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Chapter 9 Remotely Sensed Images from Above
a Shape: The objects are square at the base, but have pointed, triangular
tops, making them pyramid shaped.
Bringing all of these elements of interpretation together, we can find
three pyramids of different sizes (although still very large), in a definite fixed
arrangement, in sand near a busy city. All of these clues combine to identify
the Great Pyramid of Giza in Cairo, Egypt. Aerial images often contain clues
to try and determine the identity of the objects they contain, or to at least narrow down the options to a handful that could be determined through some
brief research through other collateral material (like investigating groupings
of large desert pyramids in that particular arrangement).
Visual image interpretation skills enable the viewer to discern more information about the nature of objects within a remotely sensed image. The
elements of interpretation can be used for closer investigation to identify (for
instance) the specific type of fighter jet that can be seen on a runway based
on the size and shape of its features, wingspan, engines, and armaments. A
forestry expert could use visual image interpretation to determine what type
of tree stand is being examined in a photo based on features associated with
the trees.
How Can You Make Measurements
from an Aerial Photo?
Once identification of objects in an image has been positively made, these
objects can be used for making various types of measurements. Photogrammetry is the process of obtaining measurements from aerial photos. Photogrammetric techniques can be used for determining things like the height
and depth of objects in an aerial photo. For instance, the lengths of visible
features in a photo or heights of buildings seen in the photo can be calculated. There are a lot of possible photogrammetric measurements, and the
following are two simple examples of how these types of measurements can
be made.
Just like the map scale discussed in Chapter 7, every photo has a photo
scale, listed as a representative fraction. For example, in a 1:8000 scale
aerial photo, one unit of measurement in the photo is equivalent to 8000 of
those units in the real world. Using the photo scale, you can determine the
real-world size of features. For instance, say you measure a length of rail on
an aerial photo to be a half-inch and the photo scale is 1:8000. Thus, 1 inch
in the photo is 8000 inches in the real world, and so a measurement of 0.5
inches in the photo would be 4000 inches (0.5 times 8000), or 333.33 feet,
in length.
The scale of a photo relies on the focal length of the camera’s lens and
the altitude height of the plane when the image is taken. The problem comes
when you’re trying to make measurements from an orthophoto but you don’t
know the photo scale. Without knowing how many real-world units equate to
photo scale the
representation used to
determine how many
units of measurement
in the real world are
equivalent to one unit
of measurement on an
aerial photo.
photogrammetry the
process of making
measurements using
aerial photos.
Chapter 9 Remotely Sensed Images from Above
a Shape: The objects are square at the base, but have pointed, triangular
tops, making them pyramid shaped.
Bringing all of these elements of interpretation together, we can find
three pyramids of different sizes (although still very large), in a definite fixed
arrangement, in sand near a busy city. All of these clues combine to identify
the Great Pyramid of Giza in Cairo, Egypt. Aerial images often contain clues
to try and determine the identity of the objects they contain, or to at least narrow down the options to a handful that could be determined through some
brief research through other collateral material (like investigating groupings
of large desert pyramids in that particular arrangement).
Visual image interpretation skills enable the viewer to discern more information about the nature of objects within a remotely sensed image. The
elements of interpretation can be used for closer investigation to identify (for
instance) the specific type of fighter jet that can be seen on a runway based
on the size and shape of its features, wingspan, engines, and armaments. A
forestry expert could use visual image interpretation to determine what type
of tree stand is being examined in a photo based on features associated with
the trees.
How Can You Make Measurements
from an Aerial Photo?
Once identification of objects in an image has been positively made, these
objects can be used for making various types of measurements. Photogrammetry is the process of obtaining measurements from aerial photos. Photogrammetric techniques can be used for determining things like the height
and depth of objects in an aerial photo. For instance, the lengths of visible
features in a photo or heights of buildings seen in the photo can be calculated. There are a lot of possible photogrammetric measurements, and the
following are two simple examples of how these types of measurements can
be made.
Just like the map scale discussed in Chapter 7, every photo has a photo
scale, listed as a representative fraction. For example, in a 1:8000 scale
aerial photo, one unit of measurement in the photo is equivalent to 8000 of
those units in the real world. Using the photo scale, you can determine the
real-world size of features. For instance, say you measure a length of rail on
an aerial photo to be a half-inch and the photo scale is 1:8000. Thus, 1 inch
in the photo is 8000 inches in the real world, and so a measurement of 0.5
inches in the photo would be 4000 inches (0.5 times 8000), or 333.33 feet,
in length.
The scale of a photo relies on the focal length of the camera’s lens and
the altitude height of the plane when the image is taken. The problem comes
when you’re trying to make measurements from an orthophoto but you don’t
know the photo scale. Without knowing how many real-world units equate to
photo scale the
representation used to
determine how many
units of measurement
in the real world are
equivalent to one unit
of measurement on an
aerial photo.
photogrammetry the
process of making
measurements using
aerial photos.