Dasymetric Map

          There are many different ways of mapping data.  One way of particular interest to me is the USDA Plant Hardiness Zone Map.  If you've ever been around gardeners talking gardening from different states one of the first things brought up in discussion is "what zone are you?"  They talk of their favorite plants specimen and like to know if the plants are hardy in the other gardeners zone.  This USDA Plant Hardiness Zone Map is a dasymetric map.  This is a map where the zones are chosen because what is occurring within these zones are similar.  This is different than using existing boundaries like in choropleth mapping.   
          In this map zones refer to the average minimum temperature over a ten year period.  Their are 11 zones separated by a ten degrees Fahrenheit.  The first map put out was in 1960 then 1965 and most currently a 1990 version which breaks the zones into two five degree subcategories.  The USDA has set the standard for this map globally.  There are problems with this map such as they don't take into account summer heat and snow cover which acts as insulation to hibernating roots allowing some plants to survive in colder zones.  Sunset Garden web site attempts to take into account many other environmental factors such as these.  In our area we are generally zones 6a or 6b.  The United States National Arboretum has "clickable" map to see different zones around the United States.