Isarithmic Mapping

Cartographic Skills - Mod 8

This week's lesson was on Isarithmic Mapping, the most widely used thematic mapping method after the Choropleth map, and also one of the oldest, dating back to the 18th century.  The most common form of an isarithmic map is a contour map.  The appropriate data type for isarithmic mapping is presumed to be continuous and smooth, to exist throughout the entire geographic region of interest and to change gradually or smoothly.  Two types of isarithmic maps are isometric, which are produced using true point data and isopleth maps produced with conceptual point data.  Isopleth maps required data to be standardized.

For our lab we created three isarithmic maps with data downloaded from the USDA Geospatial Gateway.  This data is of the average annual precipitation in Washington State for a 30 year period.  The data was prepared using the PRISM (Parameter-elevation Relationships on Independent Slopes Model) interpolation method.  This method weights the data collected from a dense monitoring network to account for physiographic factors that influence climate patterns.  We created these maps using three different symbolization approaches; Continuous tones, Hypsometric tints, and Hypsometric tints with contour lines.  The sample above is the third approach, a hypsometric tint with contour lines.

To create the continuous tone map all we really needed to do was change the symbology of the layer by changing the color ramp from Graphic View to Precipitation.  We also added the hillshade effect which added a sense of depth to the image, and adjusted the labels to present in rounded numbers instead of the raw data values.  The only difficulty came with the legend.  Because Washington is shaped horizontally it made more sense to have a horizontal legend.  But because ArcMap doesn't follow the convention of left to right reading in its defaults for the horizontal legend distribution we had to do some manual manipulation to make that come out right.  

The second map used the same data, but symbolized in Hypsometric Tints.  For this map we needed to utilize the Spatial Analyst tool Int to convert the raster vales from floating to integers.  This allowed us to create crisp contours since the cell values were truncated to whole numbers.  Once that was done we manually classified the data into 10 classes and again changed the color ramp to Precipitation and selected the Hillshade Effect.

The last map, above, was the Hypsometric Tints map with Contour Lines added.  Again we turned to the Spatial Analyst Toolbox for the tool Contour List.  With this tool we simply specified our contour values and it placed the contours on the map.  Since our contour intervals matched the hypsometric step values we did not need to notate them on this map.