Visual analysis of burn scars In this task, you will first visually analyse the burn scars by comparing the pre-fire and post-fire  imagery.  Then you  will  pre-process  and  calibrate  the  images  to  prepare  them  for  the creation

Tasks of GEOM30009:

The assignment consists of three main tasks: 

1. Visual analysis of burn scars; 
2. Creating burn index images; 
3. Creating differenced normalized burn Ratio image and mapping the burn scars.  

You should be able to complete each task in one lab session. The whole assignment should be completed within three weeks.  

Task 1:

Visual analysis of burn scars In this task, you will first visually analyse the burn scars by comparing the pre-fire and post-fire  imagery.  Then you  will  pre-process  and  calibrate  the  images  to  prepare  them  for  the creation of burn index images. 

Steps: 

1. Unzip the two datasets. Make sure you unzip the files in two separate folders: one pre-fire dataset and one for post-fire dataset. Each dataset contains 13 bands. 
2. Start ENVI. From the menu bar, select File > Open. Navigate to the pre-fire folder and select the metadata file (*_MTL.xml), then click Open. You might get an error message warning about some tiles not being displayed properly, please ignore it. Right-click on the layer name in the Layer Manager and select Zoom to Layer Extent to see the whole image.  
3. From  the  menu  bar,  try  different  histogram  stretching  options  to  enhance  the radiometric quality of the image. You can also choose the stretching to be applied to the whole image or your view extent only. Choose the option that best suits your visual analysis of the image. 
4. Use  Zoom  and  Pan  tools  to  inspect  the  different  parts  of  the  image.  Can  you  find Bakersfield, Rogers dry lake, and City of Los Angeles in the image? 
5. Open the post-fire image set by following the same instructions as above. Choose the histogram stretching method that best suits you. 
6. Compare the pre-fire and post-fire images by turning the top layer on and off. What are the main differences between the two images? Can you identify burn scars? 
7. In the Layer Manager, note that ENVI has automatically assigned the Red, Green, and Blue bands to the RGB colour channels to create a natural colour visualisation. You can try different band combinations to create a false colour visualisation. To do so, open File  >  Data Manager.  Here  you  can  find  all  wavelength  bands,  as  well  as  some additional outputs provided with the data, such as the Aerosol Optical Thickness (AOT) map  and  several  others.  We  won’t  be  using  these  additional  outputs  for  this assignment  but  will  only use  the  bands.  You  can  create  different  false  colour visualisations by selecting which bands are assigned to Red, Blue and Green channels. First, open the band selection drop-down on the bottom part of the  Data Manager window.  Then,  click  on  the  bands  on the  list  above  to  assign  them  to  different channels, but make sure the selected bands have the same spatial resolution. Try to create a false colour visualisation of the post-fire image by selecting band B7 (SWIR 2) for the Red channel, B5 (Near Infrared) for the Green channel, and band B2 (Blue) for the blue channel. This is a band combination that is often used for visual analysis of burn scars.

Save your best visualisations of each image and include these in your report. Make sure all the images are clear and are accompanied with the necessary additional information (e.g., colour bar for pseudo-colour images and band combination for false-colour images). While the  visual  observation  is  helpful  for  qualitative  analysis  of  burn  scars,  in  most  cases  a quantitative analysis of the extent of burn scars is required. In the next task, we will create burn index images which will enable quantitative analysis of burn scars.

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Task 2: Creating burn index images

Before  creating  the  burn  indices,  we  need  to  convert  the  pixel  values  to  the  top-of-atmosphere reflectance. Water pixels can interfere with this process. Therefore, we first need to mask water pixels to exclude them from the conversion.

2.1 Creating a Water Mask To create a water mask, we define a region of interest (ROI) around water bodies in the near-infrared (NIR) band, where water has a very low reflectance.  

Steps:

1. Right-click  on  the  Post  Fire  image (LC09_L1TP_041036_20250215_20250215_02_T1_MTL)  in  the  Layer  Manager  and select New Region of Interest. 
2. Change the ROI Name to Water ROI. 
3. In the ROI Tool, click the Threshold tab. 
4. Click the Add New Threshold Rule button.
5. In the File Selection dialog, select the band B5 (NIR) from the post fire image and click OK. A histogram of the NIR band  is displayed in the  Choose Threshold Parameters dialog. You will identify the water pixels by selecting the range of low pixel values in the histogram. 
6. Click and drag the red line on the left edge of the plot toward the right, covering the data values from 0 to approximately 10000. 
7. Click the Preview option. The pixels that fall within the defined range are highlighted in red. 
8. Some of the pixels in the burn scars also have extremely low NIR values, but we do not want to mark these pixels. You will need to move the slider in the histogram so that you highlight water pixels but no other features. To make it easier to move the slider and to see the histogram in more detail, hold down your middle mouse button to draw a box to zoom into. 
9. Move the right-most red slider, until only water pixels are highlighted in the image and no other features. 
10. Click OK in the Choose Threshold Parameters dialog. You may close the ROI Tool now. 
11. In  the  search  window  of  the  Toolbox,  type  build  raster  mask  and  double-click  the Build Raster Mask tool name that appears. 
12. In the Build Raster Mask Input File dialog, select the metadata file of the Post Fire Image, which contains 7 optical bands, and click OK. 
13. Click the Options drop-down list in the Mask Definition dialog and select Import ROIs. 
14. Select the Water ROI from the list, and click OK. 
15. Click  the  Options  drop-down  list  again  in  the  Mask  Definition  dialog  and  choose Selected Areas “Off”. By doing this, the water pixels will have values of 0, and all other pixels will have values of 1. 
16. Enter the output filename PostFireWaterMask.dat.  
17. Click  OK  in  the  Mask  Definition  dialog.  The  mask  image  is  displayed.  If  you  get  a Warning about the mismatch of spectral values in the header, ignore it.  

When you apply this mask to the image in the next step, the black pixels (values of 0) will be excluded from further processing, while the white pixels (values of 1) will be processed.

2.2 Converting pixel values to reflectance To  create  spectral  index  images  such  as  Burn  Area  Index  and  Normalized  Burn  Ratio,  the source  images  should  be  calibrated  to  top-of-atmosphere  (TOA)  reflectance,  where  pixel values range from 0 to 1.0 or 0 to 100. 

Steps:

1. In the search window of the Toolbox, type calibration. Double-click the Radiometric Calibration tool name that appears. 
2. In  the  File  Selection  dialog,  select  the  post-fire  multispectral  image  set,  then  click Mask … and select your PostFireWaterMask file, and click OK. 
3. In  the  Radiometric  Calibration  dialog,  select  Top-of-Atmosphere  Reflectance  from the Calibration Type drop-down list. 
4. Keep the default selections for all other settings. 
5. Enter  an  output  filename  of  PostFireReflectance.dat  and  click  OK.  Wait  for  the Radiometric Calibration process to complete.

2.3 Computing burn indices To create the burn index images we will use ENVI’s Spectral Indices tool. You must run this tool each time you create an index image.  

Steps: 

1. In the search window of the toolbox, type spectral indices. Double-click the Spectral Indices tool name that appears. 
2. In the File Selection dialog, select the file PostFireReflectance.dat, and click OK. 
3. In the Index list, select Burn Area Index. 
4. In the Output Raster field, enter a filename of PostFireBAI.dat and click OK. 
5. Repeat Steps 1-4 for the Normalized Burn Ratio (output filename: PostFireNBR.dat).  Notice that the brighter pixels in the Burn Area Index image indicate burnt areas, while darker pixels indicate burnt areas in the Normalized Burn Ratio images. Use the Zoom and Pan tools in the toolbar to further explore the images. How are the BAI and NBR images different from each other? Does one separate burnt areas better than the other? Save each index image and include these in your report.

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Task 3: Creating differenced normalized burn Ratio image and mapping the burn scars

A differenced normalized burn ratio (ΔNBR) is created by subtracting the post-fire NBR image from the pre-fire NBR image.  It  highlights burn-severity as  brighter  pixels represent  larger differences between the post-fire and pre-fire NBR images. To create a ΔNBR image, you will need to apply all the pre-processing and calibration steps for the pre-fire image that you did for the post-fire image and create a PreFireNBR.dat image file.  

Steps to create a ΔNBR image: 

1. Load PreFireNBR.dat and PostFireNBR.dat if they are not loaded already (using the File > Open). 
2. Before you can subtract one image from the other, both must be in the same spatial grid.  While both images are in the  same projection, they  might be offset  by  a  few pixels. Layer stacking will ensure that they are in a common grid. In the search window of the Toolbox, type build layer stack. Double-click the Build Layer Stack tool name that appears. 
3. In the Build Layer Stack dialog, click the Input Rasters Browse (…) button. 
4. Use the Ctrl key to select the files PreFireNBR.dat and PostFireNBR.dat. Click OK. a. IMPORTANT:  Note  the  order  the  above  layers  appears  in  the  window.  If PreFireNBR.dat is above PostFireNBR.dat, that means Band 1 will be PreFire and Band 2 will be PostFire in step 10 below.
5. Keep all of the remaining parameters at their default settings.
6. Enter an output filename of NBRLayerStack.dat, and click OK.
7. In the ENVI Toolbox, expand the Band Algebra folder and double-click the Band Math tool. 
8. In the Enter an expression field, enter float(b2 – b1). 
9. Click Add to List, then click OK. 
10. With B1 – [undefined] selected in the Variables used in expression dialog, click Layer (NBRLayerStack.dat > Band (PostFireNBR.dat from Step 4 above). 
11. Select B2 – [undefined]. 
12. Click Layer (NBRLayerStack.dat > Band (PreFireNBR.dat from Step 4 above). 13. Select Output Result to: “Memory” and click OK.

Save your differenced NBR image and include it in your report. Notice how burnt areas are highlighted by brighter pixels in the image. To create a map of fire severity we use the burn severity categories recommended by the U.S. Geological Survey FIREMON program as listed in the table below: