Unfortunately this does not solve exactly the same problem because you are only checking to see if $lat[$i] and $lat[$i+1] are close. The OP wants to find all $i and $j where $lat[$i] is close to $lat[$j].

You can get closer to what the OP wanted if you first sorted the points based on $lat[...]. However, you still would want to check if $lat[$i+2], $lat[$i+3], etc. were close to $lat[$i].