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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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definitions
Definitions
  • Resolution – ability to record and display detail
    • Spatial
    • Spectral
    • Radiometric
definitions3
Definitions
  • Spatial resolution – the amount of geometric detail
    • How close can two points be before you can’t distinguish them
spatial resolution
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
radiometric resolution
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.
slide8

6 bit

8 bit

slide9

2 bit

1 bit

slide10

2-bit

8-bit

spectral resolution
Spectral Resolution
  • Spectral resolution – the amount of detail in wavelength
    • 2 bands, 4, 6, 200 or more
temporal resolution
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
tradeoffs16
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.
tradeoffs17
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.
target variables
Target Variables
  • Contrast – the brightness difference between an object and the background
    • High contrast improves spatial detail
contrast versus spatial frequency
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

target variables22
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
target variables23
Target Variables
  • Number of objects favor higher detail
    • Orchard versus single tree
  • Extent and uniformity of background also helps distinguish things
system variables
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,
operating conditions
Operating conditions
  • Altitude
  • Ground speed
  • Atmospheric conditions
measuring resolution
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”
measuring resolution29
Measuring resolution
  • Using the target you measure the smallest pair of lines (black line plus adjacent white space)
modulation transfer function
Modulation Transfer Function
  • The Modulation Transfer Function (MTF) is response of a system to an array of elements with varying spaces
modulation transfer function33
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.
modulation transfer function35
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.
modulation transfer function36
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.
modulation transfer function37
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.
modulation transfer function38
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.
mixed pixels
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.)
mixed pixels42
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
slide43
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
slide45

Original Landsat image

Image resampled at coarser resolution

wheat (red), potatoes (green) and sugar beet (blue)

spatial and radiometric resolution
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
interactions with landscape
Interactions with Landscape
  • In a study of field size in grain-producing regions, Podwysocki (1976) showed how effectiveness of different resolutions could be quantified.
interactions with landscape48
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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