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 Band 2 Band 3 Band 4
1, 4, 5, 6 2, 5, 8, 9 105, 79, 103, 114 20, 25, 31, 26
3, 4, 4, 5 10, 3, 2, 1 114, 100, 124, 134 29, 19, 29, 50
2, 3, 9, 10 9, 8, 7, 6 130, 125, 109, 150 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. 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:
(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 Ll (Landsat 7 Science User Data Handbook Chap.11, 2002). This is what we will use for the 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 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:
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
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 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 l is computed by the following equation (Markham and Barker,1986 and Landsat 7 Science User Data Handbook Chap.11, 2002):
(3)
where Ll 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 (), ESUNl is mean solar exoatmospheric irradiances for the band l, and cosq is the cosine of the solar incident angle. Supposing a horizontal land surface is flat, the cosine of solar incidence angle (cosq) 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:
(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 Data Products/Table 11.4 of this link here at: http://landsathandbook.gsfc.nasa.gov/handbook.html
The values for ESUNl in Equation 4 are given in Table 3 below. The value of ESUNl for band 6 is not available.
Table 3. ESUNl 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 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
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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