Objective: get starting some basic operations in ENVI such as displaying an image in different ways (gray and color), getting a pixel position and value, linking multiple images, calculating NDVI, band math, statistics, and input and output image.
(2) Now you will use ENVI to do your job. ENVI is an image processing program commonly used for image processing and analysis by remote sensing scientists. Click “START” à ”Programs” à You will see there are a number of programs and documents. In this class, you will mostly use the "ENVI 4.0" and "ENVI online manuals and tutorials". The tutorials are great materials for you to learn how to use the ENVI package. In this class, some of data may come from the tutorial data. You are encouraged to explore the tutorials by yourself. If you finish all of the materials, I am sure you will be an expert in using ENVI for image processing and analysis. Click on the ENVI 4.0, the program will be running. Click “File” à ”Open Image File”. This will bring you to the default data directory (c:\RSI\IDL60\products\ENVI40\data). In this directory, there are some datasets for you to freely use. You can practise later on. Now you need to find your work directory where your data are actually locating at. There is another way to setup your work directory (you will learn it in next lab). Now go to the directory c:\RS5053\lab3\Data where your image data is, and choose the file ETMp27r40y01m7d21, which is a subset of ETM+ image with path27 row 40 acquired on 7/21/2001 prepared for this lab. This brings up a small window (called Available Bands List) that lets you pick different bands of Landsat TM data to view. You can view images in Gray Scale or RGB Color.
(4) Now you should see 3 display withows (Main, Scroll, and Zoom). Getting familar with these 3 windows
to see if you can find where Loops 410 and 1604, UTSA campus, and
are located. Click on “Tools” à“Cursor Location / Value” of the
main display window. This window tells you the location (x, y in
pixels), screen values, projection, coordinates in meters and
lagitude/longitude, and real data
values of the pixel that the cursor is pointing to.
2: Explain why this image is
called a “false” color image.
Question 3: Note the R: G: and B: values that the “Cursor Location / Value” window gives you. When you put the cursor over red pixels, what wavelength are the data largest? What objects, which are present on the ground, are responsible for these red pixels? why? (hint, see slides about absorbtion of Lecture 2). Why is there a region of relatively dark in the mountains (the very northwest part) of the image, do you know what objects are present on the ground?
(6) In the Available Bands List,
change the band composition to band 3 (R) + band 2 (G) + band 1 (B),
click Display # 1 at the bottom of the window, Click "New Display",
click "Load RGB". Click Tools à
à Link Displays à
OK, which links two images. Point your mouse to any red rectangle
of the two image diplays, click and move the mouse, and then release
what you see during the above operations? what color you see in
the #2 diplay window while it is red color in the #1 diplay window?
what color you see in
the #2 diplay window while it is dark color in the #1 diplay window
(northwest part)? why they are so different in #1 while they are looks
the same in #2? why
we call the bands 3+2+1 composition of TM image is a "true" color image?
(7) Now we will calculate a Normalized
Difference Vegetation Index (NDVI). We
will do it two different ways, first manually and then by automated
means. Click on “Basic Tools”
à ”Band Math”
from the main ENVI window. Enter the NDVI algorithm and then click “OK”. (HINT:
you can find good help on how to enter an algorithm by clicking
“Band Math, example function”). You will want to use
the float( ) function to convert all parameters in your algorithm to
floating point numbers. After getting past this hurdle, identify your
(e.g. “Band 4 (0.8300)” or “Band 3 (0.6600)” or….) with the
you named in your NDVI algorithm, then click “Memory”, then “OK
”. This will result in ENVI doing the NDVI calculation that you just
entered and spitting the result out to memory (this memory file will be
automatically removed after you close the ENVI, to keep them permanant,
you need to output it to a output file. It is better to output to
memory if you just want to see it or if the reulting image is only a
middle status image not a final one that you would like to keep). The
same calculation is automatically performed by clicking “Transform”
à NDVI (Vegetation Index),
using default parameters, and output the result to Memory.
Open these two new images in two new display windows.
Question 5: Use the Link Displays tool you just learned to see if these two new images have exactly the same pixle value? what it is the value range and the mean? (HINT: use the Statistics tool. You can find good help on how to use the tool by clicking “Help” à ”Index”, then entering "Statistics")
(8) When we use the “Transform”
à NDVI (Vegetation Index)
to calculate the NDVI, if you change the "Output Data Type" in
the NDVI Calculation Parameters window from Floating Point to Byte,
output the result to Memory. Open this image to
a new display window.
Compare this NDVI image with the previous two. Tell the difference.
Question 7: Explain the difference between Floating and Byte data types, list and explain other data types in ENVI. (HINT: you can get all these answers from the ENVI Online Help you just used previously.