4.2 Positions
127
The pair of data collections at one point is called a point value pair (class
CV _PointValuePair). According to ISO 19107, the position of a point value pair is
always a point object. In other words, the DirectPosition ofISO 19107 contains the
coordinates of a position like (x, y) and the point object (class GM_Point) uses this
DirectPosition. The point value pair of ISO 19123 (class CV _PointValurPair) uses
the point object ofISO 19107 for position information.
A common pattern of points is a quadrilateral grid. Examples of this would be a
digital image and a simple type of digital elevation matrix. For the reasons of storage
and computational efficiency, the quadrilateral pattern of points has some special
terminology and structure. The grid addressed as a whole is called a "grid values matrix" (class CV _ GridValuesMatrix). The intersections of the grid lines are called
"grid points" (class CV _GridPoint). According to ISO 19107, the position of each
grid point is always a "point object".
ISO 19107 states that a grid can also be defined as a quadrilateral pattern of simple point positions or point objects (class GM]ointObject), as in ISO 19123. The
grid is called a "point grid" (class GM _ PointGrid) and consists of a number of parallel placed point arrays (class GM _PointArray).
L.-....IS_O_19_1_0_7 _ _ _ _ _ --'IIISO 19123
GV _ GurveValuePair
GV _ SurfaceValuePair
GV _ SolidValuePair
CV _ GridPointValuePair
GV_GridPoint
CV _ GridPointValuePair
Fig. 4.2. Relations of position descriptions
Discrete
Goverages
Gontinuous
Goverages