Contents:
The
purpose:
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.
Methodology:
The
process has three major steps:

Delineating
the service areas of the existing service centers using Thiessen
Polygons:
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.

Assigning
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
formula:
1/4*20 + 1*5 + 1/2*40 = 30 
Note
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.

Assigning
the demand calculated for each service polygon back to its service
center
ET
GeoWizards functions to be used
Note
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
GeoWizards Scripting
The
task
Two
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 


The
Solution
Step1:
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
used 
Step2:
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
Spatial Join. 



How to
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
A
diagram of the process:
See ET GeoWizards
User Guide for
more information 