9.4   Image registration

Note: At the start of this tutorial subsection you should have two images open:
- A SeaWiFS climatology image of the Benguela selected from the global climatology image and saved as S19980332004040_benguela.dat.   (T9.2)
- A MERIS algal_1 image resampled to a standard grid and saved as mer20040201a1r.dat.   (T9.3)

Image registration is the process of geometrically aligning two or more sets of image data so that they can be digitally or visually superimposed and compared. In this subsection you will co-register the SeaWiFS climatology image for the first week in February and the MERIS image you were working on in the last subsection.

Having decided on new grid-specifications for the registration of the two images, close the SeaWiFS image you created earlier, and start again.

Using 'Resample' to select an area

Using 'Resample' to create a new selection requires that the original image is gridded using either a Lat/Long or UTM grid. To get started you must therefore open the original global SeaWiFS image ( S19980332004040.L3M_WC_CHLO.HDF), and edit the coordinates from [x,y] to [lon,lat].   (T9.1)

Your new selection will cover the area [ 0-30^oE, 10-40^oS ] with a pixel size of 3'.

1. From the Image menu, open the Resample dialogue ( [ALT+I] > [M] ).
2. Select the Pixel tab and change this pixel size to 3" (figure (4K)).
   Remember to apply the changes.
3. Select the Window tab and change the edge coordinates to show the boundaries of the Benguela area (figure (4K)). Apply the changes.
4. Select the Image tab and check the size of the new Image, if everything has been done correctly so far it should be 601x 601. Change this to 600 x 600.
5. Return to the Window tab and note how the end coordinates have changed.
   Note: The bottom right coordinates given in the selection are actually the position of the top left corner of the last pixel in the new selection ( T 9.1, answer 2b ).
6. Accept the settings and start the resampling.

Save the new image as S19980332004040_benguela2.dat.

Note: The new image is over-sampled (each pixel in the original will give rise to several pixels in the new image), so it has a rather blocky look, particularly along the coast. However, chosing this higher resolution allows you to retain a reasonable resolution for the MERIS image when you resample this.

Resampling to the grid of another image

To resample the MERIS image to the same grid you could simply repeat what you have just done. However, there is an even simpler way to achieve the same result.

1. Activate the MERIS image by clicking on it.
2. From the Image menu select 'Resample'
3. With the 'Interpolation' dialogue on top change the Based on option from 'Self' to S19980332004040_benguela2.dat ( figure (4K)).
4. Leave the Method as 'Nearest neighbour'. This is sufficient because both images are on a Lat/Long grid. Apply the changes.
5. Check the pixel size, window edge coordinates and image size and note how these have all changed to match the Benguela climatology image.
6. Press OK to accept the new settings, and start resampling.

The new, resampled image that appears is surrounded by a large area of black where there was no available data. The corner coordinates of this image should be identical to those of the SeaWiFS climatology image.

Save this interim image as mer20040201a1r_s.dat. (the s is for small, to distinguish it from the other, larger MERIS image you created earlier.)