To null terminate the 2 new arrays do I have to do anything? or will it be fine since I made the arrays 1 larger than they need to be so the last spot should be a null right?
Suppose the string initially contains "...,45.00034,60.9998,..." (where lat is 45.00034 and lon is 60.9932). You will copy 45.00034 into the lat array, and 60.9998 into the lon array.
You'll call atof, and the conversion to floats will work fine.
Now, the gps moves, so the string contains "...,45.0004, 61.0,...". You will copy 45.0004 into the lat array, which will then contain 45.00044. You will then copy 61.0 into the lon array, which will now contain 61.0998.
Your call as to whether you think it is necessary to NULL terminate the arrays.