M6: Isarithmic mapping

The focus of Cartography Module 6 was Isarithmic mapping. These are thematic maps where colors are used to express continuous values across an area, such as elevation or temperature. Creating isarithmic maps is a task I have done frequently when planning marine operations or displaying data gained from a sonar or magnetometer scan. 

This lab utilized data downloaded from US Department of Agriculture which displayed annual average precipitation for a 30 year span in Washington state. It makes sense to express this as an isarithmic map, however there are two primary options when determining symbology for the color gradient in an isarithmic map. 

Hypsometric Tinting (with classified values)

Hypsometric tinting with classified values assigns a specific color to individual values or a range of values in a DEM and results in hard or "stepped" delineations over an area. This is especially beneficial in situations where determination of individual values is desired, or value at a discrete point should be able to be estimated. This type of display is typically partnered with contour lines to delineate between different values. The most common type of shading on an isarithmic map, this symbology is easier to display if there is not an extremely wide range of values. For this lab switching between Continuous to classified Hypsometric was just a matter of changing the data classification and then structuring the classes and labels to reasonable parameters. 

For this lab the final product included a map showing the annual averages of rainfall in Washington State over the course of 30 years. Hypsometric tinting and contour lines create clear delineations between ranges of rainfall values, and the PRISM interpolation is explained within the map.

Continuous Tone

Continuous tone symbology displays a smooth color gradient between values in a DEM. While it may make it difficult to identify a discreet value within a gradient, this method works well for datasets with a wide range of values and in situations where the overall picture is the takeaway over any single value. Creating continuous tone symbology for this lab was fairly straightforward, with a hillshade calculation added to what was otherwise simply selecting and formatting the appropriate color ramp. 

This example of continuous tone symbology uses the same data shown above, but the smooth color gradient between values gives a broader view of rainfall patterns over the entire region. With continuous symbology the range of the total dataset is emphasized more than values within any one area. 

Interpolation and PRISM

Isarithmic maps must show continuous data, so often interpolation must be used to model the data between discrete observed points. In other courses I have discussed how DEMs may be used to interpolate elevation over an area like this. For climate and precipitation data in the United States the Parameter-elevation Regression on Independent Slopes Model (PRISM) is a widely accepted method of conducting interpolation. Originally developed by University of Oregon and then refined and expanded in collaboration with USDA Natural Resources Conservation Service, PRISM uses factors like face orientation, elevation, atmospheric layers, position, and proximity to mountains or coast to create a slope regression in each square of the DEM. For the data in this lab PRISM was given point data from several monitoring stations over the course of 30 years, and produced the continuous interpolated data housed on the USDA NRCS data portal.