Create a buffer theme

The main purpose of this utility is to enable analyses of features appearing (or not) within a certain distance from other features. A typical use is to select features within one theme that are within a certain distance from the selected features of another theme. The features selected by a buffer criterion can then be analysed further. Hence, a buffer analysis generally requires two main steps:

1. Create a buffer theme

2. Use the buffer theme for some purpose

There is also another reason for developing this utility. The ArcView's Theme-Select by Theme option in the View window works on source data co-ordinates. This doesn't matter for users wanting to select features of a theme based on whether they intersect or completely contains the selected features of another theme. However, for users wanting to select features of a theme based on whether they are within a distance of the selected features of another theme, there is a critical difference. The reason is that despite ArcView requests a distance in Distance units, the distance measure is converted to source data units. Then the features within the distance in source data units are selected. This is correct for projected source data (unprojected view), but when the source data are in decimal degrees this is incorrect as the real length represented by a decimal degree unit is not the same along the latitude and longitude axis. As a workaround to this problem, a user may create a buffer theme covering a user-specified distance around the selected features of the active themes, and then select features of a theme based on whether they intersect or completely contains the selected features of the created buffer theme.

The steps involved in creating a buffer theme are as follows:

1. Specify the buffer distance

2. Specify name of buffer theme to create

3. Create the buffer theme attribute table

4. Create the buffer features for each active theme

5. Merge overlapping buffer features into non-overlapping features

6. Update the buffer theme legend

These steps are desribed in more detail in the following:

Specify the buffer distance

The user is prompted to specify a buffer distance in the Distance units of the View. As the buffer distance is a non-directional measure it must be a positive number.

Specify name of buffer theme to create

The user is prompted to specify a shapefile to create.

Create the buffer theme attribute table

The buffer features are given three attributes: Buffer-ID, Theme, and Buffer Distance. The Buffer-ID includes an ID-number of a feature, the Theme includes the name of the theme which selected feature this buffer feature was created from, and the Buffer Distance includes the specified buffer distance used to create the buffer.

Create the buffer features for each active theme

This step processes one active theme at a time, and the creation of a buffer polygon differs depending on whether the active theme includes point, line or polygon features. The common part is that for each of the selected features in the active theme, one buffer polygon is created. If there are no selected features in the active theme, the user is prompted for whether all features are to be buffered. The reason for having the user to confirm this is to avoid time-consuming buffering of large themes due to the user forgetting to select the features of interest first.

For point features, a circular polygon centered on each point and with radius equal to the buffer distance is created.

For line features, the buffer polygon includes an area whose outline is one buffer distance from each line.

For polygon features, the buffer polygon includes an area whose outline is one buffer distance from the outline of each polygon. A positive buffer distance creates a buffer polygon larger than the source polygon, while a negative buffer distance creates a buffer polygon smaller than the source polygon.

Merge overlapping buffer features into non-overlapping features

As the final buffer theme should only include non-overlapping polygons, overlapping buffer polygons should be joined into one buffer polygon.

Update the buffer theme legend

As default, the buffer theme features are displayed in yellow, and with the buffer distance as legend label. If necessary, you may use the Legend Editor to change this display.