Cartography: Module 8 - Isarithmic Mapping

     Isarithmic maps map continuous phenomena, like precipitation. This week the assignment was to produce such a map. The aim was to map precipitation data using two different manners of symbolization, continuous tone and hypsometric tints. Contour lines were then added to a map of our choice. The exercises were carried out entirely in ArcMap. Symbolizing raster data was necessarily introduced as well as the Spatial Analyst tool, Int. My two maps are found below. The first map is the continuous tone map and the second map is the hypsometric tints map. Each map displays a short description of the interpolation method used -- PRISM. As a brief overview, PRISM (Parameter-elevation Relationships on Independent Slopes Model) is an interpolation method that incorporates elevation, other physiographic data, and proximity information when deriving a value for a given pixel (using a climate-elevation regression function). It was a method developed by Chris Daly in 1991 while a Ph.D. student at Oregon State University and serves as a way to mimic the choices climatologists made when deriving maps before digitization.  
     The first map uses continuous tones and contour lines to depict average annual rainfall over several decades for the state of Washington. This symbolization is applied by manipulating the settings in the symbology tab of the layer properties. The standard precipitation color ramp is used. Contour lines were added with the Contour List tool accessed through the Spatial Anlayst toolbox. Contours are set at predetermined values.

An isarithmic map displaying precipitation data symbolized in continuous tones.
A short description of PRISM interpolation is also included. 

      The second map uses hypsometric tinting without contour lines to depict the same precipitation information. This symbolization required an extra processing step before tinting could be applied. I used the Int (Spatial Analyst Tool) to convert the raster values from fractional to whole numbers. This helps to create distinct contours using the new whole number values. Again, from the symbology tab (layer properties) I applied a classification to assign colors to ranges of values. I used a manual classification scheme with 10 classes and the precipitation color ramp. Hillshade effects were used in both maps.

An isarithmic map displaying precipitation data symbolized in hypsometric tints.
A short description of PRISM interpolation is also included.