Sensing
Measuring Remote an object from a distance
For GIS, that means using photographic or satellite
images to gather spatial data
Fundamental Variables
Planimetric (x,y) location and
dimensions
Topographic (z) location
Color (spectral reflectance)
Surface Temperature
Texture
Surface Roughness
Moisture Content
Vegetation Biomass
Two Main Image Types
Aerial Photographs
common
relatively inexpensive
easy to interpret
small area coverage
can be geometrically
corrected
Satellite Images
large area coverage
broader spectral range
digital formats
inexpensive for large
areas
geometrically accurate
Electromagnetic energy is a mixture of
waves with different frequencies
Each wave represents energy that varies at a
given frequency.
Source:http://www.cnr.berkeley.edu/~gong/textbook/chapter2/html/sect21.htm
Plotting amplitude against the wavelength you get
an electromagnetic curve, or spectrum
Source:http://www.cnr.berkeley.edu/~gong/textbook/chapter2/html/sect21.htm
Radiation (electromagnetic energy) is emitted by the Sun,
and attenuated by the atmosphere.
Specific bands of wavelengths are used for remote
sensing
xrays
Blue .4 to.5 m
Green .5 to .6 m
Red .6 to .7 m
(micrometers)
(micrometers)
(micrometers)
Visible Light - How we see colors
Emit/Reflect
Addition of light
waves of different
frequencies
Absorb
Absorb some of the
frequencies you
see whats reflected
(paint,pigments like
chlorophyll)
http://science.howstuffworks.com/light7.htm
Energy, Atmosphere, and Surface Interactions
Sources of Information
Variations in electromagnetic fields that can be
used to identify and characterize objects:
Spectral-radiometric (color, temperature)
Spatial (pattern, size, shape, texture, ...)
Temporal
Spectral Reflectance Curves
% of energy reflected
Wavelength (m)
A
B
C
Spectral Resolution and Spectral Sampling
The three shaded bars A, B, and C represent three spectral bands.
The width of each bar covers a spectral range within which no signal
variation can be resolved. The width of each spectral band represents
its spectral resolution. The resolution of A is coarser than the
resolution of B. This is because spectral details within band A that
cannot be discriminated may be partly discriminated with a spectral
resolution as narrow as band B. The resolution relationships among
the three bands are:
Resolution of A < Resolution of C < Resolution of B
Adapted from
Source:http://www.cnr.berkeley.edu/~gong/textbook/chapter1/html/sect13.htm
Concept Spectral of Bands
A
B
Adapted from
Source:http://www.cnr.berkeley.edu/~gong/textbook/chapter1/html/sect13.htm
C
Photo Scale
Set by flying height,
focal length
Most mapping
cameras use 6
lens
Reduce scale by
flying higher
Increasing flying height reduces
scale (objects get smaller, area covered by
each photo increases)
Increasing focal length increases
scale (objects get larger, are covered
decreases)
Scale is NOT Constant
Can be over flat terrain with perfectly
vertical photographs - rarely occurs
Terrain - some objects are closer to
lens, hence larger scale
Tilt - causes perspective distortion
Perspective vs. Orthographic Views
Terrain Variation
Causes
Relief Displacement
Features are
displaced radially
from their
planimetric
position due to
differences in
relative elevation
Tilt measured
as the angle
between a
line
perpendicular
to the film and
a line
perpendicular
to the datum.
Typically
specified to
be less than
3o on vertical
aerial photos.
Parallax Relative
shift in position with a
change in
viewing location.
Closer (taller)
objects
shift
more.
We measure
parallax to
estimate
height.
Relief Displacement Height affects horizontal position
Displacement are radial
higher outward shift
Lower inward shift
In a planimetric map,
A should occur
at the same location
as B
But is displace by d
Question: how much is
d?
Use similar
triangles to find out
Big triangle CNS
Small triangle 1 - Cna
Small triangle 2 - ABS
We may see:
D/P = d / p
and
D/P = h/H
So
d/p = h/H
d=p*h/H
So
We know H
Measure p on photo
Know h from DEM
Apply equation
d = p* h/H
Apply for every spot
(cell, pixel) on the
photo, yielding an
ortho-corrected image
Time of Year and Film Type
Black and white (Panchromatic)
lower costs, wide sensitivity
"True" color Enhanced interpretation
Color infrared film (also near-IR or color-IR) Enhanced vegetation, more contrast
Two Main Types of Films
Photo
Interpretation
This is the
process of
identifying and
mapping the
features that
appear on the
photos
Use characteristics of the objects observed,
plus knowledge of acquisition (scale, time of
year, film type) to identify features
Characteristics used include:
Shape
Size
Color (or tone)
Texture
Shadows
and Context
Satellite Systems
High Resolution Sensing Systems
Ikonos, Quickbird
Spot
Medium Resolution Sensing Systems
Landsat Thematic Mapper (TM, ETM+)
Multispectral Scanner (MSS)
SPOT coarse modes
COARSE RESOLUTION LAND SENSORS
AVHRR (since the 1970s)
MODIS