converting byte to doube

Hi,
I have a byte in image.GetPropertyItem(2).Value in the format of a gps location (Propertyitem 2 is the latitude, 51.123340). I like to convert it to a double with the same decimals. When I use
System.BitConverter.ToDouble(image.GetPropertyItem(2).Value, 0)

 I get a weird number. When I use

System.BitConverter.ToInt32(image.GetPropertyItem(2).Value, 0).tostring

and convert it later to double then I get 51.0 instead of 51.123340. The latest method coud be ok except of the decimals.

Could someone help me? Thanks!
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