Use a binary editor to look at your file data - it will show you the "raw byte values", probably in hexadecimal.
If it is numeric text, each hex byte will be in the range 30 - 39, which correspond to the characters '0' to '9' respectively (the '.' in your string would be a byte containing the hex value 2E)
If it isn't in that range, it's not a number as a string, and given that it's a floating point or double precision number you are expecting, it'll be difficult to "decode" by hand:
IEEE 754 - Wikipedia[
^] - but very definitely, 2 characters would not be enough to rad the whole number anyway!
Once you've sorted out exactly what you have to work with, have a look at the code you are using to write the data to the file, and it should be obvious that you need to change one or the other, and what you need to do to it.
Sorry, but we can't do any of that for you!