Geotag – EXIF GPS Latitude field format with libEXIF

I have been developing some software to geotag jpeg images by adding EXIF GPS information using libEXIF. This is very handy as loads of applications like GIS systems and google maps etc can correctly geographically position your images.

As usual I started in the middle rather than starting at the beginning and got a bit confused by how GPS latitude and longitude fields are specified in EXIF, so I decided to try to describe it here with pictures (so that I can test out the google drawing app).

So, latitude (and longitude) can be expressed in different ways bit it is essentially just an angle. Common ways of expressing these angles are:

Degrees, minutes & seconds (with decimal places)
N 52 58 40.44

Degrees & minutes (with decimal places)
N 52 58.674

Degrees (with decimal places)
52.97790

The EXIF latitude field allows you to specify the angle in all of these forms, it is made up of 3 parts as follows:

1.) Degrees – Rational (8 bytes)
2.) Minutes – Rational (8 bytes)
3.) Seconds – Rational (8 bytes)

Each part is an EXIF Rational, it is hard to find a description of its format, but a an EXIF rational contains two 4-byte words and is like a fraction. The first word specifies the value’s magnitude while the second denominates the units. Consider the following values, (where ‘/’ should be read as ‘over’ or ‘divided by’):

a.) 52 = 52 / 1 (52 units)
b.) 40.44 = 4044 / 100 (4044 hundredths)
c.) 52.97790 = 52977900 / 1000000 (52977900 millionths)
d.) 0 = 0/1

the last value ( 0/1 ) is handy as it allows us to specify, say, 0 seconds if we only want to provide degrees and fractional minutes.

To set a rational we can use libEXIF’s set_rational() function like this:

//
// 40.44 = 4044 / 100 (4044 hundredths)
exif_set_rational(entry->data, EXIF_BYTE_ORDER_INTEL, { 4044, 100 });
//

or more generally, if, for example, you want to set a value to 6 decimal places:

//
float lat = 52.977900;
exif_set_rational(entry->data, FILE_BYTE_ORDER, { (unsigned)(lat * 1000000.0), 1000000 });
//

So now, given a latitude value in degrees, minutes and seconds all we have to do is create a EXIF_TAG_GPS_LATITUDE tag and add a rational for each. Imagine that we want to encode 52, 58, 44.44 then the tag data will then end up looking like this:

This is all very well but I don’t normally bother holding minutes and seconds in my code, instead I preferr to use a degree value to many decimal places, e.g. 52.97790, no problem, this is where our rational value 0 / 1 comes in handy – it can be represented as follows:

So wrapping all of this up, here is some example code that sets a decimal degree value for latitude:

/*
 * create_tag() is from the write-exif.c sample code that is floating
 * around the interweb - with thanks to whoever created it!
 */
/* Create a brand-new tag with a data field of the given length, in the
 * given IFD. This is needed when exif_entry_initialize() isn't able to create
 * this type of tag itself, or the default data length it creates isn't the
 * correct length.
 */
static ExifEntry *create_tag(ExifData *exif, ExifIfd ifd, ExifTag tag, size_t len)
{
	void *buf;
	ExifEntry *entry;
	/* Create a memory allocator to manage this ExifEntry */
	ExifMem *mem = exif_mem_new_default();
	/* Create a new ExifEntry using our allocator */
	entry = exif_entry_new_mem (mem);
	/* Allocate memory to use for holding the tag data */
	buf = exif_mem_alloc(mem, len);
	/* Fill in the entry */
	entry->data = buf;
	entry->size = len;
	entry->tag = tag;
	entry->components = len;
	entry->format = EXIF_FORMAT_UNDEFINED;
	/* Attach the ExifEntry to an IFD */
	exif_content_add_entry (exif->ifd[ifd], entry);
	/* The ExifMem and ExifEntry are now owned elsewhere */
	exif_mem_unref(mem);
	exif_entry_unref(entry);
	return entry;
}
// Set a decimal degree value with support for 6 decimal places
//
//
  // create our latitude tag, the whole  field is 24 bytes long
  entry = create_tag(exif, EXIF_IFD_GPS, EXIF_TAG_GPS_LATITUDE, 24);
  // Set the field's format and number of components, this is very important!
  entry->format = EXIF_FORMAT_RATIONAL;
  entry->components = 3;
  // Degrees
  float lat = 52.977900;
  exif_set_rational(entry->data, EXIF_BYTE_ORDER_INTEL, { (unsigned)(lat * 1000000.0), 1000000 });
//
//
    

I will probably do another post that details how to write EXIF data into a jpeg image’s header using libEXIF and libJPEG

The google drawing app actually worked quite well!