I spoke with ITMBs chief cartographer recently to discuss the option of using digital forms of their maps for use with raster based GPS software such as OziExplorer and MacGPS Pro. She told me that while they attempt to keep their maps as accurate as possible and continue to update them with new information, the scales they are usually produced at are too large for such a purpose (typically country maps range from 1:500,000 to 1:1.500,000). She then told me that they do sell digital (tiff image) versions of their maps on special request, for about $30.00 each. Their hardcopy maps sell for between $8 and $12 dollars each.
To prepare for our latest trip through South America, we purchased a hardcopy of each of the country maps that they produce. I scanned each map on a large format scanner, and calibrated them for use in OziExplorer. (we have access to a large scanner for free - many companies will charge over $20 for double sided scan - it might be cheaper to buy the digital versions outright).
In either case, you'll need to calibrate the map images in your GPS software. Read this article for our Video How-To explaining the calibration process in OziExplorer. The basic settings are:
- Use the full number (9) calibration points.
- As a map datum select Pulkovo (1942).
- Use a Lambert Azimuthal Equal Area Projection. This is typical for maps in the southern hemisphere. This projection requires a central meridian and latitude origin to calculate correctly.
- Choose the central meridian as the longitude that is central on the map.
- Experiment with a latitude origin that works the best. Check it by turning on the gridlines drawn by the mapping software and compare them to the gridlines on the map. If the lines don't fall on top of each other then change the latitude origin. In general, the latitude will be around the middle of the map.
So, how well do they actually work? It varies. Sometimes the software gridlines fall perfectly on the map gridlines, but the cartography varies slightly from the actually GPS track. Sometimes both are offset, and somtimes it is all bang on. The following images are screenshots of some of our tracks in Colombia and Ecuador, using OziExplorer software. The following text describes them - click the images for full-size screen shots.
The first image is a portion of an ITMB map of Ecuador, with an OziExplorer track overlaid on top. You can see that the acuracy of the track is pretty good, for the most part following the roads. This track follows major highways and smaller unpaved roads.
The second image is a portion of an ITMB map in Colombia. For some reason the cartography is shifted quite a bit - the gridlines fall properly on the map, but the OziExplorer track doesn't follow the roads.
The last image shows the same track, but this time overlaid on the Russian Military Topographic map. In this case the Russian map is incredibly accurate.
So for us the moral is to have some redundancy in your mapping information. You never know how good the data will actually be in the field. We have the original hardcopy versions of the maps, the digital scanned versions, calibrated for OziExplorer, and finally a complete independent map set. If you haven't already, take a look at our Russian Topographic Maps of South America, pre-calibrated for OziExplorer. You can download them from our site.
