9.1 From X and Y to longitude and latitude

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.

Satellite images that have been resampled and mapped to a geographical coordinate grid are variously known as gridded images, mapped images, or geocorrected images. The first image you will look at is a gridded global climatology image of chlorophyll-a derived from 6 years of measurements by NASA' SeaWiFS sensor.

1. Open the SeaWiFS climatology image for the 8 day period of 25 Jan to 1 Feb (S19980252004032*.HDF) and load the scientific data by double-clicking on l3m_data in the right frame of the 'File Structure' window. (figure (6K)).
2. Move the cursor around the image, and look at the pixel coordinates on the Bilko status bar (figure (2K)). As you can see Bilko uses [x,y] coordinates to display the image.

Finding the coordinate information

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

• 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.

Question 1:
    a) Where would you look to find this coordinate information?
    b) What map projection is used?
    c) What are the coordinates of the top left corner?
    d) What is the pixel size?

Setting the geographical coordinates

Activate the SeaWiFS image by clicking on it. You can now use the information you have collected to change the image coordinates to [lon,lat] 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')

Either of these will open the Set Coordinates dialogue window (figure (5K)).

1. Select Longitude/Latitude.
2. [TAB] (or mouse) to the Longitude(X) box and enter the top left longitude.
3. [TAB] to the Latitude(Y) box and enter the top left latitude.
4. [TAB] to the 'Pixel width (DX)' box and enter the pixel width
5. [TAB] to the 'Pixel width (DY)' box and enter the pixel height
6. 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 (^o), minutes (') and seconds ("), using W or E for longitude, and N or S for latitude (figure (15K)).