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Chapter 9 Remotely Sensed Images from Above
effects of terrain and relief displacement and enforce the same scale across
the photo. A regular aerial photo can’t be used as a map since it doesn’t have
uniform scale everywhere, but an orthophoto can. Even with uniform scale,
some of the side appearances of objects can still be seen since, when the photo
was taken, some of the sides of the building may have been hidden due to relief displacement and not visible in the photo. This effect is removed in a true
orthophoto, which gives the appearance of looking directly straight down on
all objects in the image. This is achieved by using several images of the area
for filling in the missing information, while keeping constant scale in the final
photo.
Orthophotos are used to create special products called Digital Orthophoto Quads, or DOQs. A DOQ is orthophotography that covers 3.75 minutes
of latitude and 3.75 minutes of longitude of the ground, the same area as
one-fourth of a 7.5 minute USGS topographic map (see Chapter 13 for more
about topographic maps). DOQs (sometimes referred to as DOQQs, or Digital
Orthophoto Quarter Quads) have been georeferenced (see Chapter 3 for more
information on georeferencing) so that they can be matched up with other
geospatial datasets. Also note that the USGS produces certain other DOQ
products that cover the area of the entire quad. See Hands-on Application 9.3:
Using MSR Maps for Viewing Aerial Images for an online application that is
used for viewing aerial images.
true orthophoto an
orthophoto where
all objects look as if
they’re being seen from
directly above.
DOQ a Digital
Orthophoto Quad.
Orthophotos that cover
an area of 3.75 minutes
of latitude by 3.75
minutes of longitude,
or one-fourth of a 7.5
minute USGS quad.
FIGURE 9.9 The effect
of relief displacement in
an aerial photo. (Source:
aerial photograph by VTN
Consolidated, Inc. courtesy of
J. Van Eden)
Principal
Point
Chapter 9 Remotely Sensed Images from Above
effects of terrain and relief displacement and enforce the same scale across
the photo. A regular aerial photo can’t be used as a map since it doesn’t have
uniform scale everywhere, but an orthophoto can. Even with uniform scale,
some of the side appearances of objects can still be seen since, when the photo
was taken, some of the sides of the building may have been hidden due to relief displacement and not visible in the photo. This effect is removed in a true
orthophoto, which gives the appearance of looking directly straight down on
all objects in the image. This is achieved by using several images of the area
for filling in the missing information, while keeping constant scale in the final
photo.
Orthophotos are used to create special products called Digital Orthophoto Quads, or DOQs. A DOQ is orthophotography that covers 3.75 minutes
of latitude and 3.75 minutes of longitude of the ground, the same area as
one-fourth of a 7.5 minute USGS topographic map (see Chapter 13 for more
about topographic maps). DOQs (sometimes referred to as DOQQs, or Digital
Orthophoto Quarter Quads) have been georeferenced (see Chapter 3 for more
information on georeferencing) so that they can be matched up with other
geospatial datasets. Also note that the USGS produces certain other DOQ
products that cover the area of the entire quad. See Hands-on Application 9.3:
Using MSR Maps for Viewing Aerial Images for an online application that is
used for viewing aerial images.
true orthophoto an
orthophoto where
all objects look as if
they’re being seen from
directly above.
DOQ a Digital
Orthophoto Quad.
Orthophotos that cover
an area of 3.75 minutes
of latitude by 3.75
minutes of longitude,
or one-fourth of a 7.5
minute USGS quad.
FIGURE 9.9 The effect
of relief displacement in
an aerial photo. (Source:
aerial photograph by VTN
Consolidated, Inc. courtesy of
J. Van Eden)
Principal
Point