Overwrite GPS coordinates in Image Exif using Python 3.6

Question

I am trying to transform image geotags so that images and ground control points lie in the same coordinate system inside my software (Pix4D mapper).

The answer here says:

Exif data is standardized, and GPS data must be encoded using geographical coordinates (minutes, seconds, etc) described above instead of a fraction. Unless it's encoded in that format in the exif tag, it won't stick.

Here is my code:

import os, piexif, pyproj
from PIL import Image

img = Image.open(os.path.join(dirPath,fn))
exif_dict = piexif.load(img.info['exif'])

breite = exif_dict['GPS'][piexif.GPSIFD.GPSLatitude]
lange = exif_dict['GPS'][piexif.GPSIFD.GPSLongitude]

breite = breite[0][0] / breite[0][1] + breite[1][0] / (breite[1][1] * 60) + breite[2][0] / (breite[2][1] * 3600)
lange = lange[0][0] / lange[0][1] + lange[1][0] / (lange[1][1] * 60) + lange[2][0] / (lange[2][1] * 3600)
print(breite) #48.81368778730952
print(lange) #9.954511162420633
x, y = pyproj.transform(wgs84, gk3, lange, breite) #from WGS84 to GaussKrüger zone 3 
print(x) #3570178.732528623
print(y) #5408908.20172699
exif_dict['GPS'][piexif.GPSIFD.GPSLatitude] = [ ( (int)(round(y,6) * 1000000), 1000000 ), (0, 1), (0, 1) ]

exif_bytes = piexif.dump(exif_dict) #error here
img.save(os.path.join(outPath,fn), "jpeg", exif=exif_bytes)

I am getting struct.error: argument out of range in the dump method. The original GPSInfo tag looks like: {0: b'\x02\x03\x00\x00', 1: 'N', 2: ((48, 1), (48, 1), (3449322402, 70000000)), 3: 'E', 4: ((9, 1), (57, 1), (1136812930, 70000000)), 5: b'\x00', 6: (3659, 10)}

I am guessing I have to offset the values and encode them properly before writing, but have no idea what is to be done.

Answer 1

It looks like you are already using PIL and Python 3.x, not sure if you want to continue using piexif but either way, you may find it easier to convert the degrees, minutes, and seconds into decimal first. It looks like you are trying to do that already but putting it in a separate function may be clearer and account for direction reference.

Here's an example:

def get_decimal_from_dms(dms, ref):

    degrees = dms[0][0] / dms[0][1]
    minutes = dms[1][0] / dms[1][1] / 60.0
    seconds = dms[2][0] / dms[2][1] / 3600.0

    if ref in ['S', 'W']:
        degrees = -degrees
        minutes = -minutes
        seconds = -seconds

    return round(degrees + minutes + seconds, 5)

def get_coordinates(geotags):
    lat = get_decimal_from_dms(geotags['GPSLatitude'], geotags['GPSLatitudeRef'])

    lon = get_decimal_from_dms(geotags['GPSLongitude'], geotags['GPSLongitudeRef'])

    return (lat,lon)

The geotags in this example is a dictionary with the GPSTAGS as keys instead of the numeric codes for readability. You can find more detail and the complete example from this blog post: Getting Started with Geocoding Exif Image Metadata in Python 3

Answer 2

After much hemming & hawing I reached the pages of py3exiv2 image metadata manipulation library. One will find exhaustive lists of the metadata tags as one reads through but here is the list of EXIF tags just to save few clicks.

It runs smoothly on Linux and provides many opportunities to edit image-headers. The documentation is also quite clear. I recommend this as a solution and am interested to know if it solves everyone else's problems as well.