How to get from A to B
at Minimum “Cost”
What do these functions do?
• Given:
– A starting point and
– An ending point
– A cost to move over the surface between the
two…
• Find
– The least cost pathway between start and end
– Cost???
COST?
• If you have a grid where all cells have
the same value, say 1, then it is
pretty easy to do
• If you create a layer where the
effective distance to cross a cell is
not 1 but varies depending on…
• Anything you like!
If elevation was the “cost”
• Then the path from
START to FINISH
avoids the higher
elevations
Finis
h
Start
Friction?
• IDRISI calls the “cost” surfaces and
perhaps that is a better word.
• ArcGIS
– just calls the “friction” grids whatever
comes to mind 
– But the final “Cost” is just that, a “Cost”
The model
dem
Plus
lnduse
Slope%
TotCst
Reclass
%
Costdist
1-10
Reclass
1-10
Finish
Distance
Cost_path
Direction
Start
Path
The “Friction” grids
dem
Slope%
%
Reclass
New range 1 – 10
(originally 0-20)
New range 1-10
(originally 1-23)
lnduse
Reclass
1-10
1-10
Add together
& get 2-20
20 = high
Friction
Layers (LU)
Layers (% slope)
The model
dem
Slope%
Plus
TotCst
lnduse
Reclass
%
Reclass
Costdist
1-10
Start
1-10
Total Cost
• Summation
– Reclassed
% slope
– Reclassed
LU
Cost Distance/Direction
dem
Plus
lnduse
Slope%
TotCst
Reclass
%
Costdist
1-10
Reclass
Distance
Direction
Start
1-10
Cost Distance
• Generates two grids
– Distance – Accumulated total friction so
is NOT exactly circular
– Direction – the direction from any cell to
the lowest cost cell of the 8 surrounding
cell. Really only used by the software to
find the lowest cost (total friction)
across the friction grid
Distance/Direction
The Path
dem
Plus
lnduse
Slope%
TotCst
Reclass
%
Costdist
1-10
Reclass
1-10
Finish
Distance
Cost_path
Direction
Start
Path
Something that causes problems
• You can do distance from something
like a road and get
Procedure
•
•
•
•
This is how
DISTANCE
- max distance
ABS
0
-Max
0
0
ABS
Cost min @ road