Getting GPS data from an image's EXIF in C#

Question

I am developing a system that allows for an image to be uploaded to a server using ASP.NET C#. I am processing the image and all is working great. I have managed to find a method that reads the Date Created EXIF data and am parsing it as a DateTime. That works great too.

I am now trying to read GPS data from the EXIF. I am wanting to capture the Latitude and Longitude figures.

I am using this list as a reference to the EXIF data (using the numbers for the property items) http://www.exiv2.org/tags.html

Here is the method to capture the date created (which works).

public DateTime GetDateTaken(Image targetImg) {     DateTime dtaken;      try     {         //Property Item 306 corresponds to the Date Taken         PropertyItem propItem = targetImg.GetPropertyItem(0x0132);          //Convert date taken metadata to a DateTime object         string sdate = Encoding.UTF8.GetString(propItem.Value).Trim();         string secondhalf = sdate.Substring(sdate.IndexOf(" "), (sdate.Length - sdate.IndexOf(" ")));         string firsthalf = sdate.Substring(0, 10);         firsthalf = firsthalf.Replace(":", "-");         sdate = firsthalf + secondhalf;         dtaken = DateTime.Parse(sdate);     }     catch     {         dtaken = DateTime.Parse("1956-01-01 00:00:00.000");     }     return dtaken; } 

Below is my attempt at doing the same for GPS..

public float GetLatitude(Image targetImg) {     float dtaken;      try     {         //Property Item 0x0002 corresponds to the Date Taken         PropertyItem propItem = targetImg.GetPropertyItem(2);          //Convert date taken metadata to a DateTime object         string sdate = Encoding.UTF8.GetString(propItem.Value).Trim();         dtaken = float.Parse(sdate);     }     catch     {         dtaken = 0;     }     return dtaken; } 

The value that's coming out and into sdate is "5\0\0\0\0\0\0l\t\0\0d\0\0\0\0\0\0\0\0\0\0"

And that is coming from an image that was taken by an iPhone 4 which does carry the GPS EXIF data.

I know there are classes out there that do this but would prefer to write my own - I am open to suggestions though :-)

Thanks in advance.

Answer 1

According to the link posted above by tomfanning, property item 0x0002 is the latitude expressed as a PropertyTagTypeRational. The rational type is defined as...

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.

You are trying to parse it as a string when it's actually just a series of bytes. According to the above, there should be 3 pairs of 32-bit unsigned integers packed into that byte array, which you can retrieve using the following:

uint degreesNumerator   = BitConverter.ToUInt32(propItem.Value, 0); uint degreesDenominator = BitConverter.ToUInt32(propItem.Value, 4); uint minutesNumerator   = BitConverter.ToUInt32(propItem.Value, 8); uint minutesDenominator = BitConverter.ToUInt32(propItem.Value, 12); uint secondsNumerator   = BitConverter.ToUInt32(propItem.Value, 16); uint secondsDenominator = BitConverter.ToUInt32(propItem.Value, 20); 

What you do with these values after you've got them is for you to work out :) Here's what the docs say:

Latitude is expressed as three rational values giving the degrees, minutes, and seconds respectively. When degrees, minutes, and seconds are expressed, the format is dd/1, mm/1, ss/1. When degrees and minutes are used and, for example, fractions of minutes are given up to two decimal places, the format is dd/1, mmmm/100, 0/1.