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Image Resolution. Chapter 10. Definitions. Resolution – ability to record and display detail Spatial Spectral Radiometric. Definitions. Spatial resolution – the amount of geometric detail How close can two points be before you can’t distinguish them. Spatial Resolution.

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Image resolution l.jpg

Image Resolution

Chapter 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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6 bit

8 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


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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


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