31 August 2014

How to create a raster chart for the Magellan Explorist GPS

Raster charts are the best of both worlds. You can get the most accurate graphic description of the Earth's surface existing today - in the form of topographic charts - adapted to be read by the modern portable GPS on the market.
Certainly today we have also the Vector charts. And these are, probably the future in digital cartography. These are digital files made from scratch and presented in the GPS display just like that. I mean: you don't have a paper look-a-like image. Instead, what you have is a digital rendition of the terrain. This is, certainly, the future. You can control the information you want to see, therefore avoid crowding the GPS displays with too much or excessive information. For example, and since these modern charts work by "layers", you can clear, momentarily, the exhibition of roads, or cities, or man-made constructions such as power lines, etc., thus giving a clearer presentation of the natural features of the ground. As good as they are, these charts (normally bought in digital folios - packages of charts of a specific Earth region) are not cheap. Quite often just a single folio (for example: Northwestern Africa or US West National Parks) can cost as much as the GPS receiver itself. And you still have to face the cost of the cartography updates, if you wish to be... updated.
So, if you have a folio of old topographic paper charts at home, its easily understandable that you might wanna use them on your portable GPS receiver. After all, even if they are outdated, they still remain accurate. Changes in topography are mostly of human nature. Th natural world, except some abnormal catastrophe, remains the same. Therefore, there is absolutely no reason not to use your accurate topographic charts on your GPS unit. Even if they are twenty years old. Hell, here in Madeira I'm working with Portuguese Army topographic charts, in a scale of 1/25.000, which survey was made in the late fifties and sixties. If they are good for the Army, they certainly are good for me.
How do we do this? The magical conversion from an accurate paper topographic chart to a similarly accurate digital topographic file?
Well, it ended up to be easier than expected. I'm no expert, but here is how I did it:

1. Scan your paper chart (or part of it) in a flatbed scanner and save the resulting file (picture) in .jpeg or .png format. Strangely I've found the jpegs files not color accurate on the GPS. The files in this format always shown a strong magenta cast and the colors of the original chart are lost on the GPS, giving us a monochromatic image in magenta. The .png files seem to work OK.
2. Download this file onto the free program RMP Maker and calibrate it, using for that a minimum of three geographic positions, widely separated on the picture. Don't forget that the chart datum of the chart must be the same you are using for the calibration process. Otherwise, you'll always have a positioning error, up to several hundreds of meters. Nowadays almost all the topographic charts are referred to the WGS 84. But many of the old charts are not. Create one .rmp file, already calibrated on the above-mentioned program. Now, if you have a Magellan Triton GPS receiver, at this stage you just have to export this file to the GPS and it should work faultlessly. But if you have one of the latest Explorist family, and since these equipments use different file formats, you have one additional step to make. Convert the .rmp file in a file readable by the Explorist:
3. Convert the .rmp calibrated file into a bak.rmp file (readable by the Explorist) in the free program RMP Tools.
4. Download the resulting file directly to the Explorist Maps Folder, by USB cable connection, either going directly by Windows Explorer to its directory or using the free Magellan software VantagePoint. It should work perfectly, as you can see on my previous post, about the Magellan Explorist 610.

Once again, I tell you that I'm no expert in the field, but I've found the process quite manageable, even for an ignorant like me. All the software mentioned is intuitive, and you can easily get the grip of it in just a couple of hours.The most important part of the process being the digital chart (file) calibration. If this is done right, you'll have an accurate raster chart on your Magellan Explorist. Good luck.