5.- PARÁMETROS OBJETIVOS PARA CONSIDERAR UNA OFERTA ANORMAL O DESPROPORCIONADA

In this exercise, you will mosaic two visible near-infrared (VNIR) ASTER bands with a pixel size of 15 m. The data are georeferenced to a UTM projection.

1. From the ENVI main menu bar, click on the Open folder icon. Navigate to envidata\aster, and select both NSydney.dat and SSydney.dat. Click Open. Georeferenced images are automatically positioned within the output mosaic according to their geographic coordinates. A color-infrared (CIR) image is best with ASTER VNIR data. Make sure you have that band combination for both scenes. The near-infrared band should be displayed as red, the red band displayed as green and the green band displayed as blue.

2. Click on the Zoom To Full Extent icon in the ENVI menu bar. Change the contrast stretch for each image by selecting from the stretch pull-down menu. A Linear 1% or 2% contrast stretch works well for these scenes.

Because the NSydney.img was acquired on November 26, 2000 and the SSydney.img was acquired on October 12, 2001, there is a difference in scene illumination between the images that is apparent when the two images are displayed side-by-side. If you mosaic these two images without color balancing, a noticeable seam will occur along the image boundaries where they join. To minimize these differences, you can use color balancing during the mosaic process. Color balancing matches the statistics from one image to another image. Gains and offsets are calculated

Introduction Mosaicking in ENVI

for each band from the fixed image and applied to the adjusted image(s), so the adjusted image(s) has the same statistical range. The statistics can be calculated from entire images or only from overlapping areas.

3. When scenes overlap as these do it is useful to be able to “see through” the non-data values. To do this click on Raster Management  Edit ENVI Header. Then in the Edit Header Input File choose the Sydney that is the top layer and click OK.

4. In the Header Info dialog, click Edit Attributes, then select Data Ignore Value. Type in 0, then click OK. After the header is edited, the image will have to be redisplayed. You can edit the header file for the other Sydney scene if you wish.

5. From the ENVI Toolbox expand the Mosaicking folder. Then double click on Georeferenced Mosaicking. A Map Based Mosaic dialog appears.

6. From the Map Based Mosaic dialog menu bar, select Import  Import Files and Edit Properties. A Mosaic Input Files dialog appears.

7. Select SSydney.dat and NSydney.dat and click OK. An Entry dialog appears. 8. The Entry dialog title bar includes the name of the image for which you are entering mosaic

parameters.

9. In the Data Value to Ignore field, enter 0. Overlap areas with this value will be transparent, allowing underlying images to show through.

10. In the Feathering Distance field, enter 10 pixels.

Edge feathering is used to blend or blur the overlapping seams between mosaicked images. The specified feathering distance is blended using a linear ramp that averages the two images across that distance. Cutline feathering is used to blend the boundary between two overlapping images using a specified blending distance to blend the images along the cutline. You will not use a cutline in this exercise.

11. In the Red field, enter 3. In the Green field, enter 2. In the Blue field, enter 1. The items in this block of the dialog only affect the appearance of images in the Mosaic tool.

12. If this is the entry dialog for SSydney.dat select the Adjust radio button. The SSydney.dat image will be adjusted to match NSydney.dat because it is slightly darker, and you want a brighter mosaicked image. If this is the entry dialog for the NSydney.dat image select the Fixed radio button.

13. Click OK. The next entry image dialog appears. If this is the entry dialog for SSydney.dat select the Adjust radio button. If this is the entry dialog for the NSydney.dat image select the Fixed radio button. The Background See Through and Feathering fields keep the same values as the previous entry image dialog.

14. Click OK. The Mosaic is displayed. You should see through the zero values of the top image. If you decide to change any options, right click on an image and select from the pop-up menu, or right click on a file listing at the bottom of the dialog and select an option. Note that you can change which image is on top.

Mosaicking in ENVI Introduction

15. From the Mosaic dialog menu bar, select File  Apply. A Mosaic Parameters dialog appears. 16. In the Enter Output Filename field, enter Sydney_mos.dat. Keep the Background Value of 0

if you want non-data pixels to be black. If you want non-data pixels to be white, type in a Background Value of 255 (the value to use for byte data).

17. In the Color Balance using field keep the stats from overlapping regions option.

18. Click OK to start the mosaic process. After processing is complete, the mosaic appears in the Available Bands List.

19. Open the Data Manager if it is not open already and right click on Sydney_mos.dat and select Load CIR.

20. Adjust the contrast stretch, then zoom into an area where the two images join to see how well the color balancing worked.

21. When you are finished examining the mosaic, click on the Close All Files in the Data Manager and close the Mosaic tool.

Skills Check

Skills Check

At this point, you should feel comfortable:  Using the Mosaic tool

 Adjusting the contrast of images

Self Test

1. In mosaicking, can you choose which image will be displayed on top? 2. For byte data what value do you use to make the non-data areas white?