Module 6 - Isarithmic Mapping

 

Figure 1 - Isarithmic Map for Washington State Precipitation

In this lab, we explored how long‑term precipitation patterns in Washington are modeled using the PRISM interpolation method. The dataset, originally created by the USDA Service Center Agencies and published through the USDA Natural Resources Conservation Service in 2009, was downloaded from the USDA Geospatial Gateway and includes monthly and annual precipitation rasters. PRISM begins with 30 years of rainfall measurements collected from weather stations, then fills in the gaps between stations using interpolation. Because elevation strongly influences rainfall in Washington, PRISM incorporates a digital elevation model and a regression model that relates each station’s elevation to its precipitation values. These combined factors allow PRISM to estimate rainfall for 800‑meter grid cells, producing monthly surfaces that are summed into an annual precipitation map.

Throughout the lab, we worked extensively with continuous raster data and learned how to apply continuous tone symbology to represent smooth, gradual changes in precipitation. We also used legend‑editing tools to create clear, appropriate map legends and relied on the Spatial Analyst Extension to perform raster operations. Hypsometric tinting was implemented to highlight Washington’s terrain by assigning distinct color bands to elevation ranges, and hillshade relief was added to enhance the visual structure of the landscape. We used the Int tool to convert floating‑point rasters to integers, manually classified elevation data, and generated contours using both the Contour List tool and the Spatial Analyst Toolbar. Altogether, this lab strengthened our ability to symbolize continuous surfaces, interpret terrain, apply analytical tools, and clearly communicate the processes and outcomes involved in building a complete isarithmic map.