Isarithmic Mapping: Annual Precipitation in Washington State

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The second most common type of thematic map behind the choropleth map is the isarithmic map. While choropleth maps are great for mapping phenomena existing within hard boundary lines, also known as enumeration units, isarithmic maps are perfect for mapping continuous data such as temperature, precipitation, or elevation. In isarithmic mapping, contour lines are frequently used to connect locations with equal values. Hypsometric tints can then be added to color the areas between contour lines with different shades so that map readers can easily distinguish between areas with high and low values. 

In this lab, our task was to create an isarithmic map showing average annual rainfall in Washington State. This rainfall data was interpolated using the PRISM (Parameter-elevation Regressions on Independent Slopes Model) method developed by Oregon State University. The map below was created using ArcGIS Pro. Areas with low precipitation (less than 10 in/yr) are shown in red, and areas with the highest precipitation (more than 180 in/yr) are show in blue. More information about the PRISM interpolation method can be found at the bottom of the map. 



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