5.5 MERIS and MODIS (preparing the images)

You will start by processing the MODIS composite to compare to the earlier MERIS image. Open the file l5_mod_a20040332004040.l3m_8d_chlo_4.hdf accepting the default stretches. This is an 8-day composite taken over approximately the same time period as the MERIS file. The HDF file contains metadata that you will need later to process the image.

Activity: Open the Scientific Global Attributes and use this information to answer the questions below, making a note of the answers.

Converting to a geographical grid

The [x,y] coordinates of an image allow you to identify and find specific pixels in a particular image. However, a more universal coordinate system allows the comparison of pixels in different images, even if these vary in resolution or area covered. It also allows you to compare satellite data with measurements made from ships and buoys.

Although the image uses a Lat/Lon grid the software does not yet have the necessary information to display these geographical coordinates. To change the display from [x,y] to [lon,lat] you need the following information from question 1:

• The map projection (to check what type of coordinate system to use, and whether it is supported by Bilko),
• The coordinates of the top left corner in degrees longitude and latitude,
• The pixel size in degrees longitude and latitude.

Activity: Activate the MODIS image by clicking on it. Use the information from Question 1 to change the image coordinates to [lon,lat]:

1. Open the "Set Coordinates" dialog. You can do this in either of two ways:
   Mouse: Select Edit from the menu bar, then Coords from the drop-down menu.
   Keyboard: Press [ALT + E] to access the 'Edit' menu, then [O] (for 'coords').
2. Select Longitude/Latitude.
3. [TAB] (or mouse) to the Longitude(X) box and enter the top left longitude.
4. [TAB] to the Latitude(Y) box and enter the top left latitude.
5. [TAB] to the 'Pixel width (DX)' box and enter the pixel width
6. [TAB] to the 'Pixel width (DY)' box and enter the pixel height
7. Click OK or press Enter

Move your cursor around the image, using either the arrow keys or your mouse. Observe how the status bar now shows the pixel position in degrees (°), minutes (') and seconds ("), using W or E for longitude, and N or S for latitude

To check that you have set the coordinates correctly scroll to the bottom right corner of the image. (You may want to zoom out first, to speed this up: Right-click inside the image frame, select Zoom from the drop-down menu, and click Preserve shape on the Zoom dialogue.) The image covers the whole Earth, so the bottom right corner coordinates shown on the status bar should be very close to 180°E 90°S.

Save the file as: modis_8day.dat

Repeat this for the MERIS composite, selecting the pixel size you resampled the image to in the last section and the coordinates of the study area for the latitude and longitude. Save the image.

Extracting the study area.

You now need to extract the study area from the MODIS global image. To do this:

1. Select Edit » Goto or [CTRL-G] to open the Goto dialogue.
2. Make sure that the Coords option is ticked, enter the position as x: 0°E and y: 20°S.
3. Set the selection size to DX: 30°E and DY: 20°S
4. Click ok or press Enter.
5. Right click in the image and select New » IMAGE document.
6. Click OK or press Enter

Save this file as MODIS_l3m_8d_extract.dat.

Open your MERIS composite, setting the stretch to logarithmic, the null value to zero and the upper data value to 4 and have a look at the two files. You should be able to instantly see the similarities and pick out the same features in each image. However the images are not yet comparable. Although they cover the same geographical area the pixel sizes are different.

Have a look at your answers to question one and the information from the last section of the tutorial.

You are going to resample the MERIS image to set the correct pixel size and grid.

We could just resample the image, however bilko´s resampling algorithms simply sub sample the image to create the new pixel size (they pick one pixel value and use it´s value for the new size) rather than averaging the data. This gives a 'spiky' image and results in less accurate data. To correct this issue you will need to average the data to match the pixel size of the MODIS image as closely as possible, in order to do this we need to use a mean filter on the MERIS image.

Select the image and from the image menu select filter » mean to apply a mean filter. Select an average size of three by three pixels and run the filter. Save the file as mer_200402_8day_fm33.dat.

We now need to resample our averaged image to exactly the same pixel size and grid as the MODIS image to allow further comparison.

1. Right click on the image and set the null value to zero.
2. Select Image » Resample.
3. Under the Interpolation tab set the null value to 0. Click apply.
4. Under the Window tab set the coordinates to 0°E to 30°E by 20°S to 40°S. Click apply.
5. Under the Pixel tab set the width and heigh to 0°02'30.00'' E and S
6. The image size should read 721 by 481, if not check the other settings again. Now set these to 720, 480 this will give an image the exact same size as the MODIS image.
7. Click OK or press Enter and save the image as mer_200402_8day_fm33_r.dat.