9.3   Rectification of Envisat Scenes

In this section you will resample a MERIS Level 2 scene to a standard equidistant cylindrical latitude/longitude grid. The principle is common to all satellite data, and the instructions for resampling MERIS images also apply to data from other Envisat sensors such as ASAR and AATSR.

Coordinates of MERIS pixels

All Envisat L1 and L2 scenes are distributed with each pixel already referenced to a position on the Earth's surface measured in latitude and longitude. You can see this for instance when you open a MERIS image:

1. Open the image file MER_RR__2COLRA20040201_082314*.N1
2. In the left frame of the file structure window select Bands and open the algal_1 data set by double-clicking on its icon in the right frame.
3. In the image window, move your cursor around and observe how the position given on the status bar changes. Each pixel has a position given in Longitude and Latitude (figure (31K)).

Note: The brightness values in this image represent Algal Pigment Index 1, mostly chlorophyll-a. The index is valid only for Case I waters, i.e. where phytoplankton cells are the main influence on water colour. For land pixels the brightness values represent the Top of Atmosphere Vegetation Index (TOAVI), and over cloudy areas they represent Cloud Top Pressure. This section of the tutorial will not look at these geophysical products in any detail. More information about the algal pigment index may be found in the Thematic Lessons that also form part of the Bilko Envisat Module.

Switching between [lon,lat] and [x,y]

Using the View menu (figure (2K)) you can change the coordinates shown on the status bar from   [lon,lat]   to  [x,y] :

    Mouse: Select View from the menu bar, then Coords to from the drop-down menu.
    Keyboard: Press [ALT + V] to open the 'View' menu, then [O] (for 'coords')

Try this and note the change to the pixel coordinates shown on the Bilko status bar. Repeat the exercise and see how the coordinates shown return to [lon,lat].

Question 2
Use the [x,y] coordinates to find the column containing the satellite ground track. (The MERIS product grid explains which column this is.) Switch back to [lon,lat] .
a) What was the satellite position when MERIS started recording this scene?
b) What was the position at the end of the scene?
c) How far (in degrees and minutes) did the satellite travel during the recording of this scene?

Resampling to standard equidistant cylindical grid

The MERIS image is geo-referenced, but it is displayed using the MERIS product grid rather than a standard geographical grid system, so it cannot easily be compared to other images as it stands. To compare it to the SeaWiFS climatology from the same week of the year, you must therefore resample it to a standard equidistant cylindrical grid such as that used for the SeaWiFS image.

Either option will open the Resample dialogue (figure (5K)). To accept the default settings and resample the image, just press OK. A new image opens, resampled to a standard Lat/Long grid.

Spend a bit of time moving your cursor around the image and looking at the geographical coordinates. You will probably notice that the coordinates of all the pixels are rather cumbersome. This may be avoided by changing a few of the default settings in the 'Resample' dialogue. So close the resampled image, activate the original image, and open the 'Resample' dialogue again.

As you see the see the 'Resample' dialogue has four different tabs; at present the Interpolation tab is on top. You can find out more about these tabs by clicking on them in turn, and consulting Bilko's Context Sensitive Help. (To access the help program, point your cursor at one of the dialogue boxes, and press F1).