Re: What is the standard for rain gauges based on?

In the United States, the National Weather Service developed the standard 
for "official" measurement of weather data.  When an inch of rain is 
measured, that inch covers the entire surface area of the measuring device 
(IE. rain gauge).  If your rain gauge were a cubic inch in volume (1 inch 
on all sides and an inch tall) and an inch of rain fell, then your 
supposition about a cubic inch of rain would be correct.  However, the 
actual amount of cubic inches received would be greater than a cubic inch 
since a "standard" rain guage is 8 inches in diameter.  The broader idea 
you expressed in your question is correct though.  When we say an inch of 
rain fell, that inch of rain covered the surface area represented by the 
open end of an 8 inch rain guage.