Overview
of Methods
A
view from orbit
Remote
sensing is the science
of obtaining information
about an object
or area through
the analysis of
measurements made
at a distance from
the object (i.e.,
not coming in contact
with it). The oldest
form of remote sensing
is aerial photography
where the sensor
system is the camera
and film. More recently,
the field of remote
sensing has grown
to include electronic-optical
sensors which acquire
multispectral digital
images that can
be processed and
analyzed by computers.
Many of these sensors
are on satellites
which regularly
orbit the earth.
19
Landsat
images cover
Minnesota
The
Landsat satellite
orbits the Earth
438 miles above
the surface. Its
sensor, called a
multispectral scanner,
records images of
the same 115-mile
wide path every
16 days. Nineteen
images from five
orbits are needed
to cover Minnesota
. The smallest area
recorded is a ground
resolution cell
or pixel in the
imagery measuring
30 x 30 meters (about
1/4 acre). The scanner
records digital
images of the surface
reflectance in visible
and infrared wavelengths
of the electromagnetic
spectrum. The infrared
spectral bands are
especially useful
for mapping vegetation.
The
quantity most frequently
measured and recorded
in images is the
electromagnetic
energy reflected
by the object. The
source of the electromagnetic
energy is the sun
and the spectral
reflectance properties
of many Earth surface
features, such as
soil, vegetation
and water, can be
used to uniquely
identify and characterize
them.
Determining
relationships
For
all our satellite-derived
mapping applications,
an analyst needs
to determine the
relationship between
the intensity of
reflected wavelengths
and a set of "calibration" sites.
For impervious surface
mapping, these sites
represent the true,
on the ground percentage
of impervious surface
area. For land cover
mapping, these sites
represent known
examples of different
classes of landscape
cover (e.g., forest,
agriculture, water,
etc.). The relationship
that is determined
between the calibration
sites and the electromagnetic
response is then
applied to all the
pixels in the image,
providing an image-wide
classification or
map.
The
following links
provide a more in-depth
look into our methods
for producing the
maps found on this
website:
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