How do I convert longitude and latitude to GPS Exif byte array - vb.net

Question

I am trying to put latitude = 8°50'34.46" and longitude = 125° 9'50.82" into the exif file of an image. i'm using vb.net.
I'm not having problem converting the degrees and minutes into bytes because it is a whole number but when i convert the seconds (34.46") which has decimal values into bytes. it gives different result like 0.9856.
Please help me guys how to convert numbers with decimal values into bytes.
here the code:
Private Shared Function intToByteArray(ByVal int As Int32) As Byte()
' a necessary wrapper because of the cast to Int32
Return BitConverter.GetBytes(int)
End Function
Private Shared Function doubleToByteArray(ByVal dbl As Double) As Byte()
Return BitConverter.GetBytes(Convert.ToDecimal(dbl))
End Function
Private Shared Function doubleCoordinateToRationalByteArray(ByVal doubleVal As Double) As Byte()
Dim temp As Double
temp = Math.Abs(doubleVal)
Dim degrees = Math.Truncate(temp)
temp = (temp - degrees) * 60
Dim minutes = Math.Truncate(temp)
temp = (temp - minutes) * 60
Dim seconds = temp
Dim result(24) As Byte
Array.Copy(intToByteArray(degrees), 0, result, 0, 4)
Array.Copy(intToByteArray(1), 0, result, 4, 4)
Array.Copy(intToByteArray(minutes), 0, result, 8, 4)
Array.Copy(intToByteArray(1), 0, result, 12, 4)
Array.Copy(doubleToByteArray(seconds), 0, result, 16, 4)
Array.Copy(intToByteArray(1), 0, result, 20, 4)
Return result
End Function

Answer 1

According to this specification, longitude and latitude are encoded as a
PropertyTagTypeRational
Specifies that the value data member is an array of pairs of unsigned long integers. Each pair represents a fraction; the first integer is the numerator and the second integer is the denominator.
The encoded layout should be (24 bytes total)
Byte Offset Length Encoding Field
0 4 uint Degrees Nominator
4 4 uint Degrees Denominator
8 4 uint Minutes Nominator
12 4 uint Minutes Denominator
16 4 uint Seconds Nominator
20 4 uint Seconds Denominator
Given that your input is using whole degrees and minutes and not fractions, your encoding for those two will work fine, by using the value of 1 as the denominator.
For the seconds, that you have as a floating point value, this is not the case. You will have to encode it as a rational, using a nominator and denominator part.
I am not sure what the precision is that you would like to have, but given your example of 34.46 seconds, it would seem that multiplying by 1000 and using 1000 for the denominator would be more than good enough:
Dim secondsNominator = Math.Truncate(1000 * seconds)
Dim secondsDenoninator = 1000
Then your encoding function becomes:
Private Shared Function doubleCoordinateToRationalByteArray(ByVal doubleVal As Double) As Byte()
Dim temp As Double
temp = Math.Abs(doubleVal)
Dim degrees = Math.Truncate(temp)
temp = (temp - degrees) * 60
Dim minutes = Math.Truncate(temp)
temp = (temp - minutes) * 60
Dim secondsNominator = Math.Truncate(1000 * temp)
Dim secondsDenoninator = 1000
Dim result(24) As Byte
' Degrees (nominator, and 1 for denominator)
Array.Copy(intToByteArray(degrees), 0, result, 0, 4)
Array.Copy(intToByteArray(1), 0, result, 4, 4)
' Minutes (nominator, and 1 for denominator)
Array.Copy(intToByteArray(minutes), 0, result, 8, 4)
Array.Copy(intToByteArray(1), 0, result, 12, 4)
' Seconds (1000 for denominator: ms resolution)
Array.Copy(intToByteArray(secondsNominator), 0, result, 16, 4)
Array.Copy(intToByteArray(secondsDenominator), 0, result, 20, 4)
Return result
End Function

Answer 2

The GPS latitude and longitude for exif data are "rational" data type, or two 32-bit integers. To represent 34.46, for example, you could use the two 32-bit integers 3,446 (numerator) and 100 (denominator), or 344,600 and 10,000. For the integer value of degrees, for example you could use 8 with a denominator of 1.
You can get the exif specification here.