Location, Location, Location

GeoNamesTagger provides a simple way of tagging images which lack location information.

Location services aid navigation, enhance your photography, and can even enrich your social life. Here’s how to get thoroughly geotagged.

There are lots of good reasons for knowing precisely where you are.

You can choose between a list of local shops or bars, then be shown how to reach that destination. You can work out how close you are to friends, thus decide who to invite to join you there. When you take a photo, the image can be tagged so that you know exactly where it was, perhaps to place it on Google Earth, or in your Photos album.

The more mobile your Mac or iOS device, the more valuable location services become, and a range of apps and tools now help you access and use location information.


The bee’s knees among location systems remains the Global Positioning System (GPS), first fully operational in 1994. This consists of 24 to 32 (currently 31) satellites spanning the skies of the globe, each transmitting radio signals. The crucial content in those signals is very accurate time information, synchronised against reference atomic clocks.

Because radio signals travel at the speed of light, and the satellites are over 12,000 miles above the earth’s surface, it is possible to compute your distance from a satellite if you can work out the time delay between the radio signal leaving the satellite and it being received by your device. If you can simultaneously measure each time delay from several satellites, each at known positions, it is possible to compute where you are on the earth’s surface, including your approximate altitude above the mean surface.

Although these calculations are complex, there is sufficient demand to drive down the cost of specialised GPS receivers and signal processing circuits, and for them to become packaged into tiny chip assemblies – hence their inclusion in palm-sized devices such as iPhones, vehicle SatNav systems, and handheld GPS navigators.

GPS was designed and paid for from US military funds, to enable US military aircraft and other systems precise location, and to improve the accuracy of cruise missiles and other directional weapons. From the outset, the military thought it dangerous to give to its enemies access to such a useful service, so whilst civilian electronics were allowed to use GPS, signals were deliberately degraded to increase the inaccuracy of non-military GPS devices, by ‘selective availability’.

However the international value of GPS grew as merchant ships, passenger aircraft, vehicles, and travellers in remote areas came to rely on it. Early navigational aids tried to undo the effects of degradation by linking in to existing navigational aids or subscription-based terrestrial radio systems, in ‘differential GPS’ (DGPS).

The US military relented on this deliberate degradation, and in 2000 we all gained access to an accuracy of 20 metres. This has progressively improved as the system has been upgraded since, and in ideal conditions can be as precise as a few metres now.

GPS is wonderful when it works, but to get such accurate positions your GPS receiver needs to ‘see’ at least 4 satellites. Because the crucial radio signals use wavelengths similar to those of Wi-Fi and Bluetooth, they only propagate by line of sight, and are blocked by hills, buildings, and other objects.

This often makes it impossible to get a decent ‘fix’ if you are travelling along the bottom of a valley, or in streets surrounded by high buildings. Paradoxically, the places that provide the most accurate GPS fixes are usually those furthest from habitation and Internet connections.

Another problem with conventional GPS is that it takes time to obtain an accurate fix. Key information required to compute position includes the precise location of each ‘visible’ satellite at that moment in time. GPS receivers obtain this in the ephemeris data transmitted by each satellite, and obtaining sufficient for high accuracy may take several minutes after the device is started up.


This can be greatly accelerated if the receiver is already primed with an approximate position, obtained by another means – often known as Assisted GPS (A-GPS).

Mobile phones connect to local aerials within cells, hence their early name ‘cellphones’. When your phone (or other enabled device) is connected to the aerial within a cell, it can discover the identification tag of that aerial.

Another potential source of even better localisation may be available if your device is connected to a Wi-Fi base station whose location has been determined, typically those accessible by the public or subscribers. Many cellular and Wi-Fi locations have been crowd-sourced, and iOS devices with Location Services turned on can access their location database to arrive at an approximate position.

Devices running iOS 4 or later can use any of GPS, cellular, and Wi-Fi location services, either in combination as A-GPS, or to give approximate location where GPS is not available. iOS devices that do not have 3G support lack built-in GPS, and as they cannot use cellular location, can only use Wi-Fi services (when connected to a known Wi-Fi base station).

Thus if you want to use an iPod Touch or iPad without 3G for navigation or geotagging, your best option is to connect an external GPS, ensuring that it is supported by the app(s) you wish to use. Location services are richer and have far better coverage in iPhones and iPads with 3G support.

No current Mac model, even a MacBook Air, can compete with iOS in this respect. Indeed the only common location feature that you are likely to encounter on your laptop is the ability of some websites to check your effective IP address, when browsing, and look up the location that corresponds to. You will have noticed that the result is usually the town with which your ISP’s IP addresses are associated – which can be a long way from your Mac.

You can pair a MacBook Air or other laptop with a compatible Bluetooth GPS receiver, or connect one via USB, and enabled applications such as RouteBuddy can utilise the location information for navigation.

Although this combination is relatively expensive and clumsy compared to an iPhone or iPad, and you will need to invest in commercial maps to get the best out of it, if you already own a handheld GPS unit it could be eminently practical. The one shortcoming is that currently A-GPS is not available on OS X systems, even if you also have 3G and AirPort connections.

So where am I?

However obtained, locations are normally expressed in terms of latitude (here north of the equator) and longitude (here either slightly east or west of the Greenwich meridian). This is fine for ships and aircraft, but Ordnance Survey and other national maps use their own grid references.

Converting between lat+long and grid references introduces small approximations and an increase in inaccuracy that can be large enough to put you on the other side of a building or gardens. You can sometime come across other coordinate systems, such as Maidenhead Locators, used by ham radio enthusiasts and others.

When images are tagged, Photos will show you location information.
When images are tagged, Photos will show you location information.

As mobile devices with location systems have become increasingly popular, and smartphone and tablet photography widespread, social networking, photo sharing, and other services have increased their support for location information.

Tools: Maps

Maps are graphical presentations of geospatial data, and their availability is largely determined by the licensing terms imposed on those data by their owners. The USA has determined that federal mapping is in the public domain, making high quality maps readily available and essentially free of charge. In the UK the Ordnance Survey still charges substantially for commercial provision, but most of its map data are now available under open licensing terms.

Apple's Maps has improved considerably since its shaky launch.
Apple’s Maps has improved considerably since its shaky launch.

iOS and OS X systems do still have full access to Google’s, Apple’s, Microsoft’s, and other free online maps, but these require Internet access and can quickly consume 3G data allowances. Although excellent for familiarisation with destinations, such as when planning trips, Google Earth and online map services are less immediately useful for navigation, particularly in hilly or remote terrain.

UK Map for iPad and iPhone.
UK Map for iPad and iPhone.

UK Map remains an outstanding and cheap solution for iOS, as you can pre-load your iPad, iPhone, or iPod Touch with detailed 1:25,000 and 1:10,000 scale topographic maps covering those areas for which you need high detail. You then have the option of 2D contours almost as precise as a paper map or 3D relief, without having to pay per map as you would for Ordnance Survey products.

Gaia Offline maps for the iPhone.
Gaia Offline maps for the iPhone.

After years lacking OS X support, Ordnance Survey maps can be purchased for use with RouteBuddy, giving Macs as good offline mapping and navigation as iOS devices. European coverage is also available, and if you purchase maps for standalone GPS navigation devices, you may well be able to access those from OS X applications too. For instance, France is covered in high detail in four map sets sold in Garmin’s store for its handheld GPS; Garmin RoadTrip, a free OS X application available from Garmin, can then access those purchased maps.

You can even get miniature maps on an Apple Watch.
You can even get miniature maps on an Apple Watch.

Technique: Geotagging Images

One important use for location information is in photographs. There is already good provision for location to be embedded in the metadata associated with each image, stored in EXIF or XMP format, as latitude and longitude at the moment the photograph was taken.

Beware that these metadata can sometimes become stripped from images when they are being processed: for instance, when a JPEG image is being prepared for placement on a web page, and other metadata are often removed at that time. If uploading images to Flickr or other photo-sharing sites that support geotagging of images, ensure that processing preserves metadata.

On the other hand you need to remember when your images are being tagged. Many military personnel deployed to locations in Afghanistan, for instance, posted images which contained geotags, which were then published for all to see. There are possibly apocryphal stories about some who found themselves under attack as a result.

Currently the greatest hurdle in geotagging images is getting the location data there in the first place. Sadly this is most readily available in the least sophisticated cameras, those built into mobile phones such as iPhones (and iPads with cellular network connections). Cameras with integral or added GPS systems include some in the Panasonic Lumix, Sony Alpha, Olympus, Canon and Nikon ranges.

GeoNamesTagger provides a simple way of tagging images which lack location information.
GeoNamesTagger provides a simple way of tagging images which lack location information.

If your camera cannot tag its images with location data, adding them manually is quite tedious. This can be accomplished by any application that supports editing image metadata, or more conveniently using a tool such as GeoNamesTagger, donationware from here.

Updated from the original, which was first published in MacUser volume 28 issue 06, 2012.