For the scratch variables above that are objects, re-using them like you're doing is good.  However, for these variables, there's no allocations being avoided, so the optimization actually should be the other way: keep these as normal local variables inside the constructor and pass the values through into the helper functions that need them.  Accessing a closed-over shared variable will end up being slower than accessing a local (or parameter).  In the normal scratch variable case, that access overhead is worth it to save the re-allocation cost.

I do note that stLatitude and stLongitude are like out parameters, so those two should stay.  Assigning into the closure scope is probably the fastest way to return multiple values from a function to be used later.