2.3.2 Product Grid

MERIS is an imaging instrument that produces three-dimensional data sets. The first dimension corresponds to the lines that follow the along-track chronological order - the number of lines is always product-dependent. The second dimension corresponds to the columns of the image; in the Level 0 image columns correspond to the across-track instrument pixels, and in the Level 1b or Level 2 images columns correspond to the across-track product earth pixels. The third and last dimension corresponds to the different geophysical parameters that are produced from a MERIS image.

The MERIS Product grid is intended to:

· be related to the satellite track (assuming nominal pitch and yaw attitude control of the satellite)

· provide quasi-even distance sampling of the parameters on Earth

· provide geolocation and other geophysical annotations as a regular subset of the pixels in order to improve storage efficiency

Resampling is defined on a line basis, with an even spacing grid on Earth with regard to the geoid surface in the across-track direction, it is centred at the subsatellite point as known from orbit prediction, nominal attitude control and modelled attitude perturbations. The along-track sampling is evenly spaced in time and thus provides a quasi-even distance on Earth; possible variations of the along-track sampling step may be due to the orbital motions of the satellite and/or to the ellipsoidal shape of the Earth.

There are different grids used in the MERIS processing, which are summarised below:

· The tie points grid is identical for all products. It has 71 tie points across-track, it is a 16 x 16 sub-grid of the RR product grid, a 64 x 64 sub-grid of the FR product grid, and a 4 x 4 sub-grid of the Browse and LR product grids. Image location and geometrical annotations are only provided at tie points.

· The Reduced-Resolution product grid has 1,121 RR pixels across-track.

· The Full-Resolution product grid has 4 x 4 more points than the RR product grid, in the across-track direction it has 4,481 points.

· The Browse product is obtained by sub-sampling a Level 1b product.

For the various resolutions, the across-track (at nadir) and the along-track distances between two points are shown in table 2.4 :

Table 2.5 - Across-track and along-track distances.

Figure 2.4 - Column and frame index.

The MERIS convention used for indexing column and lines is as follows. Column numbering starts at 1 and increases from east to west, and line numbering starts at 1 and increases from north to south.

The standard tools (BEAM, EnviView…) process and display the image in classical Earth representations.