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| GIS Cookbook: Recipe - Buffering an Area |
Keywords: Impact, assessment, government, buffer, analysis, regulations, influence, zone
Category: Buffers
Software: ArcView 3.2
Problem: To buffer an area of influence around an object at a specified distance
Description: A buffer refers to the area contained within a specified distance from an object in space. But why would we want to create one?
Objects in space frequently have some sort of impact on the objects and areas around them. For example, factories emit fumes that can affect people for miles around. Freeways create "noise pollution" that can be heard blocks away. Buffers can be used in these instances to depict a sphere of influence in which the people and places within this "sphere" are more significantly impacted by a given phenomenon than those on the outside.
Buffers can also be used to show the reverse -- that is places that are less significantly impacted by a given phenomenon because they are within a certain distance of an object. These sorts of buffers, known as zones of protection, are frequently drawn in relation to regulations attempting to provide protection for special places. Examples include areas around school where liquor stores are prohibited, or protected lands around which shopping malls and urban development are not permitted.
Scenario:
A story appearing in the Los Angeles Times provides an illustration of how buffers can be used to analyze and solve problems.1 The article explained that Irvine Medical Center had found it necessary to reject many of its indigent patients due to a shortage of available hosptial beds. In order to make this adjustment, UCI Medical Center decided to map a five-mile discrete buffer around the hospital, along with a two-mile buffer around the UCI-owned Family Health Centers in Anaheim and Santa Ana, California. In order to display this, buffers were created around the care centers. (In this example we are creating buffers around point features, but buffers can also be done on lines, and areas). When these buffers are in place, indigent people living outside of these buffers will be turned away.
1 "UC Hospital Capping Indigent Care." Los Angeles Times, August 1, 2002.
Methodology:
1) Start up ArcView and open a new view.
2) Add the themes that you would like to use in your view for analysis. For information on adding data/themes see How to Add Data after opening a New View in the Getting Started section.
3) Perform queries as necessary to limit your data to relevant areas. By performing queries you are simplifying your data into data relevant to your project. For example, in the illustration below, by entering [State_name]="California" you are selecting only cities in California. Because the buffers to be created only lie in California this query is helpful.
If you are getting a syntax error message on your queries, See Pitfall 1.
4) After you have selected the points, lines, and areas of interest in your project, you need to convert them to shapefiles. To do this, go to Theme->Convert to Shapefile. After you have created new shapefiles, you can delete unnecessary themes selecting those themes are clicking Edit->Delete themes.
5) Next, double click the themes in your view that you want to edit and now you can change the font, style, and symbol of themes. (In this example, the care centers were given hospital cross symbols, hospitals were changed to red to stand out on the yellow California background, and highways were given highway markers). Now you have a view with themes that are ready to buffer.
6) To create a buffer for your selected theme, go to Theme->Create Buffers; in our case we would select the care centers. The buffer wizard will first ask if you want multiple rings or one ring around the geographic area that you want to buffer. This is an important consideration, because sometimes a phenomenon tapers off with increasing distance from an object, rather than ending abruptly at the outside boundary. Multiple buffers allow us to factor in levels of gradation that would be invisible when using a single buffer. In our example, however, the care centers wish to draw a boundary that separates those indigents who are eligible from using the their services from those who are not, so a single, abrupt boundary for each is justified.
7) Next provide the distance units to use and the distance to include in your buffer. (Be sure that the distance units used in your view matches the distance units specified for the buffer. If you need to change the view distance units go to View->Properties). Here, miles were used as the distance units, as the map is of a relatively large scale. (Smaller distance units should be used when using smaller scale maps). Next specify whether or not to dissolve barriers in the buffer. (Choosing to dissolve barriers will unite that theme's buffered zones into one area. Conversely, choosing not to disolve barriers, will give each new buffer a discrete area). Finally, add completed buffer as a new theme in order to add data to your view. (To change colors on the display, go to CSISS Cookbook section Cartographic Design.)
If you have specified multiple rings and only one buffer ring is shown, See Pitfall 2
8) To make your buffer rings transparent, first bring up the Legend+Editor class=term>Legend Editor by double clicking on the theme. Then double click the symbol to open the pallete. Select the middle pallete on the left side. Next click the paintbrush symbol and choose any color foreground that you want along with no background. Click Apply.
9) To label themes, with the theme selected go to Theme->Auto-label... .
Select the field that you want to label and check themes text label placement property. You can move the labels around by clicking on the map and dragging each individual label.
If you are unable to label a certain theme, See Pitfall 3
10) In this example, people affected by the buffers are people living outside of the influence buffer. Because of this, a good application would be to select the cities that are found just outside of the buffer. In order to do this, you would first click the Select Feature tool and choose the cities that you want to select. After you have done this, Convert to Shapefile... and add it to your view. To get a description of each point, you would use the identify tool to see the attributes of each point. (Another way to select cities that are close to the buffered zone, is to make another buffer(s) around the existing buffers.
11) Again, add labels as needed. To create a map go to View->Layout. Edit your title, legend, Scale+Bar class=term>scale bar, north arrow, and neatline(s) to make the map more dynamic.
12) Finally, export your map to a format that suits your needs such as jpeg or bitmap. To do this go to File->Export....
Pitfalls:
Links:
University of Manitoba - Lecture on Buffers
ESRI Geography Network - Contains links to data, services, and information
ESRI - Website of the leading GIS software producer
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| Authored by: Ethan Sundilson Modified: 9/17/03 |
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Copyright © 2002-2015 by Regents of University of California, Santa Barbara
Cookbook: Ben Sprague, Ethan Sundilson, Carlin Wong, Sam Ying
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