What is the difference between Exif GPS DestLatitude vs Latitude

Question

In the EXIF metadata GPS schema there are two places to store GPS data:

#1-4 Latitude, LatitudeRef, Longitude and LongitudeRef

#19-23 DestLatitude, DestLatitudeRef, DestLongitude and DestLongitudeRef

In theory the first is where the photo was taken, so an iPhone will populate this data. The second are the coordinates of the object in the photo. So if you are on Westminster Bridge and taking a photo of the London Eye, you'd have two slightly different values.

Does anyone know if there's accepted usage of these properties?

Specifically, should Latitude only be set if you have GPS data from either the camera or an external logger, so this could be considered optional? But the DestLatitude would be always set on any, well organized photo collection?

Answer 1

The second is referenced as "destination point".
You can see those in W3 exif, prefixed by gps, which suggests it makes only sense when you record a position relative to a fixed destination point.

You can see that field used in tomchentw/react-google-maps for example:

withScriptjs, withGoogleMap,
    lifecycle({
        componentDidMount() {
            this.setState({
                onDirectionChange: () => {
                    const DirectionsService = new google.maps.DirectionsService();
                    DirectionsService.route({
                        origin: new google.maps.LatLng(this.props.latitude, this.props.longitude),
                        destination: new google.maps.LatLng(this.props.destLatitude, this.props.destLongitude),
                        travelMode: google.maps.TravelMode.DRIVING,
                    }, (result, status) => {
                        if (status === google.maps.DirectionsStatus.OK) {
                            this.setState({
                                directions: result
                        });
                    } else {
                        console.error(`error fetching directions ${result}`);
                    }
                    });
                }
            });
            const DirectionsService = new google.maps.DirectionsService();

In that case, this is to call Google Map in order to launch an itinerary computation.