Lab 4 -- Displaying Images
Introduction
You will display images before and after contrast stretching, images of different spatial resolutions, and true
and false color images (composites). The following images will be used.
greenmill_run_dem.img
A digital elevation model (DEM) for Greenmill Run (in front of Brewster Building)
1m_pan.img
aerial photo of 1x1 m resolution covering area around ECU football stadium
10m_pan.img
10x10 m resolutions satellite data covering Greenville area
15m_pan.img
15x15 m, and 30x30 m resolutions satellite data covering Greenville area
30m_pan.img
30x30 m resolutions satellite data covering Greenville area
ecu_campus_1m.img
1 meter resolution USGS DOQQ acquired in 1998.
ecu_campus_15m.img
15 meter resolution, Landsat 7 TM 8 image acquired on 09/23/1999.
pitt_093099_7band.img
a 7-band Lansat-7 TM image of the Tar River Basin within Pitt County, NC
to create and display true and false color composites.
Images before and after contrast stretching
You need to view images, greenmill_run_dem.img before and after contrast stretching. The contrast stretching
allows the images to be displayed and viewed in the full range of the intensity.
View a raw image
i)
Start Imagine, and split the viewer into two Views.
ii) Click on the File/Open/Raster Layer to open the "Select Layer To Add" dialog box (Fig. 1).
iii) Click (a single click) on the greenmill_run_dem.img file to select it. (DONOT double-click on the image file
name; double-clicking the file name will turn off the "No Stretch" option! You need to turn the "No Stretch"
option on.)
iv) Click on the tap of "Raster Options" to open another page of the "Select Layer To Add" dialog box. Click on "No
Stretch" button, and then OK to finish. The image is displayed without contrast stretch.
View a stretched image
i)
Click on the File/Open/Raster (or the short-cut: click on the open folder icon) to open the "Select Layer To Add"
dialog box again. Double-click on the greenmill_run_dem.img. (Note: when viewing an image by the viewer in the
IMAGINE, the default is to turn the stretch on.) Answer question 1. (The viewer windows displaying both without
and with stretched images have been resized.)
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Fig. 1.
Fig. 2. The DEM data for Greenmill Run, Greenville, NC. On the left: without the contrast stretching. On the right: with
the contrast stretching.
Compare images of the same size but with different resolutions
Open 4 viewers and display 1m_pan.img, 10m_pan.img, 15m_pan.img, and 30m_pan.img. Turn the stretch
option on for all viewed images. Answer questions 2–4.
In two viewers, display ecu_campus_1m.img and ecu_campus_15m.img. Answer question 5.
Compare images of the same ground area but with different resolutions or pixel sizes
The 1m_pan_stadium.img, 10m_pan_stadium.img, 15m_pan_stadium.img and 30m_pan_stadium.img show
primarily the football stadium of ECU with different resolutions or pixel sizes. View them in four viewers (Fig. 3). Use
the 1m_pan_stadium.img as the basis when comparing images with different spatial resolutions and answering
question 6.
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Fig. 3, 1st row: 1m_pan_stadium.img (left) and 10m_pan_statium.img (right).
2nd row: 15m_pan_stadium.img (left) and 30m_pan_stadium.img (right).
To make the images of 10x10 m, 15x15 m, and 30x30 m have 400 columns by 400 rows, one can resample
them (Fig. 4). Even though resampling reduces the pixel size or increase the number of rows and the number of
columns, it does not increase the clearness or resolution of them. Answer question 7.
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Fig. 4, 1st row: 1m_pan_stadium.img (left) and 10m_pan_statium_resampled.img (right).
2nd row: 15m_pan_stadium_resampled.img (left) and 30m_pan_stadium_resampled.img (right).
True or false color composite images
Using different bands of Landsat-7 TM data (e.g. pitt_093099_7bands.img) one can create different color
composite images. TM bands 1, 2, and 3 record the reflectance in the blue, green, and red wavelength regions,
respectively. Thus, a true color composite image of the pitt_093099_7bands.img can be created from the TM 3 (red)
displayed as Red (R), TM 2 (green) as Green (G), and TM 1 (blue) as Blue (B). (The color composite images are always
displayed in the order of red, green, and blue band display; this is the order that our computer or TV color monitor
displays color images.) To create the true color composite image:
i)
Click on the File/Open/Raster to open the "Select Layer To Add" dialog box (Fig. 5).
ii) Click, once, on the pitt_093099_7bands.img to select it.
iii) In the dialog box (Fig. 5), click on the "Raster Option" tab and set Red as 3 (TM band 3), Green as 2 (TM band 2),
and Blue as 1 (TM band 1). You have assigned TM 3 to Red color for display, TM 2 to Green color for display,
and TM 1 to Blue color for display. Click on the OK to close the dialog box.
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Fig. 5.
The color images created by any color assignments except for the true color assignment are called false color
images. For example, displaying TM 7 (near IR reflectance) as Red, TM 4 (near IR reflectance) as Green, and TM 1
(blue) as Blue, one creates a false color composite image. Use procedures above to create several false color composite
images of Pitt area. Answer question 8.
Questions
1.
(0.5 point) Comment on the differences between the images before and after stretching.
2.
(1 point) These images have the same size (# of rows by # of columns). Which image provides the most details
about the covered area, and which one the least details? Which image covers the smallest area, and which one the
largest area?
3.
(1 point) Compute the ground areas covered by 1m_pan.img, 10m_pan.img, 15m_pan.img, and 30m_pan.img
images. {Note: the ground area covered by an image = (# of rows or height)* (# of columns or width) * (pixel size
x) * (pixel size y). Pixel size x and pixel size y must have the same unit, e.g., meter. Use the m2 to measure the
area.}
4.
(0.5 point) Based on about area calculations, what are the ratios of the area covered by the 1m_pan.img to the areas
covered by the 10m_pan.img, 15m_pan.img and 30m_pan.img, respectively?
5.
(0.5 point) What is the # of rows and # of columns of ecu_campus_1m.img? Both ecu_campus_1m.img and
ecu_campus_15m.img cover the same ground area, and you already know the row # and column # of
ecu_campus_1.img, compute the # of rows and # of columns of ecu_campus_15.img.
6.
(0.5 point) What are the # of row by # of columns of the 1m_pan_stadium.img, 10m_pan_stadium.img,
15m_pan_stadium.img and 30m_pan_stadium.img?
7.
(0.5 point) What are the pixel sizes of the 10m_pan_stadium_resampeld.img, 15m_pan_stadium_resampled.img,
and 30m_pan_stadium_resampled.img?
8.
(0.5 point) Are there any unusual differences or similarities between the 3(R)2(G)1(B) image, 7(R)4(G)1(B) image,
and 4(R)7(G)1(B) image.
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Acknowledgment
This lab handout was supported by an AmericaView grant to the East Carolina University at Greenville, North
Carolina, 27858, USA.
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