Isarithmic mapping is one of the most popular methods of displaying data that is smooth and continuous.  The contour map is the most common style of isarithmic map and is widely used to present data on topography, elevation, and rainfall.  This week’s lab required that I make two isarithmic maps depicting average annual rainfall in the state of Washington.  One map would be symbolized with a continuous tone while the other would employ a hypsometric tint design.  Both maps would also use a hillshade effect to enhance the data contours.

The data for this lab was gathered from the U.S. Dept. of Agriculture through their Geospatial Gateway website but was originally created by researchers at Oregon State University by using an interpolation method called PRISM (Parameter-elevation Relationships on Independent Slopes Model).

Map 1 was titled “Continuous Tone” and was symbolized with the Precipitation color ramp provided by ArcMap.  I added a vertical legend and stretched it so the numbers and colors were easier to read.  I also added some key cities to the map to serve as a reference for viewers unfamiliar with Washington geography. 

Map 2 was titled “Hypsometric Tints” and symbolized in the same manner as Map 1.  For Map 2 I utilized a horizontal legend and manually adjusted the 10 classification values that were called for.  To make the classes easier to display in the legend we used a tool from the Spatial Analyst extension that rounded fractional numbers to whole numbers.  Another difference between the two maps was that for the hypsometric map I added contours at specified intervals related to the amount of average annual rainfall.  To add a bit of craftsmanship and refine the map I decided to make the precipitation layer 20% transparent and placed the contour layer below.  This arrangement offered the benefit of the contours layer without giving the map a cluttered appearance.  I felt this really improved the readability of the map when combined with the hillshade effect.  

I hope the audience will come to appreciate how effective and powerful isarithmic maps can be in understanding our world.