Lab Instructions
Lab 4, due 8:00 am September 30, 2006
EES5053: Remote Sensing, Earth and Environmental Science, UTSA
Student Name: ___________________
Digital Image Processing II: Radiance, Reflectance, NDVI, and
Density Display
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.
1. Preparation:
(1). Create your own folders:
c:\RS5053\studentname\Lab4, storing your work and final lab report.
(2). Connect to the server
\\129.115.25.240\XIE_misc\EES5053\Lab4
(3). Setup the ENVI data directory for lab4 as we did in last lab. Today we will use a
same image as in Lab3 (NE_p27r40_020708_12347), so we do not need to change
the data directory, but need change the output directory.
Because of the wrong order of cloud mask and atmospheric correction in lab3, we
need to repeat the similar procedure in lab3, but we need first do the Atmospheric
correction, and then mask the cloud using the ROI in Lab3 or cloud.roi.
2. Atmospheric Correction
Click Basic Tool -> Preprocessing -> General Purpose Utilities -> Dark Subtract ->
select the image (NE_p27r40_020708_12347.tif) as the input file, click OK, select the Band
Minimum, which means that the minimum value of each band will be automatically selected, and
then this value will be subtracted from all pixels in this band. Output the new image to Memory
or your directory and name it as DOC_020708.tif.
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3. Cloud Mask
We will use ROI in lab3 to build our cloud mask. Click Basic tools-> Mask->Build
Mask, click Option->import from ROI (Note: check Selected Area Off), and select
the Cloud_020708.roi or cloud0708.roi (I made it) as the input file, then output as
mask_band.roi.
Click Basic tools-> Mask->Apply Mask, select DOC_020708.tif as the input file, select
mask_band.roi as the mask band, Output to file: Mask_DOC_020708.tif.
4. Spectra radiance calculation
Equation 1 is the basic equation for calculating spectral radiance for band l (Ll) 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, 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.
For Landsat 7, However, there is a more simple way for calculating L (Landsat 7
Science User Data Handbook Chap.11, 2002). This is what we will use in this 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 is W m-2 sr-1 um-1
If you click the link LE70270400000218950.H1 in the lab4 folder. you will see
that this is the header file for Band1, 2, 3, 4, 5, 7. LE7027040000218950.H2 is the head
file for band 6. band 6 has always high gain and low gain images.
LE7027040000218950.H3 is the header file for band 8 image. Below is the table showing
all gain and offset for each band from those header files. You will just use the numbers in
this table below.
2
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
5
|
14
0.125725
-0.99999
FALSE
6
|
8
0.066823
0.000000
FALSE
7
|
10
0.043726
-0.35000
FALSE
8
|
27
0.971765
-4.70000
FALSE
9
|
8
0.037059
3.200000
FALSE
In this lab, we only do 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 as the figure 1 below, and click OK. A new window will popup, select the atmospheric
corrected band 3 (Mask_DOC_020708.tif) as b3, then save this image to your directory. In
the similar way, you can do the band 4. Output as Radn_189_b3.tif and Radn_189_b4.tif
Figure 1. Band math expressions
Question 1, calculate and show the basic statistics of the two radiance images, make comparisons
with previous results.
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5. 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)
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
incident 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)
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
Table
11.4
of
this
link
here
at:
http://ltpwww.gsfc.nasa.gov/IAS/handbook/handbook_htmls/chapter11/chapter11.html
The values for ESUN in Equation 4 are given in Table 3. 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 5, output as Reflt_189_b3 and Reflt_189_b4.
Question 2 Calculate and compare the basic statistics of reflectance Band 3 and 4 .
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6. Calculate NDVI
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 as NDVI_02189.
Question (3). Show the basic statistics of NDVI.
7. 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 4. Upload an image as RGB742, link it with NDVI Density Slice (gray).
Please make a simple discussion/comparison about the spatial distribution of the NDVI
values (you can edit the range of NDVI to match the RGB) and vegetation coverage.
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