This page summarizes various tools for analyzing shapefile relationships: buffers, feature merges by summarizing, and spatial join methods to assign attributes of encompassing or nearby features in one Theme to features in another Theme.  Combined with ArcView's select-by-theme geographic query capabilities discussed previously, these are important tools for spatial analyses of shapefiles.

Buffers

A buffer is a polygon defining the area within a specified distance of some other feature.  Buffers are typically used to define and characterize the neighborhoods around features of interest.

Use ArcView's Theme--Create Buffers tool to make buffers around selected (or all) target features in an active Theme, or around graphic elements in your View.   The buffer tool can--

• buffer to a uniform specified distance,
• buffer to distances determined by a Table attribute of the target features, or
• create multiple concentric buffers of  uniform width.

Here's and example of buffers created around New England cities with 1990 populations of 50,000 or more.  In this case, the buffer width (in feet) is determined by the "Households" field in the city Theme Attribute Table.  The buffers are not merged, and are merely drawn on the View as a graphic to indicate  "zones of urban influence."

Feature Unions via Summarize

Field--Summarize is a powerful utility that lets you aggregate the data in a Table based on values in the active field, outputting the summarized data to a new Table.  In the Table window, select the field on which to summarize by clicking on its name. Then Field--Summarize (or the Summarize button ) to access the Summarize dialog window.  Pick the fields to be summarized and the summary method for each field (average, sum, minimum, maximum, standard deviation, variance, first, last or count), and add each of these to the list on the right.

For example, you might summarize the Theme Attribute Table of States.shp based on the "Sub_Region" field, creating a new summary Table with one record per sub-region.  Make "Sub_Region" the active field in the Table, then in the Summarize dialog window add the "pop1990" and "Area" fields, by sum, to the list of fields to summarize.  This will create a new (dBase-format) Table of populations and land areas by sub-region.  Then you can edit this Table to calculate 1990 population density by region.  (Note: simply averaging states' 1990 population densities would not yield correct regional population densities!).

The Summarize utility also lets you aggregate geographic features into a new shapefile by including the "Shape" field (to be summarized by "Merge") in the list of fields to be summarized.in the dialog window.  You can then add the new shapefile to an existing View or a new View.  The other specified fields, summarized as you specified, are included in the new shapefile's attribute Table.  The resulting shapefile will have a single feature corresponding to each value category in the source field on which the merge was based.  Merging features dissolves shared boundaries.  Note that merged non-contiguous features become non-contiguous islands of the same feature.

For example, to merge states into sub-regions, activate "Sub_region" as the field to summarize on, and include the "Shape" field (by "Merge"),  along with  "Area" (by "Sum") and "Pop1990" (by "Sum") in the list of fields to summarize.  You can then add the new sub-regions shapefile as a View Theme (see below).

We have already explained how to union features by including a Merge of the "Shape" field when Summarizing by any active field in the Theme Attribute Table.

Spatial Joins

A spatial join is a join of two Theme Attribute Tables based on their "Shape" fields.  Each feature record in the destination Table is joined with the record of the nearest feature in the source record, or else the record of the source feature that completely contains it..  Spatial joins only work in unambiguous situations:

• Each destination point will be joined with data from the nearest source point, the nearest source line, or the source polygon that contains it.  For example, you could join emergency rooms to fire stations so that each fire station record would include the distance, address,  phone number of  the nearest emergency room.
• Each destination line will get joined data from the nearest source point, the source line that contains it as a segment, or the source polygon that completely contains it.  For example, you could join drainage basin polygons to the streams that drain them, so that each stream record would includes its basin's total area, percent impervious cover, etc.   You can't join intersecting lines or polygons to lines because a destination line can intersect multiple points or polygon boundaries.

Each  destination polygon will get joined data from a source polygon that completely contains it.  For example, you could estimate each county's population-weighted contribution to it's state's tax revenues by joining state population and revenue data to the counties  You can't join points or lines to polygons because a destination polygon can contain multiple source points or lines.

ArcView has a GeoProcessing Extension that supports feature dissolves (equivalent to feature summarizing), merges, clips, intersects and unions, and performs cross-Theme attribute assignments equivalent to spatial joins.  The GeoProcessing Wizard is discussed in the context of ArcView Extensions.

Hot Links

An interesting miscellaneous ArcView utility is hot-linking.  The Hot Link tool  links features in a Theme to data or files, Avenue scripts or other functions.  The action specified for a feature is triggered by a mouse-click on the feature in a View window.  For example, clicking on a reference map in one View could trigger an Avenue script that loads specified Themes into a second view and sets various display parameters for those themes, or displays an associated graphic or plays a specified audio or video clip.

To set up hot-linking for a Theme, use the Theme--Properties "Hot Link" dialog box.  If you want each feature in the Theme linked to a different parameter, file or whatever, create a new field in the Theme Attribute Table to contain the parameters or filenames, and reference this in the dialog window's Field box.   Choose a Predefined Action (Link to User Script, Text, Image, ArcView Document or Project).

For example, you might create a string field called "Hotlink" in the States.shp Theme Attribute Table, and specify the path and filename of a GIF image of the state flower for each state.  Then in the Theme--Properties Hot Link dialog window, identify the field containing the image filenames, and choose 'Link to Image File" as the Predefined Action, which refers to a canned Avenue script called "Link.ImageFile."  To test the hot link, activate the hot-linked Theme, select the Hot Link tool, and then click on one of the linked features.  The associated image file should pop up in a separate display window.