GPS

GPS: An overview

GPS technology was first developed for the US military for use in determining precise locations. A GPS receiver uses a series of geostationary orbiting satellites to compute its location by triangulation. GPS data can include latitude and longitude, as well as altitude, speed, direction, and more. Since we’re mostly concerned with information about the instant the shutter was snapped, we’ll mostly be interested in the latitude and longitude data and sometimes the altitude. This information can be used to determine the precise location of the camera when a photo was taken. (Although there is a lot of other information that can be saved in GPS metadata, it’s harder to mash-up most of it usefully with the photo. The direction information, for instance, refers to the direction of travel, not the direction the camera is pointed.)

Some digital cameras now have GPS receivers, but these tend to be point and shoot cameras. DSLR cameras frequently offering the ability to add a GPS tagging module either from the camera manufacturer or a third party. Popular image management software, such as Adobe Lightroom and Apple's Aperture offer GPS integration is various forms. And there are dozens of freestanding applications that can add GPS data to image files. While the process is generally hard to fully automate, it's getting much easier.

Precise, enduring location data

Unlike the IPTC location fields, which provide incomplete and sometimes subjective information about where a photo was shot, GPS data can provide an objectively precise position. The IPTC fields may only be able to say, for example, that you took a photo in the Tasmanian State Forest, Tasmania, Australia, but GPS data can pinpoint a location of –41° 13’ 58.10”, +147° 59’ 12.26”, which is a precise spot within the state forest (Figure 1).

Figure 1shows an image in Expression Media, along with a screenshot tagging that image on Google Maps. This is the kind of place that can't easily be described with traditional location notation, but can easily be pinpointed with GPS.

Not only does GPS data enable more precision, it can stand the test of time. Place names change: countries rise and fall, people buy and sell property, earth is turned to street, and buildings are built and torn down. With accurate map coordinates and a timestamp, you’ll never have to doubt where a photo was made.

Moreover, some databases can translate the GPS coordinates to location place names automatically, saving you the trouble of writing location names into your bulk metadata. In the near future, GPS data, along with a timestamp, will enable some good guessing about the nature and subject matter of the pictures. For instance, it will be possible to automatically tag photos shot at the time and place of the Super Bowl with the keyword “Super Bowl.”

Using GPS information

Browsers, catalog software, web-based photo sites, and other utilities are starting to let you view GPS information attached to photos. Lightroom 3 has a little arrow in the Metadata panel that pops a Google Maps window in your web browser with the image location shown (Figure 2). Lightroom 4 has a full-blown Map module that hooks onto Google maps. Expression Media can launch a Virtual Earth window with the same kind of pin. The online service Flickr has a large geotagging community sharing information, and Google has a tool called Panoramio that enables your geotagged images to show up in Google Earth.

FIgure 2If GPS tags are in the photo, you can launch Google Maps right from the Lightroom window by clicking the arrow. This one was a surprise. In looking for an example of an image from the middle of nowhere, we chose this one from Tasmania. We were astounded to see that Google has a street view of this road.

GPX and EXIF

GPS data can be stored in a number of formats, such as those created by Garmin, Magellan, or Google’s KML. Metadata embedded in the EXIF needs to be in GPS Exchange Format (GPX), an open standard XML schema for recording position and direction information, as shown in Figure 3. It includes latitude, longitude, altitude, timestamp, and a bunch of other stuff that’s relevant to a larger travel record (not the location of this particular photo). The Metadata Working Group is reviewing this schema and may come out with some new specifications before too long. For instance, some devices now include compasses and inclinometers. Coupled with information about the field of view of the lens and focus distance, it’s theoretically possible to capture lots of information about the exact position of what’s in the photo, not just where the camera happens to be.

FIgure 3Here’s what GPS data looks like inside a GPX file. Like XMP, it’s a flavor of XML data in which the tag is wrapped in headers that define what the tag means.

Geotagging your photos

There are several different ways to geotag (attach GPS data to) your photos. Here are some options.

Tagging in camera

The most effortless way to geotag images is to have the camera do the work. Since the EXIF standard includes fields for GPS data, it’s natural to have cameras write this in automatically when you take the picture. An increasing number of point-and-shoot cameras – particularly ones made for outdoor sporting use – have GPS receiver chips built into the camera so geotagging can be automatic.

Some DSLR cameras offer the capability to communicate directly with dedicated GPS units through a cable (Figure 4). When recording an image, the camera asks the unit for the current GPS reading and writes it into the file. With this method, you get automatic geotagging as well as the ability to upgrade your GPS receiver independently from your camera.

While in-camera tagging is the easiest way to get GPS data into photos, this method suffers from some drawbacks. Even the best receivers sometimes drop the signal. Tall buildings can muddle readings, and readings drop out entirely if you are below a few floors of concrete. If the GPS receiver loses its signal when the shutter is pressed, it fails to write a location to the file.

Externally connected GPS units have a couple of additional disadvantages. They may attach to the camera awkwardly, either sitting up on the hotshoe or connected by a cable that runs up the strap or into a pocket. And you need to have one attached to each camera, which makes it even more cumbersome for the multi-camera photographer.

FIgure 4 A number of the higher-end digital SLR cameras offer the ability to connect a GPS unit directly to the camera for automatic geotagging. This image shows the Geomet’r.

Using WiFi to geotag

If a GPS satellite signal is not available, it's possible to get the location data from other sources. Cell phone towers are one method that can provide location information, but at this point, only cell phones make use of this data. WiFi networks are another source of location data. The Eye-Fi memory card can use a database of WiFi access points to geotag your images, if it can find one at the time the photo is taken. This method is imprecise, and hit-or-miss, but it can offer some location tagging indoors and in urban areas that are not friendly to conventional GPS reception.

Using online maps

Some programs now offer the capability to add GPS data to images in post-production. Notably, this function is available in Aperture 3 and Lightroom 4. Expression Media 2 and Phase One Media Pro use Microsoft’s Virtual Earth to locate images and then tag them with GPS information (Figure 5). Select an image while in Expression Media 2, then launch the Virtual Earth window from the menu item. Find the location on the map and drag the image to the Virtual Earth toolbar. The location in the center of the window will be assigned to the image. If you’ve got a number of images shot in the same place, you can drag all of them at the same time to assign that location to them. You can use web applications like Flickr and Google Maps to add GPS data to pictures in much the same way.

Figure 5 Expression Media 2 can assign GPS data to photos.

Merging with tracklogs

You can also put GPS data into an image by merging the image with a saved tracklog, using the image’s timestamp to synchronize the two. We like this method for a few reasons. First, if you shoot with more than one camera, you do not have to buy three GPS units. More importantly, we have found that tagging in-camera is not the optimal way to get good information. It’s pretty common for GPS receivers to lose their signals, which means that a lot of images come up with no data at all.

With the tracklog merging method, a GPS unit travels with the photographer, saving a record of points along the way. Each saved point includes the location and time. Post-production software can compare the time that an image was taken to the timeline in the tracklog, and assign a location to the image. If the GPS unit is always getting a reading during the shoot, this method equals tagging in-camera for accuracy and it enables the use of multiple cameras, including those cameras that don’t allow GPS device connection.

More importantly, tracklog merging can fill in the gaps for those times when the GPS is not getting a signal. Perhaps on your shoot, you went into a building and lost the signal. When you exited, the signal picked up. If you use the tracklog method, all the photographs you took in the intervening time would be tagged along a line drawn from where signal was lost to where it was picked back up again. If you enter and exit a building by the same door, that door is the location that would be in the tag. (Of course, if you are traveling by subway, you'll create a tracklog that may be wildly inaccurate since it will draw a straight line from entrance to exit.)

There are a number of different programs you can use to merge a tracklog with photos. Whichever program you use, you should keep in mind that tracklog merging uses timestamp data to match up a photo with a location, so you’ll want to make sure that you have the clocks in every camera synchronized with your GPS as closely as possible. It’s an easy thing to forget to do, but if you don’t do it, sorting everything out later will be much more complicated. Test with a few files until you get all your settings right.

Figure 6GPSPhotoLinker can merge tracklogs, plus it can assign IPTC place names to the images.

Assigning place names

One of the more interesting things you can do with GPS data is use it to have it automatically assign place names to photographs. There are a number of open-source databases of place names and coordinates that can help in this process, although the technology is still developing. While databases are becoming more complete and the user interfaces are rapidly becoming streamlined, you might want to wait a little while before you attempt to use this on an entire image collection. Try it first with small groups of images to see how well you like the results.

Note that the place name that a databse assigns may be very different than the way you might think of a place. An apartment building in a large city may be tagged with the building address, when you'd really like to tag it as "Kelly's Apartment". And many locations have multiple names. For instance, a particular location in Maryland is variously tagged as Kensington, Silver Spring, Montgomery county or Homewood, depending on the database that is used.

Privacy issues

GPS tagging can pinpoint the exact location of the photograph, and that can be sensitive information, particularly in combination with names and other metadata. The exposure of location information is something that makes a lot of people nervous, particularly when it’s published to the Web. There might even be laws against publishing some location information if the photographs are of minors. If you don’t want the location information attached to a photo, you need to make sure it’s stripped.

Adobe Bridge is a very reliable application for determining what information might be in the file. If you want to have a closer, geekier look at that info, go into the Advanced panel of the File Info dialog box, and you’ll see all kinds of stuff that you can poke through. Phil Harvey's ExifTool and Camera Bits Photo Mechanic are also excellent tools to see what data is embedded in a file.
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