1.5   Selecting subareas

In this subsection we will look at different ways to select an area from within an image. There are two main ways of doing this, by eye using the mouse, or by specifying the coordinates and the window size you want.

Bilko has four ways of selecting pixels, each represented by an icon on the tool bar: You choose the type of selection you want, by clicking on the corresponding icon to activate it.

	Block Selection selects a rectangular area and inverts the display - giving a negative of the original.
	Box Selection Box selection selects a rectangular area and surrounds this with a frame.
	Line Selection selects all pixels along a line drawn from one point to another in any direction.
	Point Selection selects a single pixel (you used this in the last subsection of the tutorial).

The image on the left shows part of the January 97 image. The dark line of cooler water west of South America is the equatorial upwelling, where current divergence brings cooler water to the surface. You will select this area of the Equatorial Pacific for comparison with the same area in the January 98 image.

Visual selection

One way to select an area of interest is to identify the region you want by eye and use your mouse to draw a selection box around it.

1. Activate box selection by clicking on the toolbar icon.
2. Place the mouse cursor on the top left corner of the selected area
3. Depress the left mouse button and hold it down while drawing the box by dragging the mouse to the bottom right corner.
4. Release the mouse button when you are happy with the selection.

Selecting an area with the 'Go To' dialog

The area selected in the illustration above was a box starting at the top left corner coordinates [x,y] = [40,120], with a size of 180 x 120. It is unlikely that you managed to select exactly that area by eye. You can check on your selection using the Go To dialog. Open this with the key-strokes [CTRL+G]

Question 1
What would the setting of the Go To dialog be for the area described above?

Adjust the selection in the 'Go To' dialog so it fits exactly the area required.

Using 'Extract' to open an image subarea

Your next task is to open an extract of the January 1998 image, which matches exactly the coordinates and window size of the area you have just selected in the January 1998 image.

1. Open the 'Open' dialog ( [CTRL+O]).
2. Make sure the 'Extract' box is checked.
3. Select the file atsr199801.gif and press OK to open the 'Extract' dialog.

Question 2
What would the Window setting of the Extract dialog be to open the area of interest at full resolution?

Fill in the Extract dialog as indicated by your answer to question 2 and click OK to open the selected area of the 1998 image.