Module 8 - Isarithmic Mapping

In this week's lab we were learning about isarithmic mapping. It was one I was particularly looking forward to, as I am a meteorologist, and we use these a LOT. We learned about the PRISM interpolation method (I had heard of their datasets before but not had the opportunity to use them until now), different types of symbology for isarithmic mapping, and worked with continuous raster data. We also learned to employ hillshade relief and create contours.

What is PRISM? PRISM stands for Parameter-elevation Relationships on Independent Slopes Model. It's an interpolation method that incorporates elevation into the surface using a digital elevation model (DEM). It calculates a climate-elevation regression for each grid cell, where monitoring locations are assigned "weights" depending on the physiographic similarity of the station to the grid cell. These weights are determined by several geographic and meteorological factors. PRISM then develops spatial climate datasets to show short- and long-term climate patterns. This method is especially useful in the mountainous and coastal western United States, where there is often sparse data coverage, rain shadows, and/or sharp elevation gradients. For more information on PRISM and to access datasets, visit www.prism.oregonstate.edu.

The two types of symbology we learned were continuous tone and hypsometric tinting. Continuous tone symbology describes when colors and shades of gray smoothly merge into neighboring colors or shades, as opposed to sharply defined boundaries. Hypsometric tinting describes a technique where colors or shading are used to depict ranges of elevation, usually with contours. It allows the viewer to associate light and dark hues with low and high values, respectively.

Below are my two maps of average annual precipitation of Washington from 1981-2010. These were both created entirely with ArcMap. The first one created is using continuous tone symbology. I added the data from the PRISM dataset to ArcMap and made sure the data was already in the stretched color symbolization, and selected the precipitation color ramp. I modified the labels so that they show three values: the bottom number being the smallest value and the top number the highest. I also made sure to use "hillshade effect", as the PRISM raster data incorporates elevation into its surface. I then owned the map by adding the essential map elements: legend, north arrow, scale bar, etc., and I added some text about the PRISM interpolation method.

For the second map, I used the same template as for the continuous tone map. This was the hypsometric tinting map, and for this one I used contours. This was interesting, as I needed to use the spatial analyst tool to convert the raster values from floating to integers. This allowed for nice contours because cell values were truncated to real numbers. I also gained more experience with manually classifying the data. I used 10 classes and set the range as described in the lab. I again made sure hillshade effect was checked on, and used the precipitation color ramp. I created contours for both maps, but decided to use contours on the hypsometric tinting map only. I like the fact that creating contours using the spatial analyst tool is relatively user-friendly.

I thought these maps both look very sharp and do a good job telling the viewer at a glance the important information. I personally like the hypsometric map a little better due to the ease of reading the legend. I can look at a glance at a location on the map and see the numerical range of annual rainfall that color falls in. It's more difficult to do that with the continuous tone map, although both do an excellent job of showing exactly where the rainfall is. It's easy to see the smaller amounts of rainfall east of the mountains and the greater amounts to the west. I really enjoyed making these maps and am looking forward to next week.