Use time zone tag from photo when available

If a time zone is found in an image, use that in preference to an
automatically-determined time zone.  If a manually-specified time zone
is given, that takes precedence over all. Only the German translations
are fully updated.

Fixes #24

Author: dfandrich

README.md

@@ -42,6 +42,9 @@ version of the program.
 * The resolution of GPX positions is down to one second, but sub-second photo
   time is used (when available in the photo) to more accurately estimate the
   camera position between those points.
+* A time zone tag embedded in the photo is used to determine the local time of
+  the image when found, otherwise the time zone is assumed to be same as the
+  local machine.
 * For best results, you should synchronise your camera to the GPS time before
   you start taking photos. Note: digital cameras clocks drift quickly - even
   over a short period of time (say, a week).

RELEASES

@@ -5,6 +5,7 @@ v2.x: X
     - When interpolation is disabled, round up when exactly half way
     - Allow fractional times in --photooffset
     - Fix handling of --timeadd values between -1:00 and 0:00
+    - Read time zone from photos when available or --no-photo-tz to disable
 
 v2.2: 17 October 2024
 	- Fix metainfo nits

correlate.c

@@ -65,7 +65,7 @@ void SetAutoTimeZoneOptions(const char *Time,
 	/* PhotoTime isn't a true epoch time, but is rather out
 	 * by the local offset from UTC */
 	struct timespec PhotoTime =
-		ConvertToUnixTime(Time, EXIF_DATE_FORMAT, 0, 0);
+		ConvertToUnixTime(Time, EXIF_DATE_FORMAT, 0);
 
 	/* Extract the component time values */
 	struct tm *PhotoTm = gmtime(&PhotoTime.tv_sec);
@@ -82,11 +82,18 @@ void SetAutoTimeZoneOptions(const char *Time,
 
 /* Convert a time into Unixtime with the configured time zone conversion. */
 struct timespec ConvertTimeToUnixTime(const char *Time, const char *TimeFormat,
-		const struct CorrelateOptions* Options)
+		long OffsetTime, long *UsedOffset, const struct CorrelateOptions* Options)
 {
-	struct timespec PhotoTime =
-		ConvertToUnixTime(Time, TimeFormat,
-			Options->TimeZoneHours, Options->TimeZoneMins);
+	long TZOffset;
+	if (Options->TimeZoneFromPhoto && OffsetTime != NO_OFFSET_TIME)
+		/* Use the photo's time zone */
+		TZOffset = OffsetTime;
+	else
+		/* Use the automatic or manually-specified time zone */
+		TZOffset = Options->TimeZoneHours * 3600 + Options->TimeZoneMins * 60;
+	if (UsedOffset)
+		*UsedOffset = TZOffset;
+	struct timespec PhotoTime = ConvertToUnixTime(Time, TimeFormat, TZOffset);
 
 	/* Add the PhotoOffset time. This is to make the Photo time match
 	 * the GPS time - i.e., it is (GPS - Photo). */
@@ -112,11 +119,14 @@ struct timespec ConvertTimeToUnixTime(const char *Time, const char *TimeFormat,
  * the files - ie, just correlate and keep into memory... */
 
 struct GPSPoint* CorrelatePhoto(const char* Filename,
-		struct CorrelateOptions* Options)
+		long *UsedOffset, struct CorrelateOptions* Options)
 {
+	*UsedOffset = NO_OFFSET_TIME;
 	/* Read out the timestamp from the EXIF data. */
 	int IncludesGPS = 0;
-	char* TimeTemp = ReadExifData(Filename, &IncludesGPS, NULL, NULL, NULL);
+	long OffsetTime = 0;
+	char* TimeTemp = ReadExifData(Filename, &IncludesGPS, NULL, NULL, NULL,
+			&OffsetTime);
 	if (!TimeTemp)
 	{
 		/* Error reading the time from the file. Abort. */
@@ -141,10 +151,11 @@ struct GPSPoint* CorrelatePhoto(const char* Filename,
 		SetAutoTimeZoneOptions(TimeTemp, Options);
 		Options->AutoTimeZone = 0;
 	}
-	//printf("Using offset %02d:%02d\n", Options->TimeZoneHours, Options->TimeZoneMins);
+	//printf("Using offset %ld sec.\n", OffsetTime);
 
 	/* Now convert the time into Unixtime with the configured time zone conversion. */
-	struct timespec PhotoTime = ConvertTimeToUnixTime(TimeTemp, EXIF_DATE_FORMAT, Options);
+	struct timespec PhotoTime = ConvertTimeToUnixTime(TimeTemp, EXIF_DATE_FORMAT, OffsetTime,
+			UsedOffset, Options);
 
 	/* Free the memory for the time string - it won't otherwise
 	 * be freed for us. */

correlate.h

@@ -36,6 +36,7 @@ struct CorrelateOptions {
 	int TimeZoneHours;  /* To add to photos to make them UTC. */
 	int TimeZoneMins;
 	int AutoTimeZone;
+	int TimeZoneFromPhoto;
 	int FeatherTime;
 	int WriteHeading;
 	int HeadingOffset;
@@ -81,8 +82,8 @@ struct CorrelateOptions {
 
 
 struct GPSPoint* CorrelatePhoto(const char* Filename,
-		struct CorrelateOptions* Options);
+		long *UsedOffset, struct CorrelateOptions* Options);
 void SetAutoTimeZoneOptions(const char *TimeTemp,
 		struct CorrelateOptions* Options);
 struct timespec ConvertTimeToUnixTime(const char *TimeTemp, const char *TimeFormat,
-		const struct CorrelateOptions* Options);
+		long OffsetTime, long *UsedOffset, const struct CorrelateOptions* Options);

doc/gpscorrelate-manpage.xml.in

@@ -49,13 +49,32 @@
   <refsynopsisdiv>
     <cmdsynopsis>
       <command>&dhpackage;</command>
+      <group>
+        <arg choice="req">
+          -g <replaceable>file.gpx</replaceable>
+        </arg>
+
+        <arg choice="req">
+          <group>
+            <arg choice="plain">-l</arg>
+            <arg choice="plain">--latlong
+            <replaceable>latitude,longitude<group><arg choice="plain">,elevation</arg></group></replaceable>
+            </arg>
+          </group>
+        </arg>
+      </group>
+
       <group>
         <arg choice="plain">-z</arg>
-        <arg choice="plain">
-          --timeadd +/-<replaceable>HH</replaceable>[:<replaceable>MM</replaceable>]
+        <arg choice="plain">--timeadd
+        +/-<replaceable>HH</replaceable>[:<replaceable>MM</replaceable>]
         </arg>
       </group>
 
+      <group>
+        <arg choice="plain">--no-photo-tz</arg>
+      </group>
+
       <group>
         <arg choice="plain">-O</arg>
         <arg choice="plain">
@@ -125,38 +144,50 @@
         <arg choice="plain">--degmins</arg>
       </group>
 
-      <group>
-        <arg choice="plain">
-          -g <replaceable>file.gpx</replaceable>
-        </arg>
-
-        <arg choice="plain">
-          <group>
-            <arg choice="plain">-l</arg>
-            <arg choice="plain">--latlong</arg>
-          </group>
-          <replaceable>latitude,longitude<group><arg choice="plain">,elevation</arg></group></replaceable>
-        </arg>
-      </group>
-
       <arg rep="repeat" choice="plain">
         <replaceable>image.jpg</replaceable>
       </arg>
     </cmdsynopsis>
 
     <cmdsynopsis>
       <command>&dhpackage;</command>
-      <group choice="plain">
+      <group choice="req">
         <arg choice="plain">-s</arg>
         <arg choice="plain">--show</arg>
-        <arg choice="plain">-x</arg>
-        <arg choice="plain">--show-gpx</arg>
         <arg choice="plain">-o</arg>
         <arg choice="plain">--machine</arg>
       </group>
       <arg rep="repeat" choice="plain"><replaceable>image.jpg</replaceable></arg>
     </cmdsynopsis>
 
+
+    <cmdsynopsis>
+      <command>&dhpackage;</command>
+      <group choice="req">
+        <arg choice="plain">-x</arg>
+        <arg choice="plain">--show-gpx</arg>
+      </group>
+
+      <group>
+        <arg choice="plain">-z</arg>
+        <arg choice="plain">
+          --timeadd +/-<replaceable>HH</replaceable>[:<replaceable>MM</replaceable>]
+        </arg>
+      </group>
+
+      <group>
+        <arg choice="plain">--no-photo-tz</arg>
+      </group>
+
+      <group>
+        <arg choice="plain">-O</arg>
+        <arg choice="plain">
+          --photooffset <replaceable>seconds</replaceable>
+        </arg>
+      </group>
+      <arg rep="repeat" choice="plain"><replaceable>image.jpg</replaceable></arg>
+    </cmdsynopsis>
+
     <cmdsynopsis>
       <command>&dhpackage;</command>
       <group choice="req">
@@ -282,7 +313,8 @@
         </term>
         <listitem>
           <para>Only show the GPS data already in the given image's EXIF tags
-          instead of correlating them.</para>
+          instead of correlating them. The time shown comes directly from
+          the image without adjustments.</para>
         </listitem>
       </varlistentry>
 
@@ -312,7 +344,8 @@
         </term>
         <listitem>
           <para>Only show the GPS data of the given images in a
-          machine-readable CSV format. Images without GPS tags are
+          machine-readable CSV format. The time shown comes directly from the
+          image without adjustments.  Images without GPS tags are
           ignored. The fields output are file name, date and time, latitude,
           longitude, elevation, where the first value is the filename, as
           passed, the second is the timestamp, and the last three are floating
@@ -345,9 +378,28 @@
             in local time.  Enter the timezone used when taking the images; e.g.,
             <userinput>+8</userinput> for Perth, Western Australia or
             <userinput>-2:30</userinput> for St. John's, Newfoundland.
-            This defaults to the UTC offset of the local time zone as of the
-            time of the first image processed (versions before 1.7 defaulted to
-            00:00). </para>
+            This defaults to the time zone embedded in the image, or if that is not
+            available (or when <userinput>--no-photo-tz</userinput> is given), the
+            UTC offset of the local time zone as of the time of the first image
+            processed (versions before 1.7 defaulted to 00:00).</para>
+        </listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term>
+          <option>--no-photo-tz</option>
+        </term>
+        <listitem>
+          <para>
+            Ignore any <userinput>OffsetTimeOriginal</userinput> EXIF tags in
+            photos that specify the time zone in which the photo was taken.
+            If the tag is wrong, such as if the user forgot to update the time
+            zone manually when travelling, then this will prevent it from being
+            used. If this option is given, then <command>gpscorrelate</command>
+            reverts to using automatic time zone detection for the photo, or
+            a manually-specified one (if <userinput>--timeadd</userinput> is
+            given).
+            </para>
         </listitem>
       </varlistentry>
 
@@ -681,10 +733,10 @@
 
     <para>
     Correlate a photo taken from a camera with a fast clock (i.e., the clock
-    was 77.5 seconds ahead of GPS time):
-    </para>
+    was 77.5 seconds ahead of GPS time) and with incorrectly-specified time zone
+    tags:</para>
     <para>
-    <userinput>gpscorrelate -g Test.gpx -O -77.5 photo.jpg</userinput>
+    <userinput>gpscorrelate -g Test.gpx -O -77.5 --no-photo-tz photo.jpg</userinput>
     </para>
 
     <para>

doc/gui.html

@@ -42,6 +42,8 @@ <h3>Step 3: Set options</h3>
 
 <p>The <b>"Auto time zone"</b> checkbox, if checked, will choose an appropriate value for "Time Zone", based on the local time zone and the time of the first photo in the list to be correlated.</p>
 
+<p>The <b>"Use photo time zone"</b> checkbox, if checked, will use each photo's time zone tag to determine the local time of the image. If this tag is not available, the regular "Auto time zone" behaviour will take effect.</p>
+
 <p>The <b>"Write heading"</b> checkbox, if checked, will write tags into the picture indicating the direction of movement at the time the picture was taken, when these data are available in the GPX file. These tags are not written during times of rapid turning, determined by the <b>Max heading change</b> setting.</p>
 
 <p>The <b>"Write camera direction"</b> checkbox, if checked, will write tags into the picture indicating the direction that the camera was facing at the time the picture was taken. This is determined by adding the <b>Camera direction</b> value to the heading written in <b>Write heading</b>.</p>

exif-gps.cpp

@@ -85,7 +85,7 @@ int main(int argc, char* argv[])
 	printf("Done write, now reading...\n");
 
 	int GPS = 0;
-	char* Ret = ReadExifData(argv[1], &GPS, NULL, NULL, NULL);
+	char* Ret = ReadExifData(argv[1], &GPS, NULL, NULL, NULL, NULL);
 	if (Ret)
 	{
 		printf("Date: %s.\n", Ret);
@@ -103,7 +103,8 @@ int main(int argc, char* argv[])
 };
 */
 
-char* ReadExifData(const char* File, int* IncludesGPS, double* Lat, double* Long, double* Elev)
+char* ReadExifData(const char* File, int* IncludesGPS, double* Lat, double* Long, double* Elev,
+		long *OffsetTime)
 {
 	*IncludesGPS = 0;
 
@@ -155,6 +156,24 @@ char* ReadExifData(const char* File, int* IncludesGPS, double* Lat, double* Long
 	// Copy the tag and return that.
 	char* DateTime = strdup(Value.c_str());
 
+	// Look for time offset
+	if (OffsetTime) {
+		Exiv2::Exifdatum& OffsetTimeTag = ExifRead["Exif.Photo.OffsetTimeOriginal"];
+		std::string OffsetTimeValue = OffsetTimeTag.toString();
+		if (OffsetTimeValue.length() == 6) {
+			/* Split into hours, minutes with the same sign for both. */
+			int OffsetHours, OffsetMins;
+			if (sscanf(OffsetTimeValue.c_str(), "%d:%d", &OffsetHours, &OffsetMins) == 2) {
+				/* Can't look at sign of hours because it might be -0 */
+				if (OffsetTimeValue[0] == '-')
+					OffsetMins *= -1;
+				*OffsetTime = OffsetHours * 3600 + OffsetMins * 60;
+			} else
+				*OffsetTime = NO_OFFSET_TIME;
+		} else
+			*OffsetTime = NO_OFFSET_TIME;
+	}
+
 	// Check if we have GPS tags.
 	Exiv2::Exifdatum GPSData = ExifRead["Exif.GPSInfo.GPSVersionID"];
 

exif-gps.h

@@ -30,7 +30,13 @@
 extern "C" {
 #endif
 
-char* ReadExifData(const char* File, int* IncludesGPS, double* Lat, double* Long, double* Elevation);
+#include <limits.h>
+
+// No offset time tag was found in photo
+#define NO_OFFSET_TIME INT_MIN
+
+char* ReadExifData(const char* File, int* IncludesGPS, double* Lat, double* Long, double* Elevation,
+		long *OffsetTime);
 char* ReadGPSTimestamp(const char* File, char* DateStamp, char* TimeStamp, int* IncludesGPS);
 int WriteGPSData(const char* File, const struct GPSPoint* Point,
 		 const char* Datum, int NoChangeMtime, int DegMinSecs);

gpx-read.c

@@ -208,13 +208,13 @@ static void ExtractTrackPoints(xmlNodePtr Start)
 				LastPoint->MoveHeading = atof(Heading);
 				LastPoint->HeadingRef = HeadingRef;
 			}
-			struct timespec Timespec = ConvertToUnixTime(Time, GPX_DATE_FORMAT, 0, 0);
+			struct timespec Timespec = ConvertToUnixTime(Time, GPX_DATE_FORMAT, 0);
 			LastPoint->Time = Timespec.tv_sec;
 
 			/* Debug...
 			printf("TrackPoint. Lat %s (%f), Long %s (%f). Elev %s (%f), Time %d.\n",
 					Lat, atof(Lat), Long, atof(Long), Elev, atof(Elev),
-					ConvertToUnixTime(Time, GPX_DATE_FORMAT, 0, 0));
+					ConvertToUnixTime(Time, GPX_DATE_FORMAT, 0));
 			printf("Decimals %d %d %d\n", LastPoint->LatDecimals, LastPoint->LongDecimals, LastPoint->ElevDecimals);
 			*/