mypapers/drafts/geg2210/assignment-8.html

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   9     <p><a href="http://www.geo.uio.no/geogr/geomatikk/oppgaver/bildeforbedring_eng.html">Assigment 8</a>
  10       in <a href="http://www.uio.no/studier/emner/matnat/geofag/GEG2210/index-eng.html">GEG2210</a>
  11       - Data Collection - Land Surveying, Remote Sensing and Digital
  12         Photogrammetry</p>
  13
  14     <h1>Image enhancement, filtering and sharpening</h1>
  15
  16     <p>By Petter Reinholdtsen and Shanette Dallyn, 2005-05-01.</p>
  17
  18 <p>This exercise was performed by logging into jern.uio.no using ssh
  19 and running ERDAS Imagine.  Started by using 'imagine' on the command
  20 line.  The images were loaded from /mn/geofag/gggruppe-data/geomatikk/
  21
  22 <p>We tried to use svalbard/tm87.img, but it only have 5 bands.  We
  23 decided to switch, and next tried jotunheimen/tm.img, which had 7
  24 bands.
  25
  26 <h2>Some notes on the digital images</h2>
  27
  28 <p>The pixel values in a given band is only a using a given range of
  29 values.  This is because sensor data in a single image rarely extend
  30 over the entire range of possible values.
  31
  32 <p>The peak values of the histograms represent the the spectral
  33 sensitivity values that occure the most often with in the image band
  34 being analysed.
  35
  36 <h2>Evaluation of the different bands</h2>
  37
  38 <p><img align="right" width="40%" src="jotunheimen-truecolor.jpeg">
  39 This image show the "true colour" version, with the blue range
  40 assigned to the blue colour, green range to green colour and red range
  41 to red colour.</p>
  42
  43 <h3>band 1, blue (0.45-0.52 micrometer - um)</h3>
  44
  45   Visible light, and will display a broad range of values both over
  46   land and water.  Reflected from ice, as those are visible white and
  47   reflect all visible light waves.  Histogram show most values between
  48   30 and 136.  Mean values of 66.0668.  There are one wide peak with
  49   center around 50.  There are two peaks at 0 and 255.
  50
  51 <h3>band 2, green (0.52-0.60 um)</h3>
  52
  53   Visible light, and will display a broad range of values both over
  54   land and water.  Reflected from ice, as those are visible white and
  55   reflect all visible light waves.  Histogram show most values from 8
  56   to 120.  The mean value is 30.9774.  There are two main peaks at 20
  57   and 27.  There is also a pie at 0.
  58
  59 <h3>band 3, red (0.60-0.69 um)</h3>
  60
  61   Visible light, and will display a broad range of values both over
  62   land and water.  Reflected from ice, as those are visible white and
  63   reflect all visible light waves.  Histogram show most values from 33
  64   t 135, with one wide peak around 52.  There are also seem to be two
  65   peaks at 0 and 255.  The mean value is 34.3403.
  66
  67 <h3>band 4, near-infraread (0.76-0.90 um)</h3>
  68 <img align="right" width="20%" src="jotunheimen-band4-hist.jpeg">
  69
  70   Water acts as an absorbing body so in the near infrared spectrum,
  71   water features will appear dark or black meaning that all near
  72   infrared bands are absorbed. On the other hand, land features
  73   including ice, act as reflector bodies in this band.  The histogram
  74   show most values between 7 and 110.  The mean is 40.1144.  There are
  75   two peaks at 7 and 40.
  76
  77 <h3>band 5, mid-infrared (1.55-1.75 um)</h3>
  78
  79   The ice, glaciers and water do not reflect any mid-infrared light.
  80   The histogram show most values between 1 and 178.  The mean is
  81   49.8098 and there are two peaks at 6 and 78, in addition to two
  82   peaks at 0 and 255.
  83
  84 <h3>band 6, thermal infrared (10.4-12.5 um)</h3>
  85
  86   Display the temperature on earth.  We can for example see that the
  87   ice is colder than the surrounding areas.  The histogram show most
  88   values between 36 to 122.  The mean is 102.734.  There are one wide
  89   peak around 53, in addition to two peaks at 0 and 255.
  90
  91 <h3>band 7, mid-infrared (2.08-2.35 um)</h3>
  92
  93   The ice, glaciers and water do not reflect any mid-infrared
  94   frequencies.  The histogram show most values between 77 and 150.
  95   The mean is 24.04, and there are one wide peak at 130 and a smaller
  96   peak at 83, in addition to one peak at 0.
  97
  98 <h3>Image enhancement</h3>
  99
 100 We can get a good contrast stretch by using the histogram
 101 equalisation.  This will give us the widest range of visible
 102 separation between features.
 103
 104 <h3>Displaying colour images</h3>
 105
 106 <p><img width="40%" src="http://home.online.no/~oe-aase/jotunheimen/jotun2000topper.jpg">
 107 <!-- img src="jotunheimen-map.jpeg" -->
 108
 109 <img width="40%" src="jotunheimen-std-ir.jpeg">
 110
 111 <p>Comparing a map we found on the web, and the standard infrared
 112 image composition, we can identify some features from the colors
 113 used:</p>
 114
 115 <img align="right" width="40%" src="jotunheimen-ir-2band.jpeg">
 116 <ul>
 117
 118  <li>water is black or green
 119
 120  <li>ice and glaciers are white, while snow is light green.
 121
 122  <li>vegetation is red.
 123
 124  <li>non-vegetation is brown or dull red when closer to snow and
 125    glaciers.
 126
 127 </ul>
 128
 129 <p>Next, we tried to shift the frequencies displayed to use blue for the
 130 red band, green for the near ir band and red for the mid ir (1.55-1.75
 131 um).  With this composition, we get some changes in the colours of
 132 different features:
 133
 134
 135 <ul>
 136  <li>water is black
 137
 138  <li>ice and glaciers are light blue, while snow is dark blue.
 139
 140  <li>vegetation is light green and yellow.
 141
 142  <li>non-vegetation is red or brown.
 143
 144 </ul>
 145
 146
 147 <p>We also tried to do histogram equilization on the standard infrared
 148 composition.  This changed the colours in the image, making the
 149 previously green areas red, and the brown areas more light blue.  In
 150 this new image, we can clearly see the difference between two kind of
 151 water, one black and one green.  We suspect the green water might be
 152 deeper, but do not know for sure.</p>
 153 <img align="right" width="40%" src="jotunheimen-std-ir-eq.jpeg">
 154
 155 <h2>Filtering and image sharpening</h2>
 156
 157 <p>We decided to work on the grey scale version of the thermal infrared.
 158 This one has lower resolution then the rest of the bands, with 120m
 159 spatial resolution while the others have 30m spatial resolution.
 160
 161 <p>The high pass filtering seem to enhance the borders between the
 162 pixels.  Edge detection gave us the positions of glaciers and water.
 163 We tried a gradient filter using this 3x3 matrix.  The matrix was
 164 chosen to make sure the sum of all the weights were zero, and to make
 165 sure the sum of horizontal, vertical and diagonal numbers were zero
 166 too.
 167
 168 <p><table align="center">
 169   <tr><td>1</td><td>2</td><td>-1</td></tr>
 170   <tr><td>2</td><td>0</td><td>-2</td></tr>
 171   <tr><td>1</td><td>-2</td><td>-1</td></tr>
 172 </table></p>
 173
 174 <p>It gave a similar result to the edge detection.
 175
 176
 177 <p>We also tried unsharp filtering using this 3x3 matrix, selected
 178 also to make sure the sum of all the weights were zero, and making
 179 sure the high frequency changes had extra weight.
 180
 181 <p><table align="center">
 182   <tr><td>-1</td><td>-1</td><td>-1</td></tr>
 183   <tr><td>-1</td><td>8</td><td>-1</td></tr>
 184   <tr><td>-1</td><td>-1</td><td>-1</td></tr>
 185 </table></p>
 186
 187 <p>This gave similar results to the edge detection too.
 188
 189 <p>We started to suspect that the reason the 3x3 filters gave almost
 190 the same result was that the fact that the spatial resolution of the
 191 thermal band is actually 4x4 pixels (120 m, while the pixel size was
 192 30m).  Because of this, we tried with a 5x5 matrix, making sure it
 193 sums up to 0.
 194
 195   <p><table align="center">
 196     <tr><td>-1</td><td>-1</td><td>-1</td><td>-1</td><td>-1</td></tr>
 197     <tr><td>-1</td><td>-1</td><td>-1</td><td>-1</td><td>-1</td></tr>
 198     <tr><td>-1</td><td>-1</td><td>24</td><td>-1</td><td>-1</td></tr>
 199     <tr><td>-1</td><td>-1</td><td>-1</td><td>-1</td><td>-1</td></tr>
 200     <tr><td>-1</td><td>-1</td><td>-1</td><td>-1</td><td>-1</td></tr>
 201   </table></p>
 202
 203 <p><img align="right" width="40%"src="jotunheimen-therm-unsharp5x5.jpeg">
 204 Next, we tried some different weight:
 205
 206   <p><table align="center">
 207     <tr><td>-1</td><td>-1</td><td>-1</td><td>-1</td><td>-1</td></tr>
 208     <tr><td>-1</td><td>-2</td><td>-2</td><td>-2</td><td>-1</td></tr>
 209     <tr><td>-1</td><td>-2</td><td>32</td><td>-2</td><td>-1</td></tr>
 210     <tr><td>-1</td><td>-2</td><td>-2</td><td>-2</td><td>-1</td></tr>
 211     <tr><td>-1</td><td>-1</td><td>-1</td><td>-1</td><td>-1</td></tr>
 212   </table></p>
 213
 214 <p>This one gave more lines showing the borders between the thermal
 215 pixels.  See the included image.
 216
 217 <H2>References</h2>
 218
 219 <ul>
 220   <li><a href="http://www.cs.uu.nl/wais/html/na-dir/sci/Satellite-Imagery-FAQ/part3.html">Satellite-Imagery-FAQ</a>
 221 </ul>
 222
 223     <hr>
 224     <address><a href="mailto:pere@hungry.com">Petter Reinholdtsen</a></address>
 225 <!-- Created: Sun May  1 13:25:38 CEST 2005 -->
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 227 Last modified: Sun May  1 14:28:48 CEST 2005
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