TWOTHEME.avx
   Two Theme Analyst (v3).  This extension performs analysis between
two themes in a View.  This version has three functions; both produce
an output table, either dBase or INFO.

   The extension allows users to:

1) Create a Proximity Table.  A Proximity Table is a two column table,
with a field from each theme that displays which features in Theme A
intersect or are within a distance from which features in Theme B.
The user can specify the distance, in miles.  Negative distances are
not allowed.  A distance of 0 can be entered to specify only inter-
secting features.  Only selected features will be processed; if none
are selected, all get processed.

2) Aggregate Data based on Proportional Area.  Numeric data from a
SOURCE theme is aggregated, based on the features of a BASE theme.  The
data is aggregated based on the portion of the SOURCE theme's area that
overlays a feature in the BASE theme.  In other words, if 25% of a
SOURCE feature overlays a BASE feature, the numeric fields are weighted
by 0.25 and added to the Output table, for each record in the BASE theme.
If another SOURCE feature overlays that same BASE feature, a fraction
of its data is aggregated to that same record.  And on-and-on.
Users can choose which numeric SOURCE fields to use and can choose one
field from the BASE theme to act as an ID for the output table's records.
At the end, users can choose to JOIN the new table to the BASE theme on
that ID field.  Only selected features will be processed; if none are
selected, all get processed.

3) Calculate Area and Percentage Area.  Areas and percentages are calc-
ulated for features of the BASE theme, based on how much surface area
of the SOURCE theme overlays the BASE feature.  Both themes need to be
polygons.  The SOURCE features will be summarized by a field of the
user's choosing, before areas are calculated.  One field from the BASE
theme can be passed to the resulting OUTPUT table.  Only selected feat-
ures will be processed; if none are selected, all get processed.

   The Extension loads a new Menu item into the View GUI.  An "Instruc-
tions" menu item is included to guide users.

by Kevin O'Malley, 04/07/1998
KevOMalley@juno.com