Lab Instructions
Lab 5, due 7:00 pm October 13, 2008
EES5053/GEO4093: Remote Sensing, UTSA
Student Name: ___________________
Digital Image Processing II: Atmospheric Correction, Radiance,
Reflectance, NDVI from Landsat Image
Objective: In this lab, you will learn the basic procedure of digital image
processing from converting the digital number to radiance, and then to
reflectance by using band math. Based on reflectance, you will learn how
to calculate the NDVI, and classify the NDVI using Density slice.
Part I: Concepts and short questions:
1. Give a four bands image of 4 x 3 as the following, please replace them into BSQ,
BIP, and BIL formats for storing.
Band 1
1, 4, 5, 6
3, 4, 4, 5
2, 3, 9, 10
Band 2
2, 5, 8, 9
10, 3, 2, 1
9, 8, 7, 6
Band 3
105, 79, 103, 114
114, 100, 124, 134
130, 125, 109, 150
Band 4
20, 25, 31, 26
29, 19, 29, 50
25, 30, 33, 50
2. Summarize the differences and similarities of supervised and unsupervised
classifications. You may use examples to illustrate them.
3. Assume that two road intersections shown on a photograph can be located on a
1:25,000 scale topographic map. The measured distance between the intersections
is 47.2 mm on the map and 94.3 mm on the photograph. (a) What is the scale of
the photograph? (b) at that scale, what is the length of a fence line that measures
42.9 mm on the photograph?
4. The area of a lake is 52.2 cm2 on a 1:7500 vertical photograph. Please calculate
the ground area of the lake in m2, ha, and acres.
Part II
1
1. Preparation:
(1). Create a Lab5 directory under c:\UserData_ENVI\yourname\. Today we will use
the atmospherically corrected image that you processed in Lab4:
p27r40_July8_2002DOS.img. You do not need to copy this image from Lab4 to
Lab5, but directly open it from Lab4, while save your results to Lab5 directory.
2. Spectra radiance calculation
Equation 1 is the basic equation for calculating spectral radiance from the Digital
Number (DN) of Landsat 4, 5 and 7:


LMAX  LMIN
 * ( DN  QCALMIN )  LMIN
L  
 QCALMAX  QCALMIN 
(1)
where, DN is the Digital Number of each pixel in the image (in this lab, it will be
p27r40_July8_2002DOS.img, although a simple DOS atmospheric correction has been
performed, it is still DN, not radiance yet), LMAX and LMIN are the calibration constants,
and QCALMAX and QCALMIN are the highest and the lowest points of the range of
rescaled radiance in DN. All those parameters can be found in the image head files.
For Landsat 7, However, there is a more simple way (equation 2) for calculating
spectral radiance L (Landsat 7 Science User Data Handbook Chap.11, 2002). This is
what we will use for the Lab.
L  gain * DN  offset
(2)
In Equation 2, the “gain” corresponds to the “Gain” in the header file, and the “offset”
corresponds to the “Bias” in the header file. The unit of radiance is W m-2 sr-1 um-1
The image p27r40_July8_2002DOS.img you worked on in Lab 4 was
downloaded free from the TexasView Remote Sensing Consortium that UTSA (LRSG) is
the member of the Consortium: http://www.crgsc.org/Data/Remote.aspx. Now I would
like you to explore this website (there are many cool stuff for you to explore) and to know
how you may download data from there for your own research or for the class project.
Also you can see the header file information you need for this lab. When are in the
TexasView website, you will see the figure below:
2
This is the available Landsat 7 images for the Texas and its adjacent states. You
can see the Path (east to west) from 24-33 and Row (north to south) from 35-42. The
images you guys used for our labs are in Path 27 and Row 40, i.e. 2740 in the figure
above. Path tells you each satellite orbit, row tells the descending or ascending of the
orbits.
Question1, from this figure, is Landsat 7 a descending or ascending satellite for
the day time passing Texas (usually 10:00 am). What is descending or ascending?
Path and Row together define the one Landsat image. For San Antonio, it is
Path27, Row40 or 2740 in the image. Click the 2740 in the image, you will see there are
4 years of images available for San Antonio: y1999, y2001, y2002, y2003. The image we
are using is in July 8, 2002. Click y2002, you will see three images in different days
(Julie day): d189, d317, d365). The Julie Day for July 8, 2002 is the day 189. so click
189. you will see two different types of images. What we are using is the ‘nlaps’. Click
the ‘nlaps’, you will see the many image files there: the same file name with different file
extension. H1, H2, H3, HI are header files; I1, I2, …, I9 are real images. To understand
what they mean, please click the README.TXT file. I would encourage you to
understand all of them using your spare time, since it is the first step for you to really get
familiar with a satellite sensor and what those parameters mean. If you have any
question about them, you are always welcomed to ask me.
You should be able to find the gain and offset information from those files for the
Lab. But to save your time, I list them for you to use.
Band | Ref
DN to Radiance
Default
| Detector
gain
offset
Abs Calib?
------------------------------------------------------1
|
15
0.775686
-6.20000
FALSE
2
|
12
0.795686
-6.39999
FALSE
3
|
8
0.619216
-5.00000
FALSE
4
|
7
0.965490
-5.10001
FALSE
3
5
6
7
8
9
|
|
|
|
|
14
8
10
27
8
0.125725
0.066823
0.043726
0.971765
0.037059
-0.99999
0.000000
-0.35000
-4.70000
3.200000
FALSE
FALSE
FALSE
FALSE
FALSE
In this lab, we only applying equation 2 to band 3 and band 4. You can use the Band
Math tool to do so. From the main ENVI menu, click Basic Tools -> Band Math, type
the equation for band 3 or band 4 as the figure 1 below, and click OK. A new window
will popup, select the atmospheric corrected band 3 as b3, then save this image to your
directory as Radinace-b3.img. In the similar way, you can do the band 4 and save it as
Radiance-b4.img. Those two images will appear in the Available Band List window.
Figure 1. Band math expressions
Question 2, calculate and show the basic statistics of the two radiance images, and give a brief
analysis of the statistics. (please use the mask image you build in the last lab to exclude the
areas outside the image)
3. Spectra reflectance calculation
The reflectance for band  is computed by the following equation (Markham and
Barker,1986 and Landsat 7 Science User Data Handbook Chap.11, 2002):
 
  L  d 2
ESUN   cos 
(3)
4
where L is at satellite spectral radiance which is the outgoing radiation energy of the
band observed at the top of atmosphere by the satellite (in this lab, we use the results
calculated from step 3), d is the Earth-Sun distance in astronomical units (), ESUN is
mean solar exoatmospheric irradiances for the band , and cos is the cosine of the
solar incident angle. Supposing a horizontal land surface is flat, the cosine of solar
incidence angle (cos) can be calculated from the Sun Elevation cos(90-SunElevation).
The Sun elevation angle for the image is 65.26º (you can get this from the head file of
LE7027040000218950.H1 mentioned above)
Question 3. What is the Sun Azimuth angle for the image when it was acquired? (you
should be able to find it from the same head file. By the way, if no specific notice, the
Sun azimuth angle usually starts from the north and clockwise). What is the sun zenith
angle for the image when it was acquired?
Since the inverse of d2 (which is 1/d2) in Equation 3 is equivalent to “inverse squared
relative distance Earth-Sun, dr“, the Equation 3 can be rewritten as:
 
  L
ESUN  cos  d r
(4)
The annual averaged value of dr is 1.0, and it ranges from about 0.97 to 1.03. You can
find a real number for a special date (such as the189 day: July 8 for this image used is
1.0167)
from
Data
Products/Table
11.4
of
this
link
here
at:
http://landsathandbook.gsfc.nasa.gov/handbook.html
The values for ESUN in Equation 4 are given in Table 3 below. The value of ESUN for
band 6 is not available.
Table 3. ESUN for Landsat 4 and 5 TM in mW/cm2/μm (Markham and Barker, 1986),
and for Landsat 7 ETM+ in W/m2/μm (Landsat 7 Science User Data Handbook Chap.11,
2002)
Landsat-4 TM
Landsat-5 TM
Landsat 7 ETM+
Band1
195.8
195.7
1969
Band2
182.8
182.9
1840
Band3
155.9
155.7
1551
Band4
104.5
104.7
1044
Band5
21.91
21.93
225.7
Band6
-
Band7
7.457
7.452
82.07
In this lab, we only calculate the reflectance of band 3 and band 4, using the Band
Math tool as in step 2, output as Reflectance_b3.img and Reflectance_b4.img.
Question 4. Calculate and compare the basic statistics of reflectance Band 3 and band 4.
(please use the mask image you build in the last lab to exclude the areas outside the image)
4. Calculate NDVI
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NDVI stands normalized difference of vegetation index: the difference between the near
infrared band (~0.83 µm) and the red band (0.66 µm). For Landsat image (TM or ETM+
image), they are band 4 and band 3, respectively. Thus, the NDVI can be calculated
based on this equation: (b4-b3)/(b4+b3), using the reflectance band 3 and band 4 as input,
name your result image as NDVI.img.
Question 5. Show the basic statistics of NDVI. (please use the mask image you build in
the last lab to exclude the areas outside the image)
5. Density Slice
In the image window, click Overlay->Density Slice. In the density slice window, select
one of the item, then edit the data range and color, click apply. Then click file->Save
Range to your folder as NDVI_class.
Question 6. Upload an image as RGB742, link it with NDVI Density Slice. Please
make a simple discussion/comparison about the spatial distribution of the NDVI values
and land cover types. For example, you may do: Which types have the high NDVI?
Which types have the low NDVI?
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