st: calculating longitudinal-latitudinal distances

[Scott Cunningham <scunning@gmail.com>]

I am currently working on a project wherein I attempt to determine  
whether abortion restrictions (specifically, state-level laws  
requiring parental involvement for minors seeking an abortion and  
mandatory delays for all women seeking abortions) affects sexual  
debut and contemporaneous sexual frequency (as well as contraception  
decisions).  To do this project correctly, I have to control for the  
distance between a woman in my sample affected by the law (ie, a  
minor in a state with an parental involvement law, and all women for  
the mandatory delay law) woman's proximity to an out-of-state  
abortion provider (ie, in a state without the law).  The idea is that  
the law is not binding if a woman can travel out-of-state to elect an  
abortion.

I know the formula for calculating distances using latitude/ 
longitude, but am having trouble thinking of the most efficient way  
to do this in Stata.  My problem requires that I find not the  
distance between every county and every abortion provider, but  
rather, every county where I have a respondent living and assuming  
they live in a state with a binding abortion restriction law, the  
distance from their county and the nearest abortion provider without  
the law.  If they don't live in such a state, then I need only to  
indicate the distance from their county of residence and the nearest  
abortion provider at all.

So, my question is not about the mathematical formula for calculating  
geographic distance, but the most efficient way to do this in Stata.   
Does anyone have any insights to help me think through this?


scott
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