GPS Maps

A graphical image (such as a jpg file) is just a picture. The technical term is a Raster image. When it comes down to it, it's just a collection of different colored dots. This sort of picture isn't much use for digital mapping for a number of reasons

1. It doesn't scale well. Most GPS units allow you to zoom in or out. If you zoom in far enough, those dots that make up your picture are soon going to become very obvious.

2. It's not easy to interpret. If you tell your GPS you want to find the Hotel California, how is your GPS supposed to know which of the colored dots you are talking about?

For these reasons and others, the sort of image/maps you send to your GPS is completely different. It's not really a graphical image in the conventional sense. You may hear it called a vector image but even that's not entirely accurate.

It's probably best not to think of it as an image at all. It's more like a set of instructions of the form:

• Victoria Station is at OS grid ref. TQ 289 790

• A street called the Boulevard Jacquard starts at coordinates 50.952521N,1.852141E then goes to 50.951583,1.853148 then to 50.951129,1.853509 etc.

A GPS Digital map is a collection of:

• Lines: Any linear geographical feature such as a river or road.

• Regions: Any geographical feature, like a lake or town, which has a definite "inside".

• Landmarks: Any single point item like a spot height, building etc.

• Points of Interest: These are also single point items like Landmarks. The only real difference is that you can search for Points of Interest using your GPS unit's find menu. Landmarks are just background information.

The distinctions between these classifications are not always clear cut. You might choose to represent a lake, for example, as a single point or a region depending on how detailed a map you are making and what zoom level it is to be viewed at. A river might be a line or a region depending on whether its width is significant at the target zoom level and so on.

Note in particular that things like contour lines may form a closed loop, enclosing an area on the map, but they're still lines, not regions. That's because the thing they represent (a specific elevation) only exists where the line is, not inside the area it surrounds.

Every map item has a type which describes the sort of geographical feature it represents. For example, a line's type could be Major Road, Railway, River etc. Optionally you can also give each item a name. For example, your Major Road, might be called the A31, your river could be the Thames or the Severn etc.

Finally, but most importantly, the map has to specify where everything is. The position of a Landmark or Point of Interest is a single pair of geographical coordinates. A line is made up of two or more (usually many more) pairs of coordinates which your GPS can join up like a child's dot-to-dot picture to make the correct shape on the GPS screen. A region is similar. It has three or more pairs of coordinates which define its boundary.