Thiiink: Ideas, Imagination, and Innovation in GIS

In decision support for linear assets the most familiar result is the optimal alignment but this is far from the only useful output from a rather rigorous process. In many respects, the optimal alignment is more misleading than useful since it only provides a glimpse of what is without painting a full picture of the situation – like reading an executive summary without regard for the underlying body of knowledge. What then in the decision making process can provide us with additional insights? Cost corridors!

Before we plow into talking about cost corridors let’s remind ourselves that least cost analysis is merely a numerical technique for solving a complex system. With this now fresh in our minds we can talk a little more abstractly, numerical techniques used in solving systems operate in the realm of a solution space. Solution space is not constrained to two, three, or four dimensions but rather the dimensions are as numerous as the number of variables, constraints, and interaction rules that comprise the system.

As mentioned, the most common or expected result of a least cost analysis is the optimal path, this represents just the minimum solution, and, for those who have studied approaches for solving systems, we know that this is just one solution found within the solution set. Observing the solution set is where cost corridors come into play. The unfortunate thing is that even though solution space can handle multi-dimensions we mere mortals (okay, more to do with technical limitations) need to determine the projection of the solution set onto the practical plane of a GIS graphical display. The cost corridor is the intersection of the solution set with combined cost surface.

In earlier posts we detailed how the least cost analysis process works when all that is needed is the optimal route, quite simply:

1. establish a start point
2. accumulate cost weighted distance from the start
3. establish an end point, and
4. trace the steepest path from the end point to the start point.

The process for creating cost corridors capitalizes on the optimal route process quite heavily and is implemented as:

1. establish a start point
2. accumulate cost weighted distance from the start
3. establish an end point
4. accumulate cost weighted distance from the end
5. combine start and end accumulated cost weighted distance
6. find the minimum value within the combined accumulated cost weighted distance, and
7. using a cutoff percentage query the combined accumulated cost weighted distance for values between the cutoff and the minimum.

Check out this for some more information on this process and its usefulness.