How to measure volatility between SPY and DIA

How would you suggest to measure the volatility of SPY and DIA and compare changes in the two?

 

you could
1)look at realized historical volatility over the same time frame

2)look at ATM implied vols

3) GARCH or stochastic vol studies

i am sure there's more

 

Average True Range

Consider looking at the average true range (pick your time frames, I use 42 days 252 days and 1008 days)
as a percent of previous day's close and establish a ratio of the two.

Nutsneal

 

DIA

 

SPY