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Transcript of Image Resolution Chapter 10. Definitions Resolution – ability to record and display detail Spatial...
![Page 1: Image Resolution Chapter 10. Definitions Resolution – ability to record and display detail Spatial Spectral Radiometric.](https://reader036.fdocuments.in/reader036/viewer/2022081511/56649da05503460f94a8b1ae/html5/thumbnails/1.jpg)
Image ResolutionChapter 10
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Definitions• Resolution – ability to record and
display detail• Spatial• Spectral• Radiometric
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Definitions• Spatial resolution – the amount of
geometric detail• How close can two points be before you
can’t distinguish them
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Spatial Resolution• High spatial resolution: 0.6 - 4 m
• » GeoEye-1• » WorldView-2• » WorldView-1• » QuickBird• » IKONOS• » FORMOSAT-2• » ALOS• » CARTOSAT-1• » SPOT-5
• Medium spatial resolution: 4 - 30 m• » ASTER• » LANDSAT 7• » CBERS-2
• Low spatial resolution: 30 - > 1000 m• SeaWiFS• GOES
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Radiometric Resolution• Radiometric resolution – the amount
of brightness detail• Is the image black and white, shades of
grey• How many bits – 4, 8, 12, 16, etc.
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Radiometric Resolution
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8 bit6 bit
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2 bit 1 bit
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2-bit
8-bit
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Spectral Resolution• Spectral resolution – the amount of
detail in wavelength• 2 bands, 4, 6, 200 or more
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Temporal Resolution• Temporal resolution – the amount of
detail in time• High altitude aerial photos every 10
years, Landsat 16 days, NOAA 4 hrs • High resolution: < 24 hours - 3 days• Medium resolution: 4 - 16 days• Low resolution: > 16 days
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Tradeoffs
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Tradeoffs• There are trade-offs between spatial,
spectral, and radiometric resolution • Taken into consideration when engineers
design a sensor.
• For high spatial resolution, the sensor has to have a small IFOV (Instantaneous Field of View).
• However, this reduces the amount of energy that can be detected as the area of the ground resolution cell within the IFOV becomes smaller.
• This leads to reduced radiometric resolution - the ability to detect fine energy differences.
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Tradeoffs• To increase the amount of energy
detected (and the radiometric resolution) without reducing spatial resolution, we have to broaden the wavelength range detected for a particular channel or band. • Unfortunately, this reduces the spectral
resolution of the sensor. • Conversely, coarser spatial resolution would
allow improved radiometric and/or spectral resolution.
• Thus, these three types of resolution must be balanced against the desired capabilities and objectives of the sensor.
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Target Variables• Contrast – the brightness difference
between an object and the background• High contrast improves spatial detail
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Contrast versus spatial frequency
Sinusoidal target with varying contrast in % and varying spatial frequency left to right
Obvious resolution decrease from left to right. If your eyes are too good squint to see effect
Picture from www.normankoren.com/Tutorials/MTF.html
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Target Variables• Shape is also a significant factor
• Aspect ratio is how long the object is compared to its width• Long thin features can be seen even if
they are narrower than the spatial resolution
• Regularity of shape makes for better detail• Agricultural fields
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Target Variables• Number of objects favor higher detail
• Orchard versus single tree
• Extent and uniformity of background also helps distinguish things
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Aerial view of Olympic Peninsula facing west from Port Orchard Bay
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System Variables• Design of sensor and its operation
are important too• Air photo – have to consider quality of
camera and lens, choice of film, altitude, scale,
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Operating conditions• Altitude
• Ground speed
• Atmospheric conditions
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Measuring resolution• Ground Resolved Distance (GRD) the
dimensions of the smallest objects recorded
• Line pairs per millimeter (LPM) is derived from targets• Target is placed on the ground and
imaged
• If two obejcts are are visually separated, they are considered “spatially resolved”
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Measuring resolution• Using the target you measure the
smallest pair of lines (black line plus adjacent white space)
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Modulation Transfer Function• The Modulation Transfer Function
(MTF) is response of a system to an array of elements with varying spaces
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Modulation Transfer Function• For low spatial frequencies, the
modulation transfer function is close to 1 (or 100%) • generally falls as the spatial frequency
increases until it reaches zero.
• The contrast values are lower for higher spatial frequencies .
• As spatial frequency increases, the MTF curve falls until it reaches zero. • This is the limit of resolution for a given
optical system or the so called cut off frequency (see figure below).
• When the contrast value reaches zero, the image becomes a uniform shade of grey.
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Modulation Transfer Function
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Modulation Transfer Function• The figure represents a sine pattern
(pure frequencies) with spatial frequencies from 2 to 200 cycles (line pairs) per mm. • The top half of the sine pattern has
uniform contrast.
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Modulation Transfer Function• Perceived image sharpness (NOT
lp/mm resolution) is closely related to the spatial frequency where MTF is 50% (0.5)• i.e. where contrast has dropped by half.
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Modulation Transfer Function• Contrast levels from 100% to 2% are
illustrated on the chart for a variable frequency sine pattern.
• Contrast is moderately attenuated for MTF = 50% and severely attenuated for MTF = 10%.
• The 2% pattern is visible only because viewing conditions are favorable: • it is surrounded by neutral gray, it is noiseless
(grainless), and the display contrast for CRTs and most LCD displays is relatively high.
• It could easily become invisible under less favorable conditions.
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Modulation Transfer Function• How is MTF related to lines per
millimeter resolution? • The old resolution measurement—
distinguishable lp/mm— corresponds roughly to spatial frequencies where MTF is between 5% and 2% (0.05 to 0.02).
• This number varies with the observer, most of whom stretch it as far as they can.
• An MTF of 9% is implied in the definition of the Rayleigh diffraction limit.
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Mixed Pixels• If the area covered by a pixel is not
uniform in composition it leads to mixed pixels.
• These often occur at the edge of large parcels, along linear features, or scattered due to small features in the landscape (ponds, buildings, vehicles, etc.)
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Mixed Pixels
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Mixed Pixels• The spectral responses of those
mixed pixels is not a pure signature, but rather, a composite signature
• Can you think of an advantage to having a composite signature?
• Identify areas that are too complex to resolve individually
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• There have been a number of studies on the effect of resolution on mixed pixels
• As resolution becomes coarser• Mixed pixels increase• Interior pixels decrease• Background pixels decrease
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Resolution and Mixed Pixels Resolution Total Mixed Interior Back-
ground
A - fine 900
%
109
1.1
143
15.9
648
72
B 225
%
59
26.2
25
11.1
141
62.7
C 100
%
34
34
6
6
60
60
D - coarse 49
%
23
46.9
1
2
25
51
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Original Landsat image
Image resampled at coarser resolution
wheat (red), potatoes (green) and sugar beet (blue)
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Spatial and Radiometric Resolution• Sensors are designed with specific
levels of radiometric resolution and spatial resolution• Both of these determine the ability to
portray features in the landscape
• Broad levels of resolution may be adequate for coarse-textured landscape
• Finer resolution may help to identify more features, but may also add more detail than necessary
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Interactions with Landscape• In a study of field size in grain-
producing regions, Podwysocki (1976) showed how effectiveness of different resolutions could be quantified.
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Interactions with Landscape• Simonett and Coiner (1971)
conducted another study to determine the effectiveness of the yet to be launched MSS sensor
• Simulated by using airphotos and overlaying a grid of 800, 400, 200, and 100 feet.• Assessed the number of land-use
categories in each cell