The UAS GPS and the camera may record in different timezones, therefore, if needed a time offset was used to synchronize the camera and the GPS clocks to UTC. The raw images acquired by the Ricoh camera mounted on the 3DR Solo quadcopter were geotagged using Mission Planner software from Ardupilot.org. The sofware plans and executes the flights then was used to apply the time correction, and apply the positions and elevations logged in the tlog navigation file to the images. The geotagged images were then processed to add additional information required by USGS to the EXIF headers using ExifTools (https://www.sno.phy.queensu.ca/~phil/exiftool/
, version: 10.75) and the files were renamed to a unique identifier using Namexif (version 1.7) to avoid any possibility of duplicate names. These steps are described here:
1) ExifTools was used to update several additional tags for all the photos. The the command used to updated tags was: exiftool -CopyrightNotice="Public Domain - please credit U.S. Geological Survey " -ImageDescription="Photographs of Town Neck Beach, Sandwich, Massachusetts, USA from survey 2017-029-FA " -Caption-Abstract="Town Neck Beach, Sandwich, Massachusetts, USA from survey 2017-029-FA " -Caption="Town Neck Beach, Sandwich, Massachusetts, USA from survey 2017-029-FA " -sep ", " -keywords="Town Neck Beach, Sandwich, Massachusetts, 2017-029-FA, UAS, aerial imagery, USGS " -comment="Photo from down-looking UAS camera during survey 2017-029-FA." -Credit="U.S. Geological Survey" -Contact="WHSC_data_contact@usgs.gov " -Artist="Sandy Brosnahan" *.JPG
2) All the JPEG images were renamed with Namexif (version 1.7, http://www.digicamsoft.com/softnamexif.html
) to ensure unique filenames and compliance with USGS Coastal and Marine Geology Program's best practices for image naming convention. Images were renamed with a survey ID prefix followed by a local USGS UID, which corresponds to a distinct UAS project; a camera ID that distinguishes among USGS cameras by make, model, and camera number; the image acquisition date and coordinated universal time (UTC) in ISO8601 format; and a suffix with the original image name. For example, image name '2017050FA_U039RI02_20170918T190922Z_B0022888.JPG', 2017050FA is the survey ID, U039 is the internal identifier code, RI02 is the internal camera ID, 20170918 is the UTC date in format YYYYMMDD. A 'T' is used to separate UTC date from UTC time in format HHMMSS followed by a Z, and B0022888.JPG is the original raw photo name appended to the end of the new filename.
3) Image EXIF DateTime* fields were an hour later than the GPS time in all files. exiftool was run again with -AllDates -=1 to correct the image metadata.
4) The two 'hours' digits of each filename were decremented to match the GPSTimeStamp using a bash script.