Often, the data for Isarithmic mapping is located at distributed control points causing the lack of point information between these locations. This is where interpolation is useful because it allows for intermediate points to be assigned a value based off triangulation of the control points.
Interpolation can only be used when dealing with isometric lines. An even gradient is assumed between lines, so the value halfway between two lines is equal to halfway between the two data values. The rate of change can be easily assessed on both isometric and isoplethic maps. Lines that are closely spaced have a steep gradient while lines that are far apart have a flatter gradient.