Cartographic Skills: Isarithmic Mapping

The week we created an Isarithmic map depicting average annual rainfall for the State of Washington. We used data created from PRISM Climate data. PRISM is a climate model using weather station data over a 30 year period, applied with digital elevation model (DEM). Perception usually increases with elevation and PRISM analyses the regression of the climate data for each grid within a DEM. In other words it compares surrounding weather station data records and how they relate to each other, along with elevation data. The outcome produces a climate weather model referencing the comparison between weather and topographic features. As you can see in my map below, the higher rain fall amounts are found at higher elevation or mountainous regions. The contours on the map are not topographic contours but range differences in the amount of precipitation. The range difference is however directly related to elevation.

We used a file geodatabase downloaded from the USDA Geospatial Gateway. The PRISM data was originally created from the PRISM Climate Group but the USDA built the geodatabase using PRISM data along with other layers related to climate data. We used the average annual rain fall layer which spans yearly averages over a 30-year period.
We used Arcmap to analyze two different methods of displaying Isarithmic maps for displaying smooth continuous data. The two methods are; Continuous Tone and Hypsometric Tinting. We used the PRISM data to visually see the difference in these two methods.
Continuous tone symbology uses color tone proportional to the value of the surface at each point. I used the Precipitation color scheme to show values of warm lighter colors of low rainfall amounts and cool darker colors for higher rainfall amounts.

Hypsometric tinting visually enhances a 3-D surface by using light and dark tints for low and high values. I took the tint a step further and used a range of color schemes, rather than a single color scheme. Using a range of colors allows you to visually understand the order of colors in relation to the values, as each range value corresponds to a unique color. We manually boke our data into 10 classes by manually defining the classes. This allows the data to be smooth and continuous, necessary for hypsometric tinting.

For the final map we used hypsometric tints along with hillside effect and contours, as seen in my map above.  Using contours allows for better visualization for the breaks of ranges in the data. Hillside effect gives a 3D shadow effect with the sun’s relative position taken into account.
Having traveled through the state of Washington, you notice many areas of agriculture in the central and eastern regions. What is interesting about the actual precipitation data, is that the fertile farming regions of the state appear to have the lowest rainfall amounts. With room to grow many types of agriculture, good soil and sun, all you need is water. So it’s interesting to see low rainfall amounts for agricultural regions and the human capacity to bring water to areas that normally would not sustain agriculture.