EM 1110-2-2907
1 October 2003
5-19 Image to Image Rectification.
a. Images can also be rectified to a second projected digital image. The procedure is
similar to that performed in image to map rectification. Simply locate common, identifi-
able features in both images, match the locations, and assign GCPs. Adjust GCPs until
RMS error is less than 1.0. Enter the coordinate system that will be used and designate a
re-sampling method (Figure 5-4).
b. Rectified images can easily be converted from one coordinate system to another.
Projected images can readily be superimposed onto other projected data and used for
georeferencing image features.
5-20 Image Enhancement. The major advantage of remote sensing data lies in the
ability to visually evaluate the data for overall interpretation. An accurate visual inter-
pretation may require modification of the output brightness of a pixel in an effort to im-
prove image quality. Here are a number of methods used in image enhancement. This
paragraph examines the operations of 1) contrast enhancement, 2) band ratio, 3) spatial
filtering, and 4) principle components. The type of enhancement performed will depend
on the appearance of the original scene and the goal of the interpretation.
a. Image Enhancement #1: Contrast Enhancement.
(1) Raw Image Data. Raw satellite data are stored as multiple levels of brightness
known as the digital number (DN). Paragraph 2-7a explained the relationship between
the number of brightness levels and the size of the data storage. Data stored in an 8-bit
data format maintain 256 levels of brightness. This means that the range in brightness
will be 0 to 255; zero is assigned the lowest brightness level (black in gray- and color-
scale images), while 255 is assigned the highest brightness value (white in gray scale or
100% of the pigment in a color scale). The list below summarizes the brightness ranges
in a gray scale image.
0
= black
50
= dark gray
150 = medium gray
200 = light gray
255 = white
(a) When a satellite image is projected, the direct one-to-one assignment of
gray scale brightness to digital number values in the data set may not provide the best
visual display (Figures 5-5 and 5-6). This will happen when a number of pixel values are
clustered together. For instance, if 80% of the pixels displayed DNs ranging from 50
95, the image would appear dark with little contrast.
5-12
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