Those of us who use the information provided by satnav companies such as Garmin and TomTom to get from A to places we've never been before tend to take the map on our satnav screen pretty much for granted.
After all, the satnav contacts three or more GPS satellites, triangulates their positions (which are known) to figure out exactly where it is and the rest is just maths - right?
Wrong. It's not that simple.
The maths places the cursor in exactly the right place on the map - but those maps are made by people, and it's a very complex, multi-layered process.
Dutch-based TomTom, for instance, starts with millions of nodes, each containing the GPS co-ordinates of a data point - which could be anything from a traffic light to a hospital to your favourite Chinese restaurant.
Then a layer of lines is drawn over these, representing the roads, rivers, railways and other geographical features.
Next, each individual feature is assigned a file of information such as road names and route numbers - but also including the speed limit, vehicle restriction such as weight limits, and specific conditions such as 'No Overtaking' or 'No Right Turn'.
On top of that goes a point of interest or services layer containing schools, hospitals, garages, police stations and even restaurants and museums.
And finally, the top layer has details of 'real measured road speeds' provided by IQ routes and HD Live Traffic services; that's more like a desktop, in computer terms rather than a memory file, because it's constantly being updated in real time.
But where does all that information come from?
The basic physical data cones from satellite images and aerial photography imagery; some of the detail comes from local authorities but for most of the road signs, features and points of interest somebody has to physically go there and collect the data.
The mapping vans are fitted with cameras, gyroscopes, redundant computer systems and very accurate differential GPS - all of which record their information on a redundant hard disk. The vans also have wheel sensors to record the distance travelled while the gyroscopes record changes in direction - which really helps in places where GPS doesn't work, such as underground roads, tunnels and multi-storey car parks.
Back at the map centre, all this information is fed into a programme called Cartopia, that gives the editor an overall life-like view of the surroundings. The cameras capture images every eight metres on city streets and every 10 metres on highways with a 360° view of all road signs and roadside features.
The images are only used in-house so no privacy issues are involved.
Six photos are taken every eight metres which amounts to 750 photos per kilometre. Image recognition software detects details such as road signs and speed limits, which are then automatically applied to the database.
Community feedback from TomTom customers via MapShare also provides input on changes in the real world which is checked against reference source material.
And that's what you see when you switch on your satnav, either hand-held or in your car. On top of the GPS co-ordinates and satellite imagery, there are layers of information gathered by people who have literally been there and done that.