A Digital Mapping Primer

Richard Munn|Published 07-10-2006


Garmen Etrex Legend C<br />
Mapping GPSThe ready availability of accurate, inexpensive GPS units and digitized topographic maps has placed a powerful tool for wilderness navigation in our hands.

The concept of digital mapping can be a bit confusing, though. This article is intended to be a basic primer - a tool for understanding the different types of digital maps and how we can use them with a GPS.

Types of Digital Maps

There are two basic types of digital maps, and it's important to understand the difference between the two. The first is Raster maps, and the second is Vector maps.

Raster Maps

Raster maps are based on conventional image files. These maps are, quite simply, scans of a paper maps that are saved in an image format like .gif or .png. They are generated by running the original topo maps through a large scanner and saving the copy as a digital file. Examples of this type of product are the maps produced by SoftMap, Fugawi, Touratech and ETopo.

raster maps are scans of the<br />
original topo maps - digital images that are identical to the originalsThe advantage of this type of map? They are an exact duplicate of the original paper topo map, with all of the detail that we're used to seeing. The disadvantage? Being an image file (just like a digital photo), they don't scale well. If you try to "zoom in" on the image, it quickly becomes pixellated and fuzzy. If you "zoom out" to any significant degree, the text and details on the map quickly becomes unreadable. Examples of this type of map includes Garmin's MapSource Topo Canada and Magellan's Mapsend Topo Canada.

Because raster maps retain all of the detail of the map upon which they are based, they are a fairly large image file.

Vector Maps

Vector maps are not an image file like raster maps. Vector maps consist of a text file with coordinates describing the various points and curves on a map. When they are loaded into a mapping program, that data is used to generate a map image.

Vector maps have less<br />
detail than the original topos they were derived fromAs you would expect, the advantages and disadvantages of this map type are the exact opposite of those of raster maps. Because the image is generated "on the fly" from the data, these maps scale up and down with no loss of clarity. You can zoom out to see an entire river system, or zoom in to see a specific areas and retain the readability of the map in both.

The only disadvantage to vector maps might be the diminished level of detail. Although there is no reason that the vector data could contain all of the detail of an original topo, these maps almost always have less detail. This is because the manufacturers are trying to strike a reasonable balance between the quality of the map and the size of the data file.

Waypoints, Routes and Tracks

Before we get much further into digital mapping, we should understand the three types of data that GPS units use.

Waypoints are individual points, described by a set of north-south and east-west coordinates. Waypoints are used to store the location of campsites, rapids, portages, fishing spots and points of interest.

Routes consist of a connected series of waypoints, describing a path along a road, river or trail. We enter routes using a sequential set of waypoints, and we can use that route to navigate as we travel.

Tracks are similar to routes, but rather than being entered by the user, they are auto-generated by the GPS unit. The unit stores a waypoint at regular intervals, building up a track that we can store. We can use this track data to navigate along the path that we've saved.

Using Digital Maps

The advantages and disadvantages of the map types above govern when and where they are used.

Raster (scanned) topo maps have high levels of detail, but large file sizes and data storage demands. This makes them suitable for use on a computer, which has lots of memory and hard drive space.

Vector topo maps have lower but adequate levels of detail, and relatively small file sites. This makes them ideal for use on a GPS unit, where memory and storage are limited. The "zoomability" of vector maps is also useful, since the tiny screens on GPS units require continual zooming and panning to view areas.

Digital map products nearly always include two components - the maps themselves, and the ability to overlay tracks, routes and waypoints on those maps. Of course, there is also the ability to move those tracks, routes and waypoints from computer to GPS unit and back again.

Let's look at the maps themselves first.

If maps are raster (scanned) maps, they are designed to be used only on the computer for viewing and printing. They cannot be moved over to any GPS unit because of the large file sizes and memory demands they impose. Even if GPS units with sufficient storage were available, they would still not be an appropriate choice since they do not scale well when the user zooms in or out.

Vector maps, on the other hand can be moved over to a GPS unit because of their small file size and ability to scale (zoom) and retain their clarity. The catch? Each GPS manufacturer has their own proprietary standard for vector topo maps, and their own mapping product will only work on their own GPS units. This means if you are a Magellan GPS owner, you will have to use MapSend Topo Canada, and if you own a Garmin Product you will have to use MapSource Topo Canada. Both of these software products allow you to upload vector maps to the appropriate mapping-enabled GPS unit. The number of maps you may upload depends on the amount of memory in the GPS unit.

The second function is to allow us to work directly with the data - the waypoints, routes and tracks.

Nearly all mapping software gives us the ability to move this data from computer to GPS, and from GPS to computer. For example, we can enter waypoints and routes on our computer, then upload them to our GPS unit. Similarly, we can save waypoints or track files while we are out on a trip, then download them to the computer. In both cases, we can view those points, routes and tracks overlaid on the topo maps both on our computer and GPS unit.

Some Examples

Let's say that you're preparing for a canoe trip. You load up the maps on your computer at home and by simply clicking on the maps with your mouse pointer, you enter waypoints for access points, campsites and rapids. You click your way along the route and save that data as a route file. When you're done, you simply upload that data to your GPS and it's instantly available for use in the field when you're wondering where the next campsite is.

The opposite works just as well. When you're out on your trip, you find a couple of great campsites that weren't described in the route description. You find a great fishing spot, or an unmarked trail that climbs to a scenic lookout over the river. It's a simple matter to save that data in your GPS unit. When you get home, you plug your GPS unit into the computer and that data is uploaded and saved. You can view those points and tracks overlaid on the maps on your computer screen.

Sharing Data

Each GPS manufacturer and mapping software company has its own proprietary format for saving GPS data. Thankfully, in the past few years an open-standard data format has come to the forefront and is being used by nearly all manufacturers. That format is called GPX (GPS Exchange Format). It is an XML based file that can be exported or imported by almost all software. This has greatly simplified the process of sharing data seamlessly between users of different software and hardware platforms.