Implementation
- Geotag
- Geosync
- Geotime
Examples
Options
Troubleshooting
Tips
Reverse Geotagging
- Creating GPX log
- Creating KML file

# Geotagging with ExifTool

The ExifTool geotagging feature adds GPS tags to images based on data from a GPS track log file. The GPS track log file is loaded, and linear interpolation is used to determine the GPS position at the time of the image, then the following tags are written to the image:

    GPSLatitude     GPSLongitude     GPSAltitude     GPSDateStamp
    GPSLatitudeRef  GPSLongitudeRef  GPSAltitudeRef  GPSTimeStamp

Currently supported GPS track log file formats:

- GPX
- NMEA (RMC, GGA, GLL and GSA sentences)
- KML
- IGC (glider format)
- Garmin XML and TCX
- Magellan eXplorist PMGNTRK

### Implementation

Geotagging is accomplished in ExifTool through the use of three special tags: `Geotag`, `Geosync` and `Geotime`.

> Note: When writing these tags, order is important. If `Geotime` is written, it must be done after both `Geotag` and `Geosync`. Normally this is not a concern because the exiftool order of operations will copy all tags in batch processing after other directly assigned tags, but this should be kept in mind for cases where the value of `Geotime` is set directly from a date/time string.

#### Geotag

The `Geotag` tag is used to define the GPS track log data. Setting the value of this tag via the exiftool application activates the geotagging feature. As an example, the following command line adds GPS tags to all images in the "/Users/Phil/Pictures" directory based on GPS positions stored in the track log file "track.log":

    exiftool -geotag=track.log /Users/Phil/Pictures

As a convienience the exiftool application also provides a `-geotag` option, so the following command is equivalent to the one above:

    exiftool -geotag track.log /Users/Phil/Pictures

Multiple GPS log files may be loaded simultaneously by using more than one `-geotag` option (or by using wildcards in the filename argument for the `-geotag` option, but note that here the filename may need to be quoted -- see the examples below). This allows batch processing of images spanning different tracks with a single command.

Deleting the `Geotag` tag causes the associated GPS tags to be deleted from an image.

> Programmers: You may write either a GPS log file name or the GPS log data as the value for `Geotag`. If the value contains a newline or a null byte it is assumed to be data, otherwise it is taken as a file name.

#### Geosync

The `Geosync` tag is needed only when the image timestamps are not properly synchronized with GPS time. The value written to the `Geosync` tag may take a number of different forms, but the basic form is that of a simple time difference which is added to `Geotime` before interpolating the GPS position in the track log. This time difference may be of the form "SS", "MM:SS", "HH:MM:SS" or "DD HH:MM:SS" (where SS=seconds, MM=minutes, HH=hours and DD=days), and a leading "+" or "-" may be added for positive or negative differences (negative if the camera clock was ahead of GPS time). Fractional seconds are allowed (ie. "SS.ss").

For example, "`-geosync=-1:20`" specifies that synchronization with GPS time is achieved by subtracting 1 minute and 20 seconds from the `Geotime` value. See the Time Synchronization Tip below for more details.

> Note that a single decimal value is interpreted as seconds when written to `Geosync`. This is different from of other date/time shift values where a single value is normally taken as hours.

Camera clock drift correction:

A more advanced `Geosync` feature allows the GPS time and the image time to be specified together, facilitating a time drift correction if more than one synchronization point is provided. For this, the value written to `Geosync` takes one of the following forms:

> Format | Notes
> --- | ---
> FILE | Both GPS and image timestamps are extracted from the specified file. ie) `-geosync=image.jpg`
> GPSTIME@FILE | GPS time is taken from the `Geosync` value and the image timestamp is extracted from the specified file. ie) `-geosync="12:58:05@image.jpg"`
> GPSTIME@IMGTIME | Both GPS and image timestamps are taken from the `Geosync` value. ie) `-geosync="12:58:05@2010:01:02 12:25:26"`

The values of GPSTIME and IMGTIME specified on the command line may contain a date, but it is not necessary.

Notes:

1. If either the GPS or the image timestamp does not contain a date, the two timestamps are assumed to be on days which place them within 12 hours of each other.
2. If neither the GPS nor the image timestamps contain a date, this synchronization point may only be used for constant time offset (ie. no time drift correction will be applied).
3. Both the GPS and the image times are assumed to be local unless another timezone is specified.
4. Both the GPS and the image time values may contain decimal seconds.
5. A linear time drift correction is applied based on the closest two synchronization points if more than one `Geosync` value is written.
6. When extracting from file, timestamps are taken from the first available of the following tags:
   - Image timestamp: SubSecDateTimeOriginal, SubSecCreateDate, SubSecModifyDate, DateTimeOriginal, CreateDate, ModifyDate, FileModifyDate
   - GPS timestamp: GPSDateTime, GPSTimeStamp

#### Geotime

The `Geotime` tag specifies the point in time for which the GPS position is calculated (by interpolating between fixes in the GPS track log). Unless a group is specified, exiftool writes the generated tags to the default groups. If a value for `Geotime` is not given, it is taken from `DateTimeOriginal` for each image (as if `"-Geotime<DateTimeOriginal"` had been specified), but the value may be copied from any other date/time tag or set directly from a date/time string.

If the date/time tag does not include a timezone then one may be added (ie. `"-Geotime<${CreateDate}-05:00"`), otherwise the local system time is assumed.

By default, GPS tags are created in EXIF and the corresponding XMP tags are updated only if they already exist. However, an EXIF or XMP group name may be specified to force writing only to the specified location. For example, writing `XMP:Geotime` or `EXIF:Geotime` will write the generated GPS tags only to XMP or EXIF respectively. Note that when written to XMP, the `GPSLatitudeRef` and `GPSLongitudeRef` tags are not used, and the XMP `GPSDateTime` tag is written instead of the separate EXIF `GPSDateStamp` and `GPSTimeStamp` tags.

See the Examples section below for sample command lines illustrating various aspects of the geotagging feature.

> Programmers: Note that `Geotime` must always be specified when geotagging via the API -- the default value of `DateTimeOriginal` is implemented at the application level.

### Examples

Geotag an image ("a.jpg") from position information in a GPS track log ("track.log"). Since the `Geotime` time is not specified, the value of `DateTimeOriginal` is used. Local system time is assumed unless `DateTimeOriginal` contains a timezone:

    exiftool -geotag track.log a.jpg

Geotag all images in directory "dir" from the GPS positions in "track.log", for a camera clock that was running 25 seconds slower than the GPS clock:

    exiftool -geotag track.log -geosync=+25 dir

Geotag an image with the GPS position for a specific time. (Note that the `Geotag` tag must be assigned before `Geotime` for the GPS data to be available when `Geotime` is set):

    exiftool -geotag t.log -geotime="2009:04:02 13:41:12-05:00" a.jpg

Geotag all images in directory "dir" with XMP tags instead of EXIF tags, based on the image `CreateDate`. (In this case, the order of the arguments doesn't matter because tags with values copied from other tags are always set after constant values):

    exiftool -geotag log.gpx "-xmp:geotime<createdate" dir

Geotag images in "dir" using `CreateDate` with the specified timezone. If `CreateDate` already contained a timezone, then the timezone specified on the command line is ignored. (Note that in Windows, double quotes (`"`) must be used instead of single quotes (`'`) around the `-geotime` argument in the next 2 commands):

    exiftool -geotag a.log '-geotime<${createdate}+01:00' dir

Geotag images for which the camera clock was set to UTC (+00:00), using the time from `DateTimeOriginal`:

    exiftool -geotag trk.gpx '-geotime<${DateTimeOriginal}+00:00' dir

Delete GPS tags which were added by the geotag feature. (Note that this does not remove all GPS tags -- to do this instead use `-gps:all=`):

    exiftool -geotag= a.jpg

Delete XMP GPS tags which were added by the geotag feature:

    exiftool -xmp:geotag= a.jpg

Geotag an image with XMP tags, using the time from `DateTimeOriginal`:

    exiftool -xmp:geotag=track.log a.jpg

Combine multiple track logs and geotag an entire directory tree of images:

    exiftool -geotag a.log -geotag b.log -r dir

Use wildcards to load multiple track files (the quotes are necessary for most operating systems to prevent filename expansion):

    exiftool -geotag "logs/*.log" dir

Geotag from a sub-second date/time value with a sub-second time synchronization (only possible if the EXIF sub-second time stamps are available):

    exiftool -Geotag a.log -Geosync=+13.42 "-Geotime<SubSecDateTimeOriginal" dir

Geotag images with a linear time drift correction using the GPS time synchronization from three already-geotagged images:

    exiftool -geotag a.log -geosync=1.jpg -geosync=2.jpg -geosync=3.jpg dir

### Options

Geotagging may be configured via the following ExifTool options. These options have no command-line equivalents, but may be set using either the Options() function of the API or the %Image::ExifTool::UserDefined::Options hash of the config file. (See the sample config file for details about how to use the config file.)

> Option | Description | Values | Default
> --- | --- | --- | ---
> GeoMaxIntSecs | Maximum interpolation time in seconds for geotagging. Geotagging fails if the Geotime value lies between two fixes in the same track which are separated by a number of seconds greater than this. | A floating point number | 1800
> GeoMaxExtSecs | Maximum extrapolation time in seconds for geotagging. Geotagging fails if the Geotime value lies outside a GPS track by a number of seconds greater than this. | A floating point number | 1800
> GeoMaxHDOP | Maximum Horizontal (2D) Dilution Of Precision for geotagging. GPS fixes are ignored if the HDOP is greater than this. | A floating point number, or undef to disable | undef
> GeoMaxPDOP | Maximum Position (3D) Dilution Of Precision for geotagging. GPS fixes are ignored if the PDOP is greater than this. | A floating point number, or undef to disable | undef
> GeoMinSats | Minimum number of satellites for geotagging. GPS fixes are ignored if the number of acquired satellites is less than this. | A positive integer, or undef to disable | undef

### Troubleshooting

1. "No track points found in GPS file"

> If you see the above message, either exiftool does not yet support your track log file format, or your track log does not contain the necessary position/timestamp information. For instance, in KML files each Placemark must contain a TimeStamp. If you believe your track log contains the necessary information, please send me a sample file and I will add support for this format.

2. "0 image files updated"

> If you see this message without any other warning messages, it is likely that `Geotime` didn't get set properly.

> Be sure that the necessary date/time tag exists in your image for copying to `Geotime`. Unless otherwise specified, the required tag is `DateTimeOriginal`. The following command may be used to list the names and values of all available date/time tags in an image:

>     exiftool -s -time:all image.jpg

> Even if there is no metadata in the image you may be able to set `Geotime` from the filesystem modification date for the image (which will appear as `FileModifyDate` in the output of the above command). In this case you may also want to include the `-P` option to preserve the original value of `FileModifyDate`:

>     exiftool -geotag track.gpx "-geotime<filemodifydate" -P image.jpg

> Without the `-P` option, `FileModifyDate` is set to the current date/time when the file is rewritten.

3. "Warning: Time is too far before track in File:Geotime (ValueConvInv)"

> If you see a warning like this, you may have a time synchronization problem.

> Try adding the `-v2` option to your command to output verbose information. You should see messages like this if the GPS track log was loaded successfully:

>     Loaded 372 points from GPS track log file 'my_track.log'
>       GPS track start: 2009:03:30 19:45:25 UTC
>       GPS track end:   2009:04:03 11:16:04 UTC

> If the number of points loaded and start/end times seem reasonable, then the problem is likely in the time synchronization. Also printed will be the UTC time for the image:

>       Geotime value:   2009:04:03 10:57:01 UTC

> The "Geotime value" must lie within 1/2 hour of a valid GPS fix in the track log for a position to be calculated. (1/2 hour is the default, but this can be configured via the geotagging Options.) The time calibration relies on proper synchronization between the GPS time and your camera's clock. If a timezone is not specified, `Geotime` is converted to UTC using the local system timezone (as set by the shell's TZ environment variable). You should specify the timezone for `Geotime` if your images were taken in a different timezone (see Examples above). If the camera clock was wrong, the `Geosync` tag may be used to apply a time correction, or the ExifTool time shift feature may be used to adjust the image times before geotagging -- see the Time Synchronization tip below for examples.

### Tips

1. Time Synchronization

> One way to accurately synchronize your images with GPS time is to take a picture of the time displayed on your GPS unit while you are out shooting. Then after you download your images you can use this image to synchronize the image timestamps for geotagging. This is done by using an image viewer to read the time from the GPS display in the image, and exiftool to extract `DateTimeOriginal` from the file. For example, if the time in the GPS display reads 19:32:21 UTC and `DateTimeOriginal` is 14:31:49, then for this image the camera clock was 32 seconds slow (assuming that the timezone of the camera clock was -05:00). There are two different ways to use this time synchronization to improve your geotagging accuracy:

> A) Use the `Geosync` tag to specify the time difference while geotagging. Using this technique the existing image timestamps will not be corrected, but the `GPSTimeStamp` tag created by the geotagging process will contain the correct GPS time:

>     exiftool -geosync=+00:00:32 -geotag my_gps.log C:\Images

> or equivalently,

>     exiftool -geosync=19:32:21Z@14:31:49-05:00 -geotag my_gps.log C:\Images

> (Note that this technique may also be used for a more advanced time drift correction. See the Geosync section above for details)

> B) First fix the image timestamps by shifting them to synchronize with GPS time, then geotag using the corrected timestamps:

>     exiftool -alldates+=00:00:32 C:\Images
>     exiftool -geotag my_gps.log C:\Images

> Both examples above assume that your track log file (`my_gps.log`) is in the current directory, that the images were downloaded to the `C:\Images` directory, and that the computer and camera clocks are in the same timezone.

### Reverse Geotagging

The -p option may be used to output files in any number of formats. This section gives examples for creating GPX and KML output files from a set of geotagged images.

#### Creating a GPX track log

The following print format file may be used to generate a GPX track log from one or more geotagged images:

    #------------------------------------------------------------------------------
    # File:         gpx.fmt
    #
    # Description:  Example ExifTool print format file for generating GPX track log
    #
    # Usage:        exiftool -p gpx.fmt -d %Y-%m-%dT%H:%M:%SZ FILE [...] > out.gpx
    #
    # Revisions:    2010/02/05 - P. Harvey created
    #
    # Notes:     1) All input files must contain GPSLatitude and GPSLongitude.
    #            2) The -fileOrder option may be used to control the order of the
    #               generated track points.
    #------------------------------------------------------------------------------
    #[HEAD]<?xml version="1.0" encoding="utf-8"?>
    #[HEAD]<gpx version="1.0"
    #[HEAD] creator="ExifTool $ExifToolVersion"
    #[HEAD] xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    #[HEAD] xmlns="http://www.topografix.com/GPX/1/0"
    #[HEAD] xsi:schemaLocation="http://www.topografix.com/GPX/1/0 http://www.topografix.com/GPX/1/0/gpx.xsd">
    #[HEAD]<trk>
    #[HEAD]<number>1</number>
    #[HEAD]<trkseg>
    #[BODY]<trkpt lat="$gpslatitude#" lon="$gpslongitude#">
    #[BODY]  <ele>$gpsaltitude#</ele>
    #[BODY]  <time>$gpsdatetime</time>
    #[BODY]</trkpt>
    #[TAIL]</trkseg>
    #[TAIL]</trk>
    #[TAIL]</gpx>

This example assumes that the `GPSLatitude`, `GPSLongitude`, `GPSAltitude` and `GPSDateTime` tags are all available in each processed FILE. Warnings will be generated for missing tags. The output GPX format will invalid if any `GPSLatitude` or `GPSLongitude` tags are missing, but will be OK for missing `GPSAltitude` or `GPSDateTime` tags.

Note that the order of track points in the output GPX file will be the same as the order of processing the input files, which may not be chronological depending on how the files are named. The `-fileOrder` option may be used to force processing of files in a particular order. For example, the following command processes files in order of increasing `GPSDateTime`:

    exiftool -fileOrder gpsdatetime -p gpx.fmt -d %Y-%m-%dT%H:%M:%SZ /Users/Phil/Pictures > out.gpx

Note that since no directory was specified for `gpx.fmt`, this file must exist in the current directory when the above command is executed. (If the `gpx.fmt` file can't be found then the `-p` argument is interpreted as a string instead of a file name, and the text "`gpx.fmt`" is sent to the output, which isn't what we want.)

The `-if` option may be added to ensure that only files containing GPS information are processed. For example, the following command creates "`out.gpx`" in the current directory from all pictures containing `GPSDateTime` information in directory "`pics`" and its sub-directories:

    exiftool -r -if '$gpsdatetime' -fileOrder gpsdatetime -p gpx.fmt -d %Y-%m-%dT%H:%M:%SZ pics > out.gpx

Note: In Windows, double quotes (`"`) must be used instead of single quotes (`'`) around the `-if` argument above.

The full exiftool distribution contains this sample format file ("gpx.fmt") as well as a sample which creates GPX waypoints with pictures ("gpx_wpt.fmt").

#### Creating a Google Earth KML file

Below is an example of a print format file which generates a Google Earth KML file from a collection of geotagged images:

    #------------------------------------------------------------------------------
    # File:         kml.fmt
    #
    # Description:  Example ExifTool print format file for generating a
    #               Google Earth KML file from a collection of geotagged images
    #
    # Usage:        exiftool -p kml.fmt FILE [...] > out.kml
    #
    # Revisions:    2010/02/05 - P. Harvey created
    #
    # Notes:     1) All input files must contain GPSLatitude and GPSLongitude.
    #            2) The -fileOrder option may be used to control the order of the
    #               generated placemarks.
    #------------------------------------------------------------------------------
    #[HEAD]<?xml version="1.0" encoding="UTF-8"?>
    #[HEAD]<kml xmlns="http://earth.google.com/kml/2.0">
    #[HEAD]  <Document>
    #[HEAD]    <name>My Photos</name>
    #[HEAD]    <open>1</open>
    #[HEAD]    <Style id="Photo">
    #[HEAD]      <geomScale>.75</geomScale>
    #[HEAD]      <IconStyle>
    #[HEAD]        <color>ffffffff</color>
    #[HEAD]        <Icon>
    #[HEAD]          <href>root://icons/palette-4.png</href>
    #[HEAD]          <x>192</x>
    #[HEAD]          <y>96</y>
    #[HEAD]          <w>32</w>
    #[HEAD]          <h>32</h>
    #[HEAD]        </Icon>
    #[HEAD]      </IconStyle>
    #[HEAD]    </Style>
    #[HEAD]    <Style id="View">
    #[HEAD]      <IconStyle>
    #[HEAD]        <color>ffffffff</color>
    #[HEAD]        <Icon>
    #[HEAD]          <href>root://icons/palette-3.png</href>
    #[HEAD]          <x>160</x>
    #[HEAD]          <y>128</y>
    #[HEAD]          <w>32</w>
    #[HEAD]          <h>32</h>
    #[HEAD]        </Icon>
    #[HEAD]      </IconStyle>
    #[HEAD]    </Style>
    #[HEAD]    <Folder>
    #[HEAD]      <name>Waypoints</name>
    #[HEAD]      <open>0</open>
    #[BODY]      <Placemark>
    #[BODY]        <description><![CDATA[<br/><table><tr><td>
    #[BODY]        <img src='$directory/$filename'
    #[BODY]          width='$imagewidth' height='$imageheight'>
    #[BODY]        </td></tr></table>]]></description>
    #[BODY]        <Snippet/>
    #[BODY]        <name>$filename</name>
    #[BODY]        <styleUrl>#Photo</styleUrl>
    #[BODY]        <Point>
    #[BODY]          <altitudeMode>clampedToGround</altitudeMode>
    #[BODY]          <coordinates>$gpslongitude#,$gpslatitude#,0</coordinates>
    #[BODY]        </Point>
    #[BODY]      </Placemark>
    #[TAIL]    </Folder>
    #[TAIL]  </Document>
    #[TAIL]</kml>

[These example `-p` files are included in the full ExifTool distribution.]

Created Apr. 2, 2009
Last revised Mar. 13, 2010

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