9.5   Adding coastlines and country borders

You can make the resampled MERIS image look a little more attractive by applying a coastline. The procedure has two steps:

1. Opening the coastline based on the grid of the MERIS image.
2. Combining the coastline and the satellite data to create a new image with the coastline superimposed.

Opening the coastline with a custom grid

The coastline map you will use has been downloaded from the National Geophysical Data Centre (NGDC) shoreline/coastline database using the on-line coastline extractor. ( http://www.ngdc.noaa.gov/mgg/shorelines/shorelines.html ) It is on a Lat/Long grid, so you can extract the area you want at the desired spatial resolution when you open it.

1. Open the 'Open file' dialogue ( [CTRL +O] )
2. Make sure the Extract box is checked (figure (7K)).
3. Select the coastline map USGS_10S8E_40S30E.DAT from the list and open it. The extract dialogue that appears has 3 tabs: Window, Pixel and Image.
4. Select the Window tab and enter the window coordinates. Remember to apply the changes.
5. Select Pixel tab, set the pixel size and apply the changes.
6. Select the Image tab and check that the size matches that of the resampled MERIS image. If it does not, go back and check your Pixel and Window settings (you may have forgotten to apply the changes you just made).
7. When all settings are correct, click OK to accept and open the coastline

Applying the coastline to the satellite image

To combine the coastline with the MERIS image you must first connect them:

1. From the image drop-down menu, select 'Connect' ( [ALT+ I ] > [C] )
2. When the Connect dialogue opens, select both images, and check the Stack box (figure (2K)).

This creates a set of two images, one on top of the other. Use the Selector (figure (1K)). to pick which image to display, or switch between them using the [TAB] key. The selector also tells you what order the images appear in the stack. The first image is @1 (and is coloured red), the second image is @2 (green), and so on.

Question 1.
What number is given to the USGS coastline map?

Note: If you have problems connecting the two images it may be because they are not on the same grid. Check the pixel size and corner coordinates of each image, and if necessary open the coastline map again, taking care to match the pixel size and area covered by the MERIS image.

Open the formula called coastline.frm, and spend a moment examining it.

You will now apply the formula and carry out the calculations that will draw coastlines and borders onto the MERIS image.