Vertex-Centered and Cell-Centered Grids:
The mathematical process of gridding data results in calculated values that must be mapped onto the earth's surface. These calculated values are usually displayed as colored rectangles ("pixels") in graphic displays. The association between grid cells and pixels is so thoroughly ingrained in our knowledge of gridded data that we see the grid in our mind as the physical extent of the graphic display, that is the colored pixels on the screen or the print copy. But the exact placement of the colored pixel on the earth surface, in relation to the geographic coordinate system of the original data, can be accomplished in two different ways. The physical difference would appear to be slight, but misunderstanding of the methods can lead to considerable error and anguish.
The array of latitude and longitude lines, universally depicted as whole-number values, is called the graticule. It is the basic concept upon which "areas of interest" are defined, and data gridding methods are usually controlled by reference to graticule concepts, e.g. minimum and maximum longitude and latitude, cell spacing in the x-direction, and cell spacing in the y-direction. In this figure, an arbitrarily selected graticule offshore Namibia, the longitudes at the vertices are 10, 11, 12, 13, 14 and 15. The latitudes at the vertices are - 25, -26, -27, -28 and -29. Obviously the cell dimensions are 1 degree in each direction.
If we have scattered oceanographic data over the region of the graticule, then we can grid it to obtain calculated values at each of the vertices of the graticule, as shown below. We have even colored the area assigned to each calculated value to illustrate that the cells of the graticule are not the same. This type of grid is called a Vertex-Centered Grid. Notice that the number of rows and columns are both one greater than the number of rows and columns in the graticule. Notice also that the grid image extends 1/2 cell dimension beyond the graticule in every direction. This means that for global grids there is may be "overlap" or duplication of data in the far western and eastern columns. Many difficulties and misunderstandings have occurred over the years due to incorrect reading of vertex-centered data.
We can also grid the data to obtain calculated values at the centers of the graticule cells, as shown below. This grid does not "extend" beyond the limits of the graticule, and the number of rows and columns is the same as the graticule. This type of gridding is preferred for global grids, because the far western and eastern columns of the grid do not overlap.
