UNIT 33: USING BUFFERS

Example Application

Tidewater National Park which includes historical, archaeological, and natural values wishes to improve access to a scenic waterway which runs through the Park. The park management team sees this as an opportunity to examine ways of protecting sensitive areas while creating the best possible access to important features. The stream has a bluff on one bank which has many historical and archaeological sites associated it, and has low marshlands on the other bank which are sensitive to trampling and erosion. The managers would like to identify locations where either paths or scenic lookovers could be provided without damaging the park resources. In addition there are public safety issues which must be addressed. The following constraints have been selected to appropriately protect resource value.

1. No access can be provided in areas which are less than 1 meter above the stream surface.
2. No access can be provided which are within 2 meters of the bluff top.
3. Active archaeological digs must be surrounded by a 50 foot buffer.
4. Inactive archaeological sites must be surrounded by a 10 foot buffer.
5. Historical sites do not need to be buffered, but may need to be redesigned or closed if they impinge on archaeological buffers.
6. New paths and scenic overlooks must be easily attached to existing paths without traveling through sensitive areas.

The GIS technician will be responsible for checking the provided data layers for projection consistency, and providing support to a GIS manager for general data accuracy checking. In addition the technician will perform buffer operations as directed by the manager, intersect buffered data layers, and perform simple analytical tasks.

Awareness:

The expected learning goals of this section are to achieve a general understanding of buffer implementation, the potential uses of these operations, and a working knowledge of basic buffer operation vocabulary.

Competency:

The learning goals of this section are to develop the ability to perform specific buffer operations, and to manipulate buffer command parameters for buffer output control.

Mastery:

The learning goals of this section are to be able to integrate knowledge of buffers to perform high level tasks, and post-buffer analyses.

1. Unit 28 - Editing polygon data(Building and Protecting topology only)
2. Unit 34 - Pre/Post overlay tasks; Types of overlay operators
3. Unit 39 - Performing statistical analyses
4. Unit 41 - Using boolean search techniques

Complementary:

1. Unit 7 - Using and interpreting metadata
2. Unit 10 - Projecting data
3. Unit 47 - On screen visualization

1. Unit Concepts
   
   Data model capability and functionality vary widely amongst software systems.
   
   
   • Name of the buffer command, or the location of the menu item?
     
     
   • Parameters of the command
     
     
   • Right/left/full buffering functionality
     
     
   • Data model dependent buffering functionality
     
     
   • Attribute value dependent buffering functionality
     
     
   • Proper use of these functions and parameters
     
     
   • Pre or post buffer processes that must be run in specific software
     
     
     • These might include registration, or topology building processes.
       
       

1. View descriptions of each layer to be used. Check to be certain that all layers to be intersected share the same projection and the same projection parameters.
   If areas are to be calculated and compared, this projection must be an equal area projection.
   
   
   
   
2. Look at the layers using on-screen visualization capabilities.
   This step should be repeated often in order to control errors.
   
   
   • Check for proper registration, missing data, and known objects in appropriate locations.

   
   
   

1. Create a uniform buffer around all points in a point layer.
   
   
2. Create a full uniform buffer around the selected arcs.
   
   
   • From a line layer select a set set of lines according to an attribute value.
     
     
   • create a full variable buffer around these lines. (This can be done with points, polygons, and rasters as well.)
   
   
3. Create a right side buffer around the selected arcs.
   
   
   
   Software may require that you recreate topology or update data tables before using the buffered data.
   
   

1. Intersect the point buffered and the full variable line buffered layers.
   
   
2. Calculate the total area of buffered regions in a single layer.
   
   
3. Calculate the area of intersection for the two layers.
   
   
4. Repeat the same process for the buffered points and the right side buffered lines.
   
   

1. Create a variable buffer around points in a layer based on the variation in values of an attribute.
   
   
2. Analyze a verbal problem description, and implement appropriate buffers.
   
   
3. Perform distance, area, and other spatial analyses on buffered and intersected data sets.
   
   
   
4. Perform descriptive statistical analyses on buffered and ancillary data sets.
   
   All GIS systems do not have this functionality. Data can be exported to many spreadsheet software systems if necessary.

1. Unit 30 - Validating databases
2. Unit 31 - Managing database files
3. Unit 34 - Using overlay operators; Pre/Post overlay tasks
4. Unit 35 - Point in polygon operations; Line in polygon operations
5. Unit 40 - Using reclassification operators

A list of GIS Resources.


An example of site selection relative to water resources.


An example of a forestry buffering application (tutorial) .