9.2 Selecting areas with known location

Note: In the previous subsection ( T9.1 ) you opened S19980332004040.L3M_WC_CHLO.HDF and set its geographical coordinates. For this subsection the image should still be open.

A quick way to check that the Lat/Lon grid of a global image has been set correctly is by moving to the origin where the Meridian crosses the Equator: Position [0,0].

Finding a specified location

To find a specified position such as 0^oE, 0^oN you use the Bilko 'Go to' function and enter the desired coordinates.

1. Use [CTRL + G] to open the Go to dialogue. Notice how it shows the coordinates in degrees (^o), minutes ('), and seconds (").
2. Remember to make sure the selection type is set to Point selection
3. Edit both Position boxes (X and Y) to show 0 (figure (6K)).
4. Click OK or press Enter

Your cursor should now be in the tropical Atlantic, in the Gulf of Guinea, on a pixel with DN=98.   Is it?

Quick ways to enter positions

Postions given in latitude and longitude may follow either of two conventions:

• the traditional degrees, minutes and seconds (used by the Bilko display)
  
• decimal degrees (accepted alternative for use in dialogue windows).
  

The decimal position [-87.1875, 5.625] can thus be entered into the 'Go to' position boxes without converting it to degrees, minutes and seconds (figure (5K)).
Note: More information about these conventions in Equidistant cylindrical (Lat/Lon) grids.

Question 1
Go to position [87.1875]   [-5.625] What is this position in degrees, minutes and seconds? Where is it?

If a position is given in in [^o ' " ] you don't have to enter the full notation to find the pixel. For instance, the position [ 009^o11'05.60"W, 02^o37'30.05"N ] can be entered

both quicker than typing the full notation.

Areas of known location and size

You can also use the 'Go to' function to select an area of the image.

Question 2.
The Benguela coastal upwelling area covers an area of the South Atlantic, very roughly from 0^oE, 10^oS to 30^oE,40^oS.
a) What is the top left corner of this area? How would you enter this position?
b) What is the width of this area? How would you enter this in the 'Go to' dialogue?
c) What is the height of this area, and how would you enter it?

Use the'Go to' dialogue ( [CTRL + G] ) to select this area from the SeaWiFS image:

1. Fill in the position boxes X and Y with the coordinates of the top left corner
2. Make sure the 'Selection type' is Box selection.
3. Fill in the 'Selection size' boxes with the DX and DY values that correspond to width and height of the wanted area.
4. Click OK or press the 'Return' key on the keyboard.

The area you have selected should contain the sea areas to the West and South of Southern Africa (figure (18K)).

Note: The most common mistake when selecting an area using Lon/Lat coordinates is forgetting that positions in the Southern hemisphere and directions running southwards are negative. If the area you have just selected does NOT include the southwest tip of Africa, you have probably made that mistake. (Check how to enter the coordinates - 35K).

Saving a geo-referenced selection

You can use the selected area to create a new image of just the Benguela:

1. Copy the content of the selection onto the clipboard ( [CTRL + C] ).
2. Open a new file ( [CTRL + N] ).
3. In the New dialogue select IMAGE document (figure (5K)).
4. Click OK or press 'Return'.