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 imageas 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.
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 asmask_band.roi.
Click Basic tools-> Mask->Apply Mask, select DOC_020708.tifas 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:
(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.
(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 linkLE70270400000218950.H1in 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.
Band | Ref DN to RadianceDefault
| 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.
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):
(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 ofLE7027040000218950.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:
(4)
The annual averaged value of dris 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:
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)
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 2Calculate and compare the basic statistics of reflectance Band 3 and 4.
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 band4 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->SaveRange 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.
1