Isarithmic Mapping: Annual Rainfall in Washington

 Isarithmic Mapping

Today we are going to talk about Isarithmic Mapping!

This type of mapping is a way to explain smooth or continuous phenomenon such as a heat map, weather, temperature or rainfall. 

There are two types of these kind of maps, Isopleth and Isometric...

Isopleth Maps: These are made from perceptual poi nt data such as the population of an area or the rate of crime in an area. These maps should be normalized if using raw data and should come from averages, densities, and ways to standardize the data. Such as the number of Covid cases per every 10,000 people, rather than coloring polygons of counties based on the number of covid cases. 

Isometric Maps: These come from true point data, such as crime locations as points at a certain address or X and Y coordinate

The two rainfall maps of Washington below are Isopleth maps as they are using perceptual data of areas regarding their average rainfall in the state of Washington. This data is already standardized due to calculating the averages or rain for the last 30 years. 

Hypsometric Tint: (Shown in the map below)

The map above shows the average annual precipitation for a 30 year period in the state of Washington. The elevation contours are also displayed on the map to provide context of the elevation and how it relates to the annual rainfall averages. 

By using the data provided by the USDA Geospatial Gateway, we can use hill shade and a continuous Tone of rainfall to establish a classified annual precipitation for the state. 

Instead of laying the rainfall as a continuous layer, we can achieve greater accuracy by classifying the data into 10 classes, and defining our own custom intervals to better color code the different areas of rainfall. As you can see above, the more steep areas in elevation have higher averages of annual precipitation than the flat areas located on the southeast portion of the map. 

This style of mapping used the PRISM model (Parameter-elevation Regressions On Independent Slopes Model) which uses an elevation grid (DEM - digital elevation model) and point data to display predictions of annual rainfall. The Int Spatial Analyst Tool was used to calculate the contours clearly in classified intervals to allow  the tone of elevation to be displayed. 

What is hypsometric tinting?

This method of coloring different elevation values to enhance relief cues or make it easier to see the depiction of relief areas. In clearer terms it means that an analyst will display colors in between contour lines to represent different elevation between areas. This allows the map reader to make out the contour differences and it can be an accurate representation of the elevation.

Refer to the map above…

This is an example of hypsometric tinting in which we calculated contours which the Int Spatial analyst tool. The data is classified as discrete and not continuous. This identifies areas of rainfall with natural breaks using 10 data intervals. We can clearly see the difference in the two maps above and below, as the one below has more color, defined areas of rainfall, contours, are more visible, and we do not loose visual detail on the map. 

The advantage of this method is that each cell in the dem is given a value so that its color is represented clearly, and we do not lose the visual detail on the map.
The data is accurately portrayed as it layers over the different elevation areas with contour lines. This gives the analyst full control on the colors used, data intervals, elevation ranges for the contours, and it shows the differences between features.
A major disadvantage of this method is that the analyst will have to go in and classify the data, create intervals for each data classification, assign colors, and the map can look pixely or busy to a map reader. 

ESRI Example Map: (link is at the bottom)

An example to best use the hypsometric tint would be if I was making a map over an area for navigation purposes and I needed clear distinction in contours to show differences in elevations. Maybe a hiking trail map would be best for this situation, so that hikers at a state park know how steep a trail is going up and down a mountainside.  We would want clear distinctions, with a lot of intervals and colors so that the hikers can clearly understand the slope differences in elevation, and we can control how many to use very easily. Or in the example above with the lake and land features, we would want to differentiate between elevation near the land and water features. 

a)     Continuous Tone: (Shown in the map below)

What is continuous tone symbology?

This method can be applied to a dem or raster for a smooth appearance. This is basically a color ramp for the data and allows a smooth display from the lowest to highest values. It does not differentiate all the data in different tones, but rather displays the map data as a single tone proportional to the value of the area or surface at that point. 

This allows the data to be represented as a fluid, smooth, continuous surface. As you can see in the image above, the map is a fluid surface with red to blue indicating annual rainfall over a 30 year period.

Example From NOAA: Heat Map
https://www.climate.gov/news-features/event-tracker/heat-wave-broils-us-southeast-over-memorial-day-weekend-2019

The continuous tone approach would best be used if I were making a map containing the United States and I wanted to show a dataset representing temperature such as a snowstorm coming into the SE from the NW. This is because the color ramp would be a fluid white to blueish purple and can easily stretch across the states. The symbology would be easily readable to map viewers watching the weather channel and it would lay across the landscape to show the density of snow. 

Advantage of this color tone method is that the visual of the thematic data on the map is smooth, so it is not choppy and there is no stepped appearance to the tinting. A major disadvantage to this method is that when you assign the color ramp, you have less control over the colors used to classify the data. We can also loose detail on the map due to the tones not classifying as many intervals as you would with the discrete method, therefore loosing visual appearances. However, sometimes the continuous method is great for a fluid like look to a map. 

Follow these links below for more information:

http://wiki.gis.com/wiki/index.php/Isarithmic_map

https://www.esri.com/arcgis-blog/products/product/imagery/hypsometric-tinting/