Shading on the MRCC freeze maps in VIP
The freeze maps in VIP are shaded using a different method than the shading methods used on many maps. Typical shading methods use some type of interpolation to get values between known points. For many situations, this is a valid way to interpolate. In the case of freeze dates, the episodic nature of the events can lead to situations where nearby stations have dates that vary considerably, and the intervening dates may not be valid dates for the field being mapped.
Therefore, the shading method used in VIP is called Thiessen polygons. Rather than interpolate between stations, the shading is determined by mapping to the nearest station with a known value. In this scheme, it is possible to have shading jump from one value to another without the intervening shading appearing on the map. This is the desired behavior so that values that did not occur will not appear on the map.
As an example, two neighboring stations (A and B) have a first fall freeze date that differs by several weeks. An early freeze occurs at A on 9/28 while B remains above freezing. A period of warmer weather keeps the area above freezing until 11/3 when the entire area experiences a freeze. In this example, the first freeze date for A is 9/28 and for B is 11/3. Shading methods that interpolate would give values throughout October to the area between the stations despite the temperatures remaining above freezing throughout the month. Using Thiessen polygons, the areas of the map closest to A would be shaded as 9/28 and the areas closest to B would be shaded as 11/3 switching at the midpoint between the two stations. This method does a better job of representing what happened in the example by not introducing October freeze dates where no October freezes occurred.
Click here for more on the ArcGIS geoprocessing tool that creates these polygons.