Module 6: Isarithmic Mapping- Elevation and Annual Precipitation Rates in Washington State

This week concentrated on mapping smooth continuous data types, such as elevation, precipitation, and  temperature. Using ArcGIS Pro software, a map was created from provided annual precipitation data for the state of Washington. This data was created using the PRISM, or Parameter-elevation Relationships on Independent Slopes Model interpolation method. In this method, precipitation data is collected at precipitation stations and surrounding precipitation rates are then extrapolated based on elevation cell values, found by comparing Digital Elevation Model (DEM) cell values of the same region, along with other environmental factors. DEM elevation values, provided in raster cells, are compared to the cells containing the precipitation station location, and are then assigned a precipitation rate based on its similarity to that elevation value. This is based on the positive relationship between elevation and precipitation rates, or the fact that as elevation increases, so does the rate of precipitation. The final map is included below: 

To create this map, precipitation rates were classified into ten classes using the natural breaks classification method, ranging in color from red (low) to blue (high). A contour layer was created using the Contour List tool within the Spatial Analyst toolbox, with varying elevations from 10 to 180. 

A hypsometric tinting method was also used. Hypsometric tinting involves creating a transparent layer in which colors vary based on the elevation values. This is then placed over another surface layer to produce an image that looks 3-D and more efficiently shows differences in relief. In this map, hypsometric tinting was used to create highlights and shaded areas to resemble the mountains and valleys of the state. By overlying both elevation and rainfall data, a positive relationship is revealed to show that the higher elevations towards the western side of the state have higher rainfall rates (blue).