Geophysical data acquisition, processing and interpretation - Geophysics for the Mineral Exploration Geoscientist - page 55

Look-up
table

a)

Linear

255

209

Stretch function

Data histogram

Display
histogram

0

G

TMI

Min Max

Low High

Class values

Look-up
table

b)

Histogram equalised

255

249

Display
histogram

Data histogram

0

TMI

G

Min Max

Low High

Class values

c)

Look-up
table

Histogram normalised

255

195

Display
histogram

Data histogram

0

TMI G

Min Max

Low High

Class values

0 2

Kilometres

Figure 2.33 Images of the data shown in Fig. 2.32 created using the same look-up table, but with different colour stretches. (a) Linear stretch,
(b) histogram equalised, (c) histogram normalised (Gaussian stretch). The frequency histogram of the data and the display histogram are
shown for each image. Note how class value is mapped to different colour values with the various stretch functions. Data are aeromagnetic

data from the vicinity of the Mount Polley alkalic Cu

' G '

Au deposit, British Columbia, Canada. Pit outlines are shown. Data are reproduced
with the permission of the Minister of Public Works and Government Services Canada, 2006, and courtesy of Natural Resources Canada,
Geological Survey of Canada.

-

broadened relative to the data histogram. This means there
is a greater range of colours comprising the image.
Alternatively, the histogram equalisation or histogram
linearisation stretch adjusts the size of the data class inter-
vals so that the display histogram has approximately equal
number of data points in each class, i.e. the data points are
more uniformly distributed across the classes ( Fig. 2.33b ).
The

contains approximately equal numbers of pixels of each
colour. Value G now maps to display value 249, a pale red.
The histogram normalisation stretch adjusts the size of the
data class intervals so that the display histogram resembles
a normal (Gaussian) distribution ( Fig. 2.33c ). This
increases the number of colours assigned to the central
part of the data range, where usually most variation occurs.
Value G now maps to colour value 195, a shade of orange.

flattened display histogram shows that the image