GPS Q&A
By Greg May

Q.How can I integrate my digital camera with GPS data collection? T. Fleming Norfolk, Virginia

A.Digital imagery has become an indispensable aid in the collection and interpretation of spatial data. As plan maps and as perspective photos, digital imagery enhances traditional vector spatial databases of point, line, and polygon data. The various vector-oriented methods of GPS data do not lend themselves directly to the collection or use of digital imagery. For instance, I have just surveyed a subdivision, but how do I create and link a photograph of it to my data? I own scanned maps of my area of interest, but how do I use them while collecting GPS information?

Digital Raster Backgrounds
Digital maps in plan form, i.e. aerial photographs or scanned paper maps, have a significant role to play in the enhancement of spatial vector data, during both its collection and its interpretation. Using a pen or notebook computer or a handheld data collector with appropriate GPS software, it is possible to display raster images as a background to GPS data collection. This allows you to see the data you have collected or need to collect in the context of the real world outside your immediate line of sight. For instance, when collecting soil samples a background map of soil types allows you to see where to sample and not to. Background aerial photos can also assist when navigating with GPS to features of interest for inventory.
    A fundamental constraint of using background maps in GPS data collection is the fact that any scanned map or aerial photograph must be georeferenced, i.e. tied to real-world coordinates, and that these coordinates are in a particular coordinate system. Problems can arise when you are required to work in a different coordinate system to that of the raster image. Vector data is easily transformed into different coordinate systems - each coordinate is simply recomputed using the appropriate formula - however the same transformation on a raster image requires each pixel of that image to occupy a new location in space. Depending on the coordinate transformations involved, this may constitute distortion of each pixel by translation, rotation, skewing, and scaling - a very processor-intensive task considering that the end result must also be an image of square pixels. This task is rarely done on the fly as it may require minutes or even hours of calculation. Without the appropriate software to perform this transformation, you may be required to work in the field using the same coordinate system as your background map so that GPS data overlays correctly. A good GPS data collection program will allow you to collect data in a different coordinate system than that required for data analysis, meaning that you can benefit from a background image without being locked permanently into its own coordinate system.

Digital Photographs
Increasingly, digital photographs of objects encountered in the field are being used to enhance the spatial data being collected. Unlike plan maps, a digital photo requires no georeferencing as it is usually a perspective image, i.e., a regular photograph. Take for example a photograph of a utility pole: The coordinates of the pole are not an intrinsic part of the photo, the value of which is in its descriptive information such as the condition of the pole or the presence of guy wires. However without a link between the photo and its geographic location the photo can be valueless.
    There are two ways of linking positions with photos when using GPS: by storing the GPS position on the camera itself, or by linking the digital photo to a regular GPS data file.

Storing GPS within a Camera
Some of today's sophisticated digital cameras can store additional information about each photo in the camera's memory. One of these pieces of information may be the GPS position. GPS-enabled digital cameras allow a GPS receiver to send a position to the camera at the time the photo is taken. Usually this is in the form of a NMEA string - NMEA is an ASCII format emitted by many GPS receivers which contains, among other things, GPS position information.
    If you intend to perform this type of digital photo collection you will need both a digital camera capable of receiving NMEA strings, and a GPS receiver capable of emitting NMEA strings. An important point to note is that once a position is embedded in the digital photo it cannot be differentially corrected. Therefore it is important that your GPS receiver is emitting real-time differentially corrected positions in order to get a position more accurate than 100 meters.

Storing a Photo within a GPS File
Any digital photo can be linked to a regular GPS file. This may be a manual process of entering information about the image, or it can also be automated with the appropriate software. As digital photos are stored in numerical order on a digital camera, a GPS feature can be defined to include this number. Alternately after the digital camera has been downloaded, the filename of the image can be stored as a text attribute or comment. Admittedly these two pieces of information have to be entered manually, however more advanced GPS data collection programs running on a pen or notebook computer have the ability to automatically select a file from a digital camera, download it to the computer, and store its filename in a field for the current GPS feature.
    Because the digital photo is simply another attribute of a GPS feature, the GPS positions themselves can be differentially corrected with appropriate software. In fact, much more information about a feature can be collected this way, and the data can be exported to a GIS system much more easily. However it is important to update the filename links if the digital files are moved to a new computer location.

Conclusion
There are numerous ways to combine digital imagery with GPS data collection. As we have seen, one use is to display a georeferenced background image with the GPS data to aid in navigation and site selection. Digital photographs and GPS positions can also be stored together, either by storing a position within a digital image (camera and GPS receiver must support this) or to store a link to the digital photo inside a GPS file.

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