"At any latitude and zoom level, you can determine the scale by using the following equation"
Map resolution = 156543.04 meters/pixel * cos(latitude) / (2 ^ zoomlevel)
is a quote from (http://msdn.microsoft.com/en-us/library/aa940990.aspx)
Can anyone explain to me the relevance of the cos(latitude) part? I am having difficulties visualising this.