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Author Topic: Synchronizing GPS clock and camera Clock for geotagging  (Read 13457 times)

Santiago

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Synchronizing GPS clock and camera Clock for geotagging
« on: May 09, 2013, 01:27:36 AM »
Hi all,

I have the following inquiry. I work with multirotor UAV's for mapping purposes. I have to geotag each photo by using the route I planned so I just plot in the lat and long of the waypoints I planned. I know this final method may result in an error augmentation do to positioning errors the kopter has in the air.

I've seen in some websites that the camera's clock should be synced to the GPS UTM clock. To be sincere I have no idea how to do this, I don't even know how to look at the clock of the MK's GPS. I've looked in many forums, but I haven't found anything useful. Also another problem I see is that the way the time is displayed in the pictures my camera takes (Sony Nex 5 and 5N) has only precision of minutes. For example 12:34, not 12:34:23. Hope someone can help me out by telling me how to synchronize them and use this data to geotag the pictures with more precisi?n, probably using the GPX data from the mikrokopter.

Thanks and best regards,

Santiago

RalfH

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Re: Synchronizing GPS clock and camera Clock for geotagging
« Reply #1 on: May 09, 2013, 12:39:46 PM »
Synchronising GPS and camera clock is quite easy: just take a photograph of the GPS time displayed on a handheld GPS device (including seconds) before and/or after the flight. It does not have to be the same device as the one that's flying with the UAV, because all GPS devices are synchronised via the satellite signal. Just make sure that the GPS already sees satellites and has established its position, otherwise its time may not have been synchronised to the satellites yet. After the flight, look at the photograph showing the GPS time: subtracting the GPS time from the photograph's time stamp (under file... properties... details) will give you the corect offset. Then, you can use this offset to put it into a geotagging software together with all photographs and your GPS track to assign the correct GPS position to each photograph. Alternatively, you can calculate the correct time and assign the correct GPS position manually or using a spreadsheet software like Excel.

P.S.: Be aware that the flight velocity of you UAV will have an influence of position accurracy: camera time stamps do not include fractions of a second, so your synchronisation is only approx. +/- one second. At 20 m/s flight vecocity, of course this results in a position error of +/- 20 m in addition to the GPS error of +/- a few metres. Flying a lawnmower pattern you can hope (bout not be sure) that much of that error will be cancelled out.
« Last Edit: May 09, 2013, 12:58:28 PM by RalfH »

Santiago

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Re: Synchronizing GPS clock and camera Clock for geotagging
« Reply #2 on: May 13, 2013, 08:14:56 PM »
Synchronising GPS and camera clock is quite easy: just take a photograph of the GPS time displayed on a handheld GPS device (including seconds) before and/or after the flight. It does not have to be the same device as the one that's flying with the UAV, because all GPS devices are synchronised via the satellite signal. Just make sure that the GPS already sees satellites and has established its position, otherwise its time may not have been synchronised to the satellites yet. After the flight, look at the photograph showing the GPS time: subtracting the GPS time from the photograph's time stamp (under file... properties... details) will give you the corect offset. Then, you can use this offset to put it into a geotagging software together with all photographs and your GPS track to assign the correct GPS position to each photograph. Alternatively, you can calculate the correct time and assign the correct GPS position manually or using a spreadsheet software like Excel.

P.S.: Be aware that the flight velocity of you UAV will have an influence of position accurracy: camera time stamps do not include fractions of a second, so your synchronisation is only approx. +/- one second. At 20 m/s flight vecocity, of course this results in a position error of +/- 20 m in addition to the GPS error of +/- a few metres. Flying a lawnmower pattern you can hope (bout not be sure) that much of that error will be cancelled out.

Hi Ralph,

Thanks for your reply. I did try using a handheld device to synch the camera to it, but my handheld GPS doesn't display the time or at least I wasn't able to find it, also there's the issue with the camera which doesn't show me more that minutes. It's a sony nex 5 so if anyone knows if the time could be displayed as 12:00:00 that would be great. Thanks anyway Ralph at least I know how it must be done now. Also which geotagging software do you use, right now I'm using geosetter, but with it I have to geotag one image at a time, I would want a software that allows me to do a more efficient geotagging.

Best regards and thank you,

Santiago

RalfH

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Re: Synchronizing GPS clock and camera Clock for geotagging
« Reply #3 on: May 13, 2013, 09:24:18 PM »
Santiago,

usually the camera time stamp including second will be somewhere in the file info details or in the EXIF data, and geotagging software should be able to read it. Alternatively, if you want to work with the time stamps and GPS logging data in a spreadsheet software (which is what I usually do), there's software that can read (and export as text file) all the EXIF data from your images. I don't remember the name of the software at the moment and can't look it up because I'm at a different computer now. Same for the geotagging software; have to look it up, but it was able to synchronize many photographs with GPS log files. The handheld GPS I use is a GT-11, and it displays seconds. Don't know what other devices do.


frank.stremke

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Re: Synchronizing GPS clock and camera Clock for geotagging
« Reply #4 on: May 15, 2013, 10:12:44 AM »
hi
you can use GPSprune for writing the gps position from a gpx or text file to the pictures it can even handle offsets and does whole folders at one go.
you can either set the time on your camera corectly or work with ofsets.
if your gps unit does not display the time then you can use any other source for UTC time (not utm by the way)
usualy your pc can do this for you with the time sync
or see here
http://www.bipm.org/en/scientific/tai/time_server.html
to work with ofsets do as discribed and take a picture of a corect watch radio controled watch will work as well you will just have to aply hour corections.
most cameras will not allow to see or set seconds however they will be in he exif file
to sett the clock corectly have a look on a corect utc clock and sett the camerea watch to the next minute pressing enter or ok once the seconds are up and the counter on the watch jumps to the new minute which you have sett on your camera now the cammera should have the correct time.
make shure to set the right date as well or gps prune will have trouble.
best regards
frank

Santiago

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Re: Synchronizing GPS clock and camera Clock for geotagging
« Reply #5 on: June 04, 2013, 03:30:50 AM »
Thanks guys

You both have been very helpful I'll give it a try and let you know.

Best regards,

Santiago