Will Patterson
Integrating ArcView and the Spatial Analyst Extension with the PRISM
Climate Expert System
Geographic Information Systems (GIS) are increasingly being associated with
climatological research as flexible utilities in which to develop and explore
modeled data. PRISM (Parameter-elevation Regressions on Independent Slopes
Model) is a climate-mapping expert system developed by Dr. Christopher Daly
and staff from the Oregon Climate Service (OCS) at Oregon State University
(OSU). PRISM utilizes a digital elevation model (DEM) and a weighted
regression technique to distribute point climate measurements to a geographic
grid. PRISM exhibited superior results over various kriging methods in a
comparative study of regional precipitation mapping. While the PRISM
software is not available to the public, many products and datasets produced
by PRISM are available over the Internet. This paper will provide a brief
overview of climate mapping with PRISM and describe how ArcView and the
Spatial Analyst extension have been customized with Avenue to assist with
formatting PRISM input files and assessing PRISM output data. The
availability of GIS-compatible PRISM datasets will also be discussed.
Introduction
GIS has not only enhanced data visualization within disciplines, but perhaps
more importantly, greatly improved data relations across discipline
boundaries. For example, with the power of GIS overlays, a soil scientist
can now directly compare categorized soil-type polygons with precipitation
spatial data developed by a meteorologist. This basic yet essential
functionality has helped fuel the rapid demand for GIS data. In particular,
spatial distributions of climate parameters are proving invaluable as base
data for overlay analysis and as inputs for other agricultural, natural
resource, and hydrological models.
Recent advances in the provision of spatial climate data have been
accomplished through development of the PRISM expert system. PRISM is a
group of seven stand-alone programs that work together to produce gridded
estimates of climate parameters. PRISM has been termed an expert system
because it attempts to duplicate the procedures an expert would use to
develop climate maps. The system is based on the philosophy that
topographic orientation and elevation are primary driving factors of climate
distributions. PRISM was developed originally to address the problems of
precipitation mapping in complex terrain. However, as the system evolved,
it was discovered that other climatic properties such as snow and temperature
could also be successfully mapped using similar modeling assumptions.
The author of this paper has been using PRISM in a precipitation modeling
project for the National Weather Service Office in Eureka, California.
ArcView has been used as an assistive tool for formatting necessary files
to input into PRISM and also for viewing PRISM output data. Both PRISM
input and output data are ASCII files with specific formats. It was
discovered that Avenue, ArcView's object-oriented programming language,
provides several features for reading and writing ASCII data. By bundling
together related Avenue routines for developing and importing PRISM-format
files, a PRISM Tools extension for ArcView was developed to assist users
of the expert system.
The PRISM Method
PRISM uses a DEM to estimate the elevation of each input weather station.
Next, each DEM cell is assessed for slope orientation and assigned to a
topographic "facet" (a group of cells with a common topographic orientation).
Localized climate-parameter/elevation regression functions are then formed
from nearby weather stations and a parameter estimate is produced. In the
regression equations, stronger weights are assigned to weather stations with
locations, elevations, and topographic positionings similar to that of the
DEM cell. Finally, PRISM produces estimate prediction intervals to
approximate the levels of uncertainty involved.
PRISM Versus Kriging
Kriging has become a popular method for interpolating many types of data,
including precipitation and other climate themes. Kriging utilizes a
semivariogram to determine the optimal station weights for interpolation
purposes. PRISM was compared to kriging, detrended kriging, and cokriging
in a regional precipitation mapping exercise for the Willamette River Basin,
Oregon (Daly, et al., 1994). Of these methods, PRISM exhibited the lowest
overall bias and mean absolute error. The latter two methods also require
area-wide relationships between the climate parameter and elevation (i.e.
between precipitation and elevation). This relationship does not often
exist when modeling large domains such as states or nations. PRISM avoids
this problem by continually adjusting its cell neighborhood to adapt to
changes in orography at multiple scales.
Acceptance of PRISM
The PRISM Evaluation Group (PEG) was formed to critique and review PRISM
methods and products. PEG consists of State and Regional Climatologists,
representatives of national agencies such as the Natural Resources
Conservation Service (NRCS), and other potential users from state and local
governments. After considerable evaluation and scrutiny, PEG determined
that PRISM-generated precipitation maps were of equal or greater quality
than the best manually prepared maps available. Under an agreement with
NRCS, PRISM is currently being used to produce climate maps for all fifty
states.
The Seven PRISM Programs
The PRISM expert system consists of the following seven programs (Daly,
et. al., 1996). The programs can be used separately for specific tasks but
are more commonly used sequentially in a complete modeling session.
1. LATTICE utilizes a Gaussian filter to smooth and/or convert grids to
different spatial resolutions. It is typically used to convert a DEM
to a resolution that is appropriate for the climate parameter being
modeled. LATTICE will also convert GRASS or Esri-format ASCII Grids
to PRISM-format ASCII grids used within the expert system.
2. FACET uses the output DEM from LATTICE to develop grids of slope
orientation at up to six separate spatial scales. Adjacent cells with
similar slope orientations are grouped together to form contiguous
topographic zones called facets. Nine different orientations are possible,
including eight compass directions and a flat orientation.
3. ASSAY uses an iterative jacknife cross-validation technique to determine
optimal modeling settings.
4. PRISM is the core interpolation module that produces gridded estimates of
climate parameters based on input station data, FACET grids, and a DEM.
Gridded predictions are produced from a weighted parameter-elevation
regression function calculated for each DEM cell.
5. POLISH is a postprocessing routine for evaluating parameter-elevation
regression slopes produced by PRISM. Some smoothing of an output grid may be
performed in cases where POLISH detects abrupt transitions between adjacent
topographic facets with opposing climate characteristics.
6. MIRROR produces point value estimates from POLISH output grids and various
performance summary and diagnostic statistics.
7. GISLINK performs unit conversions if necessary, and also converts native
PRISM-format output grids to one of two GIS compatible formats, GRASS or Esri
ASCII Grid (also known as ArcInfo ASCII Grid).
PRISM Flowchart
The following flowchart illustrates a typical PRISM modeling session.
Click on the image to see the full size version.
(Image courtesy of Wayne Gibson, Oregon State University)
The Challenge
The LATTICE and GISLINK programs within the expert system provide the
necessary transitions between internal PRISM-specific file formats and
GIS-compatible data. However, it was hypothesized that some customized
utilities within a GIS environment could supplement these programs with
additional routines for creating or reformatting PRISM data. The ability
to view various intermediate PRISM files produced during a modeling
session without having to put each one through several GIS conversion and
importing steps could prove to be helpful.
Some examples of PRISM input and output data include:
Input Data:
- Digital elevation models
- Weather station files, including coordinates and climate data
Output Data:
- FACET grids
- Diagnostic weather station files, reflecting model adjustments
- Diagnostic grids
- Final polished climate grids
ArcView was chosen to try to fulfill the challenge of providing a unified
platform for managing PRISM input files and evaluating PRISM output data.
When used with the optional Spatial Analyst extension, ArcView provides
a robust set of raster tools that includes the capability of importing
ASCII Grids in an Esri-format. However, it wasn't clear whether ArcView
would provide adequate customization features for the task at hand.
Avenue Provides a Solution
It was soon discovered that Avenue provides the exact utilities necessary to
develop shortcuts for dealing with PRISM data. Avenue is a programming
language that utilizes objects and requests to complete user-desired tasks.
An object is basically something that you are working with, and requests are
used to initiate a particular function or property of the object. For
example, a linefile is an object that allows Avenue to read through ASCII
files line by line. Linefile objects are controlled by specific requests
that will read, store, and write new ASCII files. By using these objects
and requests, conversion between different ASCII file formats is made
possible.
The following is an example algorithm to import a PRISM-format ASCII grid
directly into ArcView, thereby creating a new Esri Grid:
1. Convert the input PRISM-format ASCII grid to a linefile object.
2. Read the header of the PRISM grid, calculate cell size information,
and store the coordinate data.
3. Create a temporary linefile object to act as an Esri-format ASCII grid.
4. Using the stored coordinate and cell size data, write out an Esri-format
ASCII grid header into the temporary linefile.
5. Read the cell values from the PRISM grid linefile, line by line.
6. Write the cell values to the temporary linefile, line by line.
7. Import the temporary linefile as an Esri-format ASCII grid directly into
a view; this will create a regular Esri Grid.
8. Delete the temporary linefile.
Essentially, this algorithm converts a PRISM-format ASCII grid with this header:
GEO
north: 42.686631944580
south: 40.979166666666
east: -130.230034722646
west: -125.02083333334
rows: 145
cols: 217
To a temporary Esri-format ASCII grid with this header:
ncols 217
nrows 145
xllcorner -125.02083333334
yllcorner 40.979166666666
cellsize 0.04166666667
And then imports the temporary Esri-format ASCII grid directly into ArcView,
thereby creating a regular Esri Grid. Before finishing, the routine also
deletes the temporary Esri-format ASCII Grid.
This entire procedure can be represented by one Avenue program, or script.
When executed, the grid conversion and importing process is instantaneous
and transparent to the user. By using similar approaches, a variety of
assistive conversion routines were developed for use with PRISM, ranging
from importing weather station files as shapefiles to exporting Esri Grids
directly to PRISM-format ASCII grids.
The PRISM Tools Extension
To simplify the installation and accessibility of the PRISM shortcut
scripts, an ArcView "PRISM Tools" extension was developed. An extension is
essentially a group of Avenue scripts and customized interface features that
can be turned on or off at the discretion of the user. Eleven separate
scripts for working with PRISM data as tables (databases), Grids, and
shapefiles were included in the extension. Because PRISM Tools also uses
the Spatial Analyst extension, many other powerful features are available.
For example, several handy climate-style color ramps are available for
viewing Grids, including some specifically for use with precipitation
and temperature.
The following images illustrate the look and feel of the PRISM Tools
extension, and include some background examples of various diagnostic
and final products produced by the expert system.
Click on the images to see the full size versions.
ArcView Project-Extensions Menu with PRISM Tools extension available:
ArcView Table-PRISM Menu with FACET grid and weather station shapefile in background:
ArcView View-PRISM Menu with final precipitation grid in background:
Conclusions
The PRISM expert system has provided unique and powerful techniques to
produce climate GIS data. In a similar way, ArcView, Avenue, and the Spatial
Analyst extension are providing outstanding environments for viewing and
evaluating climate data. The lessons learned from the integration of these
technologies not only apply to climate research, but also to any undertaking
where ASCII format data needs to be visualized.
Availability of PRISM Data
A wide variety of final polished PRISM products and metadata are in the
public domain and are available for download from the Oregon Climate Service
web site. The climate grids are available in Esri ASCII Grid formats, and
can be used directly with the ArcView Spatial Analyst extension or the
ArcInfo GRID module. Various polygon interpolations of the climate grids
are also available in ArcInfo coverage format.
Oregon Climate Service web site:
http://www.ocs.orst.edu
Specific URL for PRISM Products:
http://www.ocs.orst.edu/prism/prism_products.html
Acknowledgments
Dr. Christopher Daly of Oregon State University and staff from the Oregon
Climate Service for assistance with this project and allowing me to
test the PRISM expert system.
Mr. Mel K. Nordquist for meteorological advice, data classification,
and academic consultation.
Dr. Lawrence Fox III and Dr. Steven A. Carlson, both of Humboldt State
University, for research and advisory guidance.
The California Department of Fish and Game, Technical Services Branch, GIS
Unit for GIS assistance and programming advice.
References and Additional Readings
Daly, C., Neilson, R.P. and Phillips, D.L., 1994: A statistical-topographic
model for mapping climatological precipitation over mountainous terrain.
Journal of Applied Meteorology 33, 140-58.
Daly, C., et. al. 1996: Overview of the PRISM model - an Internet
publication.
Daly, C., G.H. Taylor, and W.P. Gibson., 1997: The PRISM approach to
mapping precipitation and temperature. In: Proceedings of the 10th AMS
Conference in Applied Climatology, American Meteorological Society, Reno,
NV, Oct. 20-23, 10-12.
Esri Educational Services, 1997: Programming with Avenue, three-day course
notebook with exercises and training data.
McDonnell, R.A., 1996: Including the spatial dimension: using geographical
information systems in hydrology. Progress in Physical Geography 20, 159-177.
Author Information
ArcView PRISM Tools Extension Developer and Author of this Paper:
Will Patterson, GIS Analyst
California Department of Fish and Game
Technical Services Branch
1730 I Street, Suite 100
Sacramento, California 95814
Telephone: (916) 323-1484
Fax: (916) 323-1431
E-mail: wpatters@hq.dfg.ca.gov
Also representing:
Spatial Analysis Laboratory
Department of Natural Resources, Planning, and Interpretation
Natural Resources Graduate Program
Humboldt State University
Arcata, California 95521
Other Contacts
PRISM Author and Primary Contact:
Dr. Christopher Daly
Oregon State University
USFS Forestry Sciences Lab
3200 SW Jefferson Way
Corvallis, OR 97331
E-mail: daly@fsl.orst.edu
PRISM GIS Products Contact:
Wayne Gibson
Oregon Climate Service
326 Strand Ag. Hall
Corvallis, OR 97331-2209
E-mail: oregon@ats.orst.edu