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Overview
References:
List of journal references
Images:
MER_RR_1COLRA 20030424~.N1 Description
Download image
(31 MB)
Useful information:
The Benguela Current System
Coccolithophore blooms
Differences in MERIS and MODIS data
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At the end of the last section you prepared two images for comparison, you are now going to look them in more detail, using point measurements, histograms, transects and a difference image.
Open the file modis_8day_chla.dat and mer_200402_8day_mf_r.dat. At present the images are barely visible, you need to set the stretches. To decide on appropriate stretches you need to create a histogram for each image.
Select the first image and select all using [CTRL-A] then right click and select New » Histogram document, repeat for the second image. Right click on the new histograms and select scale » ignore 0 (nulls are set to 0). You may want to change the scale to better see the data spread. Answer question one, then close the histograms and apply your stretches.
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Question 1.
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| a) |
What is the data range in the images? What percentage of the values are below 4? below 3?
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| b) |
How are the chlorophyll values distributed in the histograms? What type of stretch does this suggest?
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In the last section you mapped the images to the same grid, resampled the images, ensured that the data was in the same format and you have now selected appropriate stretches to make the detail visible. The images are now ready for comparison.
Exploring the image
We will start by using the point tool. Browse around the images, picking points at the same places, using the goto tool (Edit » Goto) . Have a look at both high and low chlorophyll areas to get a feel for the images and any differences.
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Activity/Question 2.
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| a) |
At the coordinates 17°48'E, 31°48'S what is the chlorophyll concentration in each image?
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As you explore the image you will become aware that the point tool is of limited use, other than to check that the values are not the same. We really want to look at the data over a larger area. To do this you are going to use the box tool to examine data spread over a small area.
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Use the point tool to select an area of intrest.
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Select the box tool from the tool bar, and select Edit » GoTo or press [CTRL-G] to bring up the goto dialogue.
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In the goto dialogue untick coords and set the selection area Dx-Dy to three by three pixels and click ok.
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Right click on the image and select New » Histogram document. Repeat steps 1-3 for the second image.
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Repeat steps 1-4 this time using a selection 20px by 20px. Minimise the images and select Window » Tile Vertically to display your histograms.
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| b) |
What do these histograms tell you about the data?
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Looking at data over a geographical area
We are now going to have a look at how the data compares over a geographical area using transects. The best way to do this is to stack the images.
You can then create a transect document that will allow you to see how the two image compare over the main upwelling area.
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Right click on one of the images and select connect, add the MERIS and MODIS images. Make sure Stack is ticked and click ok.
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Select the line tool from the tool bar, and select Edit » GoTo or press [CTRL-G] to bring up the goto dialogue. Create a transect from 013°50'00.0''E to 26°00'00.0''S; Dx=4°E, Dy=8°S. Click Ok.
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Select New from the File menu [CTRL+N], choose TRANSECT document, making sure the Apply stretches checkbox is unchecked before clicking OK.
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To see the legend corresponding to the coloured lines, click the down arrow in the selector.
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If you want to examine the chlorophyll values at a particular point along the transect, place the cursor in that position and [TAB] through. The vertical line representing the cursor will change colour, telling you what line is currently displayed on the Status Bar.
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Question 3.
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| a) |
What can you tell from the transect?
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| b) |
Have a look slightly North to 14°E x 25°S MODIS shows very high chlorophyll concentrations, MERIS doesn't, can you think of any reasons for this?
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| c) |
From your histograms, point measurements and transects how well do you think the two sensors agree?
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You may also want to look at some of the other features and try a transect from outside into the inside of the upwelling area.
Creating a difference image
You have already seen that the two sensors 'see' different values, and you may have also noticed that the MODIS image appears slightly smoother. We are now going to create a difference image. This is a very useful tool that, as it name suggests, highlights the differences between two images.
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Activit/Question 4.
A difference image is easily created by simply deducting the pixel values in one image from the corresponding pixel values in another image. It can be done the 'quick and dirty' way by linking the two images and applying a formula of only one line @1-@2. This will open a the difference image as a new image unconnected to the other two. However, here you will do it the proper way, creating a set of three images, one of them a blank, and using constant declarations and comments to explain what your formula does. It may seem a bit cumbersome for such a simple calculation, but it's a good discipline to get into.
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How would you set up a Bilko set to carry out these calculations?
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How would you write a formula to carry out the difference calculation?
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What numerical format should your output be for this calculation?
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Open a New Formula document and write the text of your formula. To carry out your calculations:
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Stack the MERIS and MODIS images (right click on an image and select Connect), add the correct number of blank image(s) and make sure that Stack is ticked.
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Activate your formula and check the Options! to see that the output is set correctly (either to Same as @1 or to 32-bit float; then copy the formula and paste it onto the stack to carry out the image calculations.
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Tab to see the result of your caluclations, and save the new images as meris_modis_diff.dat then close the stacks and the Formula document.
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Reopen the recently created image, setting no Null values. You should spend some time thinking about the contents of this image.
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