A very common GIS task is to analyze the location
of service centers (hospitals, stores, cell phone towers etc.) based on the
demand (people that are using the service). The purpose of this document is to describe a method for
allocation of customers (census polygons) to service centers (points) using
ArcGIS (any license) and the functions available in ET GeoWizards.
process has three major steps:
the service areas of the existing service centers using Thiessen
Thiessen polygons (also referred to as Voronoi Diagram), define
the individual regions of influence around each point of a set of
points. Thiessen Polygons divide a plane, assigning the area to point in
the set such that any location within a particular polygon is nearer to
that polygon's point than to any other point.
the demand from the census polygons to the service areas using the
spatial intersection of the two sets of polygons.
||Example: Three source
polygons with values 20,5 and 40 in the field to be transferred.
The target polygon overlaps a quarter of the first polygon, the
entire second polygon and a half of the third polygon
The value for the target polygon will be calculated using the
1/4*20 + 1*5 + 1/2*40 = 30
that the assumption is that the value is evenly spread over the area
which is not always the case. The more representative the source data
is, the better the results will be.
the demand calculated for each service polygon back to its service
GeoWizards functions to be used
that the links above are to the functions available via the interface of ET
GeoWizards. If the procedure is to be performed in the Model Builder - refer
to the corresponding tools available in the ET
GeoWizards ToolBox. If performed via a VBA script - refer to ET
input datasets - a polygon dataset with population attribute and point
dataset representing the available service centers. We need to assign for
each point the population that will be serviced by this point
ZIP code dataset for
California and some Service points
The ZIP code polygons
have a population attribute
Build Thiessen polygons.
In order the entire area to be covered, the "Use buffered
convex hull" option with large enough buffer distance has to be
Clip the Thiessen polygons with the ZIP code polygons
|The resulting service area
for a smaller area
||The Service area overlaid
with the Demand polygons
|Step 3: Transfer
the population attributes from the ZIP polygons to the Thiessen
polygons. Use the "Count" (sum proportion) option
||Step 4: Transfer
attributes from the Thiessen polygons to the Service points using
get it done?
ET GeoWizards offers
three ways of achieving the above procedure
- Via the User Interface - just perform
the steps one by one using the Wizards available
- With a simple VBA script - many of the
functions of ET GeoWizards are available for use within VBA scripts
or custom applications written in any COM language.
- Create a Geoprocessing Model in the
Model Builder (ArcGIS 9.0 and above only) using the ET GeoWizards
geoprocessing tools made available in version 9.2. A model
performing the task is included in the download of ET GeoWizards
for ArcGIS 9.x
diagram of the process:
See ET GeoWizards
User Guide for