Natural boundaries, such as coastlines, rivers, lake shores, cliffs, man-made obstacles, geological intrusions and fault lines are widespread and are frequently significant in terms of understanding spatial processes. When conducting spatial analysis it is often important to examine datasets for the existence of such features, and to take explicit account of their impact where appropriate. A large number of structures and arrangements may be encountered, and the standard analytical tools provided within your preferred GIS package may not handle these adequately for your purposes. For example, simply masking out regions, such as lakes, areas of sea, regions where the underlying geology is not igneous etc., is often misleading, if all this procedure does is to hide those areas which are covered. Ideally the analytical procedure applied should be able to take account of the breakline or natural boundary in a meaningful manner. In the case of breaklines there are three different types that tend to be supported, principally in connection with surface analysis and modelling (see further, Chapter 6).

Terminology usage varies here, so we shall use the form provided within ArcGIS 3D Analyst for convenience:

·         Case 1: hard breaklines, which are well-defined surface structures, whose shape and heights are known and fixed, and which mark a significant change of surface continuity — for example, indicating a road, a stream, a dam or a major change of slope which has been surveyed along its length (e.g. a ridge). Such breaklines are usually specified as a set of triples {x,y,z} and intermediate values are usually determined by simple linear interpolation

·         Case 2: soft breaklines are similar to hard breaklines, but do not imply a change in surface continuity. They indicate that the known values along the breakline should be maintained, for example when creating a TIN of the surface, but no sharp change in the continuity of the underlying surface is implied

·         Case 3: faults are more complex, in that they often have a spatial extent due to displacement and not just a simple linear form

The surface analysis package, Surfer, models breaklines as 3D polylines and faults as 2D polylines or polygons, reflecting their variable form. ArcGIS models breaklines as 3D polylines and faults as 2D polylines. Software packages treat these types of boundary in very different ways. Cases (1) and (2) take into account all data points provided, although in Case (1) z-values computed for the linear breakline take precedence over points that may lie on the other side of the line from the point being analysed. Case (3) is quite different, since fault lines and zones are treated as barriers. In ArcGIS data values beyond a barrier are not included in computations, where the notion of beyond is defined by visibility from the source point; in Surfer linear barriers are treated as obstacles, which may be skirted around (involving considerable additional distances and extra processing), whilst polygonal faults are treated more like self-contained regions, with interpolation inside and outside the polygonal regions being treated separately.