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Georeferencing with GPS

If you haven't found out about Geo-referencing your Travel Photographs yet, you need to sit up and start taking some notice. Geo-referencing is 'the new Black' where photo sales are concerned, and having your photographs geo-referenced these days can often make the difference between sale and no sale.

What is Geo-referencing? It is the technique of recording the location that the photograph was taken, and storing this information within the photo's EXIF GPS Location tags. Normally, just the Latitude and Longitude are stored, but optionally the Height above mean sea level may be added, and even the GPS bearing value. How do you do it? There are a variety of ways to record positional information with the EXIF tags of your photograph. Some are more complicated than others. Some are more costly than others. Complicated and costly are not always better! First of all, lets look at receiving the positional information. This is normally done with a GPS receiver. Many types are available, to suit all pockets. Some cameras, such as my Nikon D300, have the ability to record GPS data on to a photo when it is taken, but this involves attaching a GPS receiver to my camera, meaning more weight, batteries, leads trailing, and general complication - this is the last thing I need. One of these days, the camera bodies will include a BlueTooth wireless link to an adjacent GPS receiver (seems obvious to me - why does no one do it - HINT HINT) but until then, there is an easy way: you just have to co-ordinate (also known as synching) your camera's clock with the clock in your GPS receiver. They must be correct to within a second or three. Once this has been done, you use a GPS receiver to record a 'track' file: in other words a file of every position you move to throughout the day. Each stored position will have a time, longitude, latitude, and altitude. Its very easy to do: when you step outside in the morning, you turn on your GPS receiver, and once it has located the satellites and knows where it is, you tell it to start recording the track file. You then put the GPS receiver in the top pocket of your backpack, and forget about it. At the end of the day, when you've taken your last photograph, you tell it to stop recording, and turn it off. Easy, isn't it!

Lets now look at the second part in the process of geo-referencing: recording the location information on to your photographs. First of all, we must connect our GPS receiver to our computer, and download the track file that we are interested in. We then use a program that will cross-reference the time that each photograph was taken (the photo already has that information) with the location that we were in at that moment (the track file has that information). The resultant position for each photo will be displayed on the screen, just to make sure everything is OK, and when we are happy that everything is correct, the position will be added to the photo's EXIF GPS Location tags. Job done!

Why do I prefer this way of doing it? First of all, as mentioned above, there are no leads to catch when I take the camera out of its bag, and no extra weight to un-balance the camera. Secondly, I am not tied to a particular GPS receiver - any type will do ,as long as it can record a track file that can later be transferred to a computer. This means I can go for cheap-and-cheerful if cost is an issue, or I can go for top of the range to choose features such as improved weak signal handling, greater memory capacity, uploadable maps, battery compatibility, and whatever else is important to me. The track file that all the GPS receivers generate will be of the same format.

Likewise, there are a variety of different software packages that I can use to do the coordination. Some are free, some must be paid for. They all offer different features, some of which may be important to you. After having tried most of the GPS coordination programs that are available, I prefer RoboGeo, which you can find on the RoboGeo External link opens a new Browser window website. Although this must be purchased, it gives a range of features and an ease of use that I have not found matched in other packages, so would recommend other Travel Photographers to try it as well - a Demo download is available. Not only can it Georeference photographs as described, it also allows you to superimpose the track on a Google Map or on Google Earth, and provide markers that show the location of each photograph. You can them upload the map to your website, where all can see the location of the track and photos, plus zoom in with the map, terrain, or satellite image turned on to give you even more information about where you have been and where the photos came from. Its extremely useful!

For further information on the mechanics of Geo-referencing, plus some discussion about GPS receivers, other coordinating software packages available, and other methods of coordinating such as using Google Earth, please see the Geo Referencing External link opens a new Browser window page on one of my other websites: www.MapAbility.Com External link opens a new Browser window. You will find my Travelling Tim's Travel Blog External link opens a new Browser window there as well.

NB: for cross-referencing between GPS and camera to work properly, it is essential that the camera's clock is identical to satellite time, as provided by your GPS receiver. If there is an errror, the photos won't match up with the correct position. For a photo taken out of a moving vehicle, the error could be quite substantial! I find the clock in my Nikon D300 keeps quite good time, so calibrating it once a week is sufficient. Don't forget when changing time zones that both camera and GPS need this resetting too. The GPS will keep recording in UTC, but at least it will display the correct time. If there is a disparity between photo and GPS track file, you can adjust settings in RoboGeo to compensate, as long as you know what the error is!

If you are changing time zones alot, as I do, you will find it all too easy to forget to keep all of your electronics up to date. It is very annoying to remember half-way through the day's shooting that the clock in your camera should have been reset, and that all the photographs that you have already taken will have the wrong EXIF times. Or what happens if you shoot a few photos, then move into a new timezone that same day: this is a frequent occurrence for travellers on the Overland trucks. For this reason, some travel photographers leave their cameras set to UTC all of the time. This works well until you are in the Antipodean regions of our planet, where you will find that the day's shoot could be split between folders, or that part of one day and part of the next day end up in the same folder: very confusing. For a complete guide to photo times, dates, and time zones, please see my tutorial: When was your Photo Taken? and prepare to be surprised.


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