gridDimensions: indicate the size of the grid - in terms of columns, rows, etc - first in the x direction, then the y direction, etc gridSpacing: spacing in real world units between the grid positions, first in the x direction, then the y direction, etc.. All units are typically of the same type. However, if spatio-temporal data is being considered, the units may for example consist of a list of metres, metres, years. The spacing is given with respect to grid positions rather than grid lines. For example, a two-dimensional grid with a grid spacing of 1,1 will have equal distance bewteen succesive grid postions in both dimensions regardless of whether it is a recangular grid or a triangular grid, the spacing between succesive lines, however, will be different: offsetStructure: an enumeration with possible values of none, in or out which defines the shifting of alternate grid rows with respect to the second dimension spcified by gridDimensions. A rectangular grid has a offsetStructure value of none (default), a triangular grid structure will have a value of either in or out describing the relationship between alternating rows. Alternating rows are offset with respect to each other by one half the spacing specified by the second grid dimension. Case 1-originCorner: nw offsetStructure:out first row of grid x x x x x x x x x
second row x x x x x x x x x
third rowx x x x x x x x x
Case 2-originCorner:se
offsetStructure:out
x x x x x x x x x third row
x x x x x x x x x second row
x x x x x x x x x first row of grid
Case 3-originCorner:nw
offsetStructure:in
first row of grid x x x x x x x x x
second row x x x x x x x x x
third row x x x x x x x x x
Case 4-originCorner:se
offsetStructure:in
x x x x x x x x x third row
x x x x x x x x x second row
x x x x x x x x x first row of grid