[GRASS-user] distances between points in two different UTM zones

[Stephen Sefick]

All:

I apologize if this is not the correct place to post this question and
will gladly redirect this to the appropriate place if prompted.

I have sample sites located in UTM/NAD83 zones 16 and 17 in the United
States.  As I understand UTM I can't use the raw coordinates to
calculate distances from pairs of coordinates among zones.  What is the
most accurate way of calculating distance between these two points to
investigate spatial auto-correlation.  What coordinate system,
projection, etc. should I use?  What distance formula(s) should I use?
Thanks for all of your help in advance.  

Stephen Sefick