Spatial Feature Definitions

• Point - uses 3 or 4 dimensions (x, y, time and optionally z). If the spatial definition does not include z then the point is assumed to lie on the surface and is simply plotted planimetrically on a map.
• Lines  - definition as for points. A 3D line can also be used to store depth values at a point. The line is loaded as a feature and then attributes are stored against any element/co-ordinate of that line.
• Polygons - uses 3 dimensions (x, y, and time).
• Polypolygons - uses 3 dimensions (x, y, and time). Used when a feature has multiple parts, for example the islands of a country, buildings of a farm or where an area or polygon has a 'hole' (e.g. a doughnut shape). Attributes should only be loaded onto this type of feature if the attribute relates to all the components, otherwise the feature should be loaded as individual polygons.
• Surfaces - represents a 3D surface (e.g. topography, water surface) as a series of points stored as one feature. uses 4 dimensions (x, y, z and time). This type of feature may be used in conjunction with lines/contours to describe bathymetry or elevation.
• Trapezia - these are specifically used in conjunction with surface features to apply datum correction to height/depth data. Uses 3 dimensions (x, y and z) - the z dimension has a before and after correction value. Corrections are interpolated within the trapezia using triangulation so the trapezia must be convex in shape.
• Grids - used for images such as photographs, remotely sensed images, gridded model output or any other raster format maps.

For guidance on how best to store features and load attributes onto the features see Efficiency Issues.