Mapping Distance and
Density
Distance functions allow
you to determine the nearest location of something or the least-cost path to a
particular destination. Density functions, on the other hand, allow you to see
the highest and the lowest concentrations of features in your data.
Distance functions may be
directly or indirectly used in an analysis. The Straight Line Distance
function, for example, computes the straight-line distance to a particular
location, and is often the whole of an analysis (e.g., locating the nearest
hospital). The value of each cell in a surface generated with the Straight Line
Distance function equals the measured distance of a straight line from the
cell's closest source.
The Cost Weighted
Distance function, however, calculates the least-cost path to a particular
location, a process that usually represents one part of a larger analysis
(e.g., finding the least-cost path for constructing a pipeline). The cell
values in a surface created with the Cost Weighted Distance function represent
the accumulated cost of travel from the closest source. Distance is measured
from each cell to the closest source, such as from a campsite to a store
location.
In density surfaces, however, the cell values always indicate the number of features per unit of area, such as the number of people per square mile. Density surfaces are great for visualizing patterns in the data that otherwise might not be apparent.
Distance surfaces
You may have heard the
expression "as the crow flies" used to describe distance or
direction, for example, "The campsite is located 8 miles due south of
town, as the crow flies." Since a crow can easily fly above obstacles such
as forests, rivers, and gated communities, this expression implies a straight
line path, or the shortest distance between two points.
For those of us who can't
fly, forests, rivers, and gated communities are obstacles that will impede
travel to the campsite, so we must decide the best way to navigate around or
through them. You may even choose a campsite that is farther away because
getting to it requires less effort. So, as the crow flies, the nearest campsite
might in fact be 8 miles due south, but for you it could be a 20-mile hike.
With that in mind, consider
two types of raster surfaces: one that measures distance as the crow flies, and
the other that measures distance based on the cost of travel. These types of
surfaces will help you find the nearest location of something or the least-cost
way of getting there.
All distance surfaces are
calculated based on a source, which is the location of some object of interest,
such as your home. If there is more than one source, as in the case of city
fire stations, the calculation is based on the nearest one.
Density surfaces
Density surfaces are good
for illustrating concentrations of point or line locations. For example, if on
an annual basis lightning strikes a hilltop more often than the valley below,
then the density of lightning strikes will be concentrated near the hilltop.
Density is a calculation of
the quantity of something per unit of area, such as the number of annual
lightning strikes per acre. You can calculate the density of countable things:
discrete objects or events (e.g., businesses, trees, car thefts, earthquake
epicenters) and their attributes (e.g., employees of businesses, types of
trees, property value of car thefts, magnitude of earthquakes).
The Density function has
two methods for calculating density: simple and kernel. Both methods employ a
circular neighborhood or search area to make the density calculation.