M6 - Computer Cartography - Isarithmic / Flow Maps

 

The map above illustrates average annual precipitation across Washington State using 30 years of data prepared by the PRISM interpolation method.

This week’s lab focused on isarithmic mapping to visualize continuous raster data, particularly average annual precipitation in Washington over a 30-year period using advanced interpolation methods. Our analysis was based on the Parameter-elevation Regressions on Independent Slopes Model (PRISM), which highlights how elevation impacts climate variables like rainfall and temperature. By gathering data from various weather stations and using inverse distance weighting (IDW), PRISM helps us calculate climate-elevation relationships for each raster cell, showing how elevation affects precipitation patterns.

We created two types of isarithmic maps: continuous tone maps, which display smooth transitions of data across an area, and hypsometric tint maps, which group data into bands with a color gradient. For our precipitation map, we sorted values into ten ranges from ≤10 inches to >180 inches, using a color ramp from red to deep blue. While hypsometric tinting clearly illustrates changes in elevation, it can sometimes mask local variations due to generalization.

The lab also broadened our understanding for different interpolation methods, using 30 years of sensor data to calculate annual averages while considering factors like terrain and rain shadows. Isarithmic maps, including isometric and isopleth types, are key for showing continuous geographic phenomena and rely on the idea that nearby locations tend to share similar characteristics.