Environmental Remote Sensing GEOG 2021

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Environmental Remote Sensing GEOG 2021

• Lecture 2
• Image display and enhancement

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Image Display and Enhancement

• Purpose
• visual enhancement to aid interpretation
• enhancement for improvement of information
  extraction techniques

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Image Display

• The quality of image display depends on the
  quality of the display device used
• and the way it is set up / used
• computer screen - RGB colour guns
• e.g. 24 bit screen (16777216)
• 8 bits/colour (28)
• or address differently

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Colour Composites

• Real Colour composite
• red band on red
• green band on green
• blue band on blue

Swanley, Landsat TM 1988
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Colour Composites

• Real Colour composite
• red band on red

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Colour Composites

• Real Colour composite
• red band on red
• green band on green

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Colour Composites

• Real Colour composite
• red band on red
• green band on green
• blue band on blue

approximation to real colour...
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Colour Composites

• False Colour composite
• NIR band on red
• red band on green
• green band on blue

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Colour Composites

• False Colour composite
• NIR band on red
• red band on green
• green band on blue

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Colour Composites

• False Colour composite
• many channel data, much not comparable to RGB
  (visible)
• e.g. Multi-polarisation SAR
• HH Horizontal transmitted polarization and
  Horizontal received polarization
• VV Vertical transmitted polarization and
  Vertical received polarization
• HV Horizontal transmitted polarization and
  Vertical received polarization

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Colour Composites

• False Colour composite
• many channel data, much not comparable to RGB
  (visible)
• e.g. Multi-temporal data
• AVHRR MVC 1995
• April
• August
• September

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Colour Composites

• False Colour composite
• many channel data, much not comparable to RGB
  (visible)
• e.g. MISR -Multi-angular data (August 2000)

0o 45o -45o
RCC
Northeast Botswana
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Greyscale Display

• Put same information on R,G,B

August 1995 August 1995 August 1995
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Density Slicing
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Density Slicing
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Density Slicing

• Dont always want to use full dynamic range of
  display
• Density slicing
• a crude form of classification

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Density Slicing

• Or use single cutoff
• Thresholding

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Density Slicing

• Or use single cutoff with grey level after that
  point
• Semi-Thresholding

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Pseudocolour

• use colour to enhance features in a single band
• each DN assigned a different 'colour' in the
  image display

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Pseudocolour

• Or combine with density slicing / thresholding

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Image Arithmetic

• Combine multiple channels of information to
  enhance features
• e.g. NDVI
• (NIR-R)/(NIRR)

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Image Arithmetic

• Combine multiple channels of information to
  enhance features
• e.g. NDVI
• (NIR-R)/(NIRR)

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Image Arithmetic

• Common operators Ratio

• Landsat TM 1992
• Southern Vietnam
• green band
• what is the shading?

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Image Arithmetic

• Common operators Ratio

topographic effects visible in all bands FCC
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Image Arithmetic

• Common operators Ratio (cha/chb)

• apply band ratio
• NIR/red
• what effect has it had?

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Image Arithmetic

• Common operators Ratio (cha/chb)

• Reduces topographic effects
• Enhance/reduce spectral features
• e.g. ratio vegetation indices (SAVI, NDVI)

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Image Arithmetic

• Common operators Subtraction

An active burn near the Okavango Delta,
Botswana NOAA-11 AVHRR LAC data (1.1km pixels)
September 1989. Red indicates the positions
of active fires NDVI provides poor
burned/unburned discrimination Smoke plumes
gt500km long

• examine CHANGE e.g. in land cover

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Top left AVHRR Ch3 day 235 Top Right AVHRR
Ch3 day 236 Bottom difference pseudocolur
scale black - none blue - low red -
high Botswana (approximately 300 300km)
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Image Arithmetic

• Common operators Addition
• Reduce noise (increase SNR)
• averaging, smoothing ...
• Normalisation (as in NDVI)

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Image Arithmetic

• Common operators Multiplication
• rarely used per se logical operations?
• land/sea mask

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Histogram Manipluation

• WHAT IS A HISTOGRAM?

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Histogram Manipluation

• WHAT IS A HISTOGRAM?

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Histogram Manipluation

• WHAT IS A HISTOGRAM?

Frequency of occurrence (of specific
DN)
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Histogram Manipluation

• Analysis of histogram
• information on the dynamic range and distribution
  of DN
• attempts at visual enhancement
• also useful for analysis, e.g. when a multimodal
  distibution is observed

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Histogram Manipluation

• Analysis of histogram
• information on the dynamic range and distribution
  of DN
• attempts at visual enhancement
• also useful for analysis, e.g. when a multimodal
  distibution is observed

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Histogram Manipluation
Typical histogram manipulation algorithms Linear
Transformation

255
output
0
0
255
input
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Histogram Manipluation
Typical histogram manipulation algorithms Linear
Transformation

255
output
0
0
255
input
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Histogram Manipluation
Typical histogram manipulation algorithms Linear
Transformation

• Can automatically scale between upper and lower
  limits
• or apply manual limits
• or apply piecewise operator

But automatic not always useful ...
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Histogram Manipluation
Typical histogram manipulation algorithms Histogr
am Equalisation

Attempt is made to equalise the frequency
distribution across the full DN range
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Histogram Manipluation
Typical histogram manipulation algorithms Histogr
am Equalisation

Attempt to split the histogram into equal areas
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Histogram Manipluation
Typical histogram manipulation algorithms Histogr
am Equalisation

Resultant histogram uses DN range in proportion
to frequency of occurrence
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Histogram Manipluation
Typical histogram manipulation algorithms Histogr
am Equalisation

• Useful automatic operation, attempting to
  produce flat histogram
• Doesnt suffer from tail problems of linear
  transformation
• Like all these transforms, not always successful

• Histogram Normalisation is similar idea
• Attempts to produce normal distribution in
  output histogram

• both useful when a distribution is very skewed
  or multimodal skewed

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Histogram Manipluation
Typical histogram manipulation algorithms Gamma
Correction

• Monitor output not linearly-related to voltage
  applied
• Screen brightness, B, a power of voltage, V
• B aV?
• Hence use term gamma correction
• 1???3 for most screens

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Summary

• Display
• Colour composites, greyscale Display, density
  slicing, pseudocoluor
• Image arithmetic
• ??
• Histogram Manipulation
• properties, transformations

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Summary

• Followup
• web material
• http//www.geog.ucl.ac.uk/plewis/geog2021
• Mather chapters
• Follow up material on web and other RS texts
• Learn to use Science Direct for Journals