The Digital Geographer

Posted by raper at 01:43 PM | Comments (0)

July 23, 2007

Wild georeferencing

One problem you may face with mobile GIS is a lack of georeferenced mapping for your GPS-enabled device. If the device uses proprietary data formats then you just have to buy it... this is usually a factor in the purchase. If the device accepts 'open formats' (ones you can create yourself), then you can acquire the maps from various sources (often free for personal use) and load them into your system as a backdrop for your GPS position. This makes all the difference to GPS use.

Since this is not a tutorial on georeferencing per se, let me just highlight one quick and easy way to do this: you can learn the details elsewhere. The way to do this is to acquire a map in image form like JPEG, and to georeference it by entering relevant data into an ESRI World file. Several mobile GIS can import or use this kind of data.

There are two cases:

Note that solutions to both of these situations are going to use projected coordinates to georeference mapping, not latitude/ longitude, as most regional or local mapping is projected, that is, flattened out by subtle distortion to deal with the earth's curvature. Map projections are mathematical transformations from sphere to plane: they reference the planar space by using coordinates (usually in metres) to address all the space inside the usable area of the map projection. There's a lot more to tell about this process (e.g. the earth is not a true sphere): but that will do for now.

In the first case: grab any metrically accurate map (i.e. not a sketch) from an online provider as a JPEG. Use screen dumping to get the map (PrintScreen and paste into Paint on Windows, Ctrl+Shift 4 on a Mac and paste into GraphicConverter). Guess the map projection (perhaps there is a standard one in the country concerned) or look at the map it appears to be derived from in a shop or online. Now trim to the size you want in a graphics program, making the upper left (NW) point at a coordinate value you know or can find out. If there are grid lines trim along the lines and work out the coordinates at upper left. If not, find out the position of some mapped location either by going there and using your GPS to find out a coordinate, or googling a location that publishes its position, or by reading it off a paper map with grid lines or ticks along the margins to indicate coordinate values. Now trim the map to that known location. Note that the use of a small amount of public mapping for your own purposes is usually free under copyright fair use terms (check the fine print). Finally, work out the resolution of the JPEG map by finding some feature whose dimensions you know/ can measure or by looking at the grid lines, and count the pixels in your graphics program corresponding to the known distance. Divide one by the other to get the distance on the ground represented by one pixel (= the resolution). Create a text file and enter the resolution and the upper left coordinates in the ESRI World file (Google the precise format): change the filename to the same as the map image but with an extension of .JGW. Now copy the ESRI .prj file for the projection concerned (available in /Coordinate Systems from any ArcGIS install or Google it), and change its file name to the same as the map image but with an extension of .PRJ. Move all 3 files onto your device and load.

In the other cases: getting the map is primary problem, so you have to be ingenious. One solution is to take a digital photo of any metrically accurate map e.g. wall poster or a paper map. Stand with the camera lens at the centre of the poster/ map, keep the frame square onto the map and make sure the light is even across it. Take the photo, and then trim it as above. Now usually you have no idea about coordinates: so research the probable projection online e.g. on survey or ministry web sites. Or go to a map shop/ library and try to read the key details off the margins of the map. You will need to know the true origin of the projection in latitude/ longitude and the 'false origins' and the scale factor (often safe to assume this latter value is 1.0). Now you can get or create the appropriate ESRI .prj file and rename it to the same as the map JPEG. Now create the ESRI World file by working out the resolution, and just guess values for the coordinates of the upper left e.g. 100000/ 100000. Now start your GPS and see how far out your current position is from the correct one (you need to know where you are for this!). Measure the offsets and revise the ESRI World file. Keep doing this until you have a rough approximation. Now move some distance (kilometeres) with the GPS running (or load a previous track) and see if the position remains correct away from your current location. If not, then adjust the resolution... and then you will have to change the upper left value again. This can take some time, but ultimately you will get an acceptable solution.

This is a time-consuming but free solution to mobile map acquisition, IF you can get to photograph a map, and IF you guess or find out the applicable map projection details. You might need these techniques in places where there are no good topographic maps available for security or availability reasons. You might have to be circumspect about map use on your device in these situations (someone might wonder how you know where you are, or see the map on screen), and it can be tantamount to spying in some places, though it might be acceptable at home. And you should be using a small amount of mapping for personal navigation purposes otherwise you breach copyright, unless the mapping is in the public domain. Don't rely on maps georeferenced this way for anything vital, as 'wild georeferencing' will get progressively more inaccurate away from the place where you tested it.

It works for me: I have been able to track myself on remote Pacific islands and in African safari parks where positioning was really useful for planning and re-assurance.

Jonathan

Posted by raper at 03:23 AM | Comments (0)