GEOG 372: Introduction to Remote Sensing
October 10th and 11th, Fall 2007
Laboratory 6
Reflectance Spectra in Remote Sensing Images
No late labs can be accepted. Please type the answers just below the questions and hand the printouts at the beginning of the next lab: October 17th (0102) and 18th (0101). Also do not forget to mention your MAJOR on your labs
The purpose of this laboratory exercise is to analyze spectral responses of various objects in the EM spectrum and understand the underlying physical principles for these responses. This lab will require a fair amount of additional work on your own analyzing the spectral responses, using lecture notes and your text.
Task 1. Identify the features at given pixel locations, obtain their spectral profiles in visible, NIR and midIR range and explain the spectral response of a given feature. Open ENVI 4.2. Go to File – Open Image File, navigate to U:\\g372\372-fall07\lab6data and open file 2001_10_05image. When the file opens in the “Available Bands List” you will notice that the file contains 6 bands. These are bands 1 through 5 and band 7 of the Landsat image stacked in the same file. These bands have already been converted from DN to reflectance values for you.
Note: This is a large file, so be patient letting things load up.
Load a true color composite (band 3 into red, band 2 into green and band 1 into blue) into a new Display. Using Tools – pixel locator find the required pixels as listed below. As an example find pixel (7018, 6140). To obtain the spectral profile go to Tools – profiles – Z profile (spectrum), or right click on your image and select Z profile (spectrum). You will get a spectral profile which shows reflectance of the pixel in all 6 bands (bands 1-5, and band 7).
- Once you open your Z profile, in that window, go to Edit – Plot Parameters. Next to where it says Axis, click in the Y-Axis button. Then, in the boxes next to Range, change the values to 0.001 and 0.400 and click Apply. You can close the Plot Parameters box after this.
- Next, in your Z profile window, go to Options and make sure you remove the check mark next to Auto Scale Y-Axis.
The reason for performing these extra steps is to make sure the profiles from each location are plotted along a common x and y axis so you can more easily see the changes at each location.
Take a screen shot of your profile. (Ctrl – Print Screen). You can then open the screenshot by pasting it into Paint, then select just the spectral profile and paste that into a text document. Make sure you name it so that you remember what it is - it is a good idea to use the pixel coordinates in the name of the file. Then using your textbook and lecture materials analyze the spectral signature of the area following the example answer below.
Figure 1. Spectral profile of pixel (7018, 6140).
Example Answer: From the overall position of the pixel in the image it appears to be within a wetland. The pixel has an overall low reflectance in all bands. It has a small peak in reflectance in the blue band (band 1) which is characteristic of shallow water features and reflectance from the bottom of the water feature. Low reflectance of the green and red (bands 2 and 3 respectively) also indicates the presence of water in this pixel. Since the reflectance in band 3 is low, it is unlikely that it has bare soil above the water surface. A small increase in the NIR (band 4) implies that there is probably some above water vegetation present in this pixel. The subsequent drop in reflectance in bands 5 and 6 is also characteristic of water bodies. Conclusion: the pixel lies within a shallow water wetland that has some vegetation above the water level.
Note: Be sure to mention reflectance in every band !
Your write up will include the X,Y position of a pixel, a picture or neat drawing of the profile, and explanation similar to the above example for each of the following points. (Note: as an example, pixels a, b, and c are all water pixels with similar spectral profiles, but instead of giving three identical answers, describe how the minor variations between each profile give you more information about the pixel).
a) 5803, 7315
b) 6800, 6644
c) 3841, 5911
d) 2867, 2196
e) 2968, 2220
f) 7195, 3959
g) 5361, 5387
h) 3586, 4855
i) 3135, 1736
j) 5341, 2508
Copy your screenshots to a CD or USB jump drive, email them to yourself, or print them using your print account.
Log off your computer and turn off the monitor when you are finished.
Typed answers with screenshots are due October17th (0102) and 18th (0101), 2007.