7. Non-Topographic Points

Some data points should not be included in the TIN because they do no affect terrain configuration. These points are collected primarily for planimetric purposes - horizontal location. A good example is a fire hydrant. The top of the hydrant is measured to establish its horizontal location and map an underground water line.

Figure E-31 shows five data points, four which are ground points, the fifth a fire hydrant top.

(a) Top	(b) Perspective
Figure E-31 Five Data Points

Figure E-32 shows a TIN which includes the hydrant.

(a) Top	(b) Perspective
Figure E-32 TIN With All Points

Because the hydrant is part of the TIN, contours are created towards its top, Figure E-33.

(a) Top	(b) Perspective
Figure E-33 Phantom Contours

These are phantom contours because they do not really exist.

The hydrant should not be included in the TIN, Figure E-34, to prevent it affecting the terrain model.

(a) Top	(b) Perspective
Figure E-34 Fire Hydrant Exclusion