merging multiple data frames based on X and Y coordinates

Hi all,
I'm a little stumped on this.
I have 5 data frames with continuous GPS coordinates at 5 to 10 m  
spacing. Due to the different GPS units used, data collected within  
one to two sec apart have coordinates that differ by 3 to 5 m on  
average. I'd like to aggrate the data frames relative to the one with  
the most continuous data stream, but I have no ideas how to specify a  
distance tolerance, (say 5 m ?) when using "merge"
Does anyone know of a suitable function that might accomplish this?

Thanks in advance...