I can answer some of this, but please remember that I'm only a consumer of
this technology, not someone that has designed it, sells it or has any other
vested interest!
>> I'm not sure I understand exactly what this unit does. Does it add GPS
>> coordinates to the EXIF headers of photos on the camera, or does it
>> download them to its own memory and add the data there? Either way,
>> when does it do this - immediately after each photo is taken, or later
>> on when you connect the unit?
There are several devices on the market. The one that Jill identified is in
the most common category. These are devices that sit in your pocket,
independent of the camera, and simply record a GPS track. Later you plug it
into the computer to transfer the track and run some special software that
updates the EXIF headers in the photos. This works well for JPEG photos only
- I am unaware of any software on the market that will add tags to any of
the RAW formats or TIFF.
There are some other devices that plug directly into cameras. For example, I
know of a cable that will plug into the prosumer and professional Nikon
cameras and into a GPS device that will stamp all photos taken with the
current Lat and Long at the time of taking the photo. There are also
Bluetooth devices that do the same sort of thing (by communicating with
Bluetooth-enabled GPS devices).
>> I'm not sure I'd be very comfortable with something trying to modify
>> photos in the camera's own memory, in case it corrupted them. I guess
>> you'd also want to check that your camera will allow it to do that -
>> some cameras only allow downloading and deleting, not uploading or
>> modifying, but I guess there are less and less of these with each
>> release.
As I said, I am only aware of devices for Nikon cameras that allow this
level of sophistication. I do believe that there are some point-and-shoot
digital cameras either just released or just about to be released that have
GPS inbuilt, but don't hold me to this.
>> If it has to read the entire file from the camera, modify it and put it
>> back, that could take a considerable amount of time. Maybe only a few
>> seconds but perhaps enough to disturb rapid shooting.
I don't think that any of the devices use this approach.
>> If the file modification happens later on, and this applies to the
>> software Paul described too, how does it know which coordinates from the
>> track to apply to each photo? I assume it must look at the time stamps
>> on the photos, but that would be dependent on both time clocks being
>> synchronised. These will inevitably be at least a little different, and
>> quite commonly an hour different because of Daylight Savings. This
>> might mean a small position error or a very large one depending on how
>> fast you're moving.
The way the software works is that it looks at the date and time that the
photo was taken and then interpolates the GPS track to estimate the location
that the photo was taken (based on the date and time of the captured GPS
readings). In practice this means that there is likely to be slight errors
in position, however the size of the error is directly proportional to your
speed, so whilst walking the error would be in the vicinity of a metre or
two.
If I adjust my camera to Daylight Savings Time, I adjust all my camera gear
and the GPS simultaneously. In actual fact, both camera and GPS record UTC
and the timezone offset, so this tends not to matter.
>> Regarding Paul's problems getting GPS tracks to work with Google Maps,
>> etc, there is an excellent free program called GPSBabel
>> (http://www.gpsbabel.org) which can convert between a large number of
>> GPS track file formats, including filtering, etc.
Thanks Peter, I wasn't aware of this software - however the Garmin software
saves the tracks in a standard interchange format that Google whatever can
read.
>> People should also be aware that any data stored in the EXIF headers of
>> photos is easily lost. It's not as true as it once was, but many
>> editing programs simply discard this data when you save an edited file.
>> So if the position is valuable to you, it's well worth having a
>> secondary method of getting it back, e.g. writing photo numbers and
>> possibly coordinates in your notes. I suppose you could photograph your
>> GPS screen occasionally too.
I use Photoshop and Lightbox and these two at least preserve the EXIF
headers - but it is worth remembering.
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