers
(DNs).