Digital elevation model (DEM) is very often generated by measuring terrain points along contour lines using a digitizer. DEM with contour points should be provided with an algorithm interpolate elevation at arbitrary points.

There are several interpolation methods as follows.

Profile Method
A profile passing through the point to be interpolated will be generated and linear or spline curve applied, as shown in Figure 3.9 (a). However, sometimes improper profiles as shown in Figure 3.10 (a) are introduced. In case of spline curve, transition areas from steep to gentle slope will be a problem of wave shape profile.

Proportional Distance Method
According to distance to two adjacent contour lines, as shown in Figure 3.9 (b), the elevation is interpolated proportionally with respect to the distance ratio. However a point in an island contour, as shown in Figure 3.10 (b), would be a problem.

Window Method
A circular window is set up around a point to be interpolated as shown in Figure 3.9 (c) and adjacent terrain points are used to interpolate the value using second order or third order polynomials.

The interpolation accuracy is better than other methods, but searching of adjacent points within the window is time consuming.

TIN Method
TINs are generated using terrain points along contour lines. The interpolation is very easy but TINs within an island, as shown in Figure 3.10 (c), would be a problem.

Buffering based on proportional method with additional independent terrain points will be the best interpolation method.