ABSTRACT
A large proportion of topographic surveying, earthwork surveying, site surveying and mapping, involves the drawing and manipulation of lines. In computing terms, these vector data have a fundamentally different type of data structure to the gridded, raster, data of APs, satellite images and geophysical plots described above (see Infobox 1 on page 14). The essence of vector data is working within a coordinate system so that individual points can be spatially positioned, and if required, joined to create lines. An example of using vector data is the plotting of contour lines as shown in Figure 2.10a and b, a topographic survey of Alfred’s Castle hillfort in Oxfordshire. Using electronic surveying equipment (a Total Station) a series of readings are taken which record not only the position of the reading point relative to a spatial datum but also its height above a predetermined height datum. These readings are stored digitally (often in a piece of equipment attached to the Total Station and known as a Data Logger) and can be downloaded into a computer either on-site or back at base. Contouring software positions each data point spatially and interpolates between recorded heights to produce contour lines at any chosen contour interval; it is simple to alter the contour values and redraw the display.
