GIS Files 2: Geographical data

2.6: The third dimension (2)

3-D GIS data

Height information can be captured in a GIS in exactly the same way as the shape and location of objects. The spectacular ability of today's computers to perform calculations means that 3-D models of the ground surface can be constructed from data recording the height at different points across an area. The typical way this information is stored is an extension of the conventional grid coordinate system: as well as recording the latitude (the x axis) and longitude (the y axis) for a given point, the elevation (the z axis – usually as height above sea level in metres) is also stored. Thus the height information for an area is often referred to in terms of z values. The fluctuations in ground height across an area are a continuous phenomenon – every point on the ground has a z value irrespective of whatever physical features are present.

Point height information can be collected by surveyors out in the field, or more commonly by using remote sensing, including photogrammetry (section 2.2). Points of the same height can be joined to form a line or contour.

Once created, most 3-D GIS data is stored as a grid of points, with x, y and z values stored as attributes, often referred to as a digital terrain model (DTM) or digital elevation model (DEM). From this grid a computer can build a 3-D model.

An alternative method of representing a surface is to create a triangulated irregular network (TIN). A TIN model forms a continuous surface by connecting irregularly spaced spot heights to form triangles, keeping a flat surface within each triangle.

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