Thomas Sutcliffe: How Casablanca put us on the map

The first thing you see in Blood Diamond, Edward Zwick's consciousness-raising thriller, is a world map, an outline of dark yellow on black with just one country - Sierra Leone - solidly blocked in. What does it tell us, this map?

Well, it tells us where the film will be set, of course - something the average multiplex audience couldn't, in all fairness, be expected to work out for themselves; even, perhaps, the Sierra Leoneans in the audience, given that Blood Diamond was largely filmed in South Africa, where the star-pampering infrastructure is a little more reliable. More significantly, though, it also tells us that the producers feared a verbal title-card was not going to be enough without a primary school visual aid. After all, to the Californian ear Sierra Leone sounds dangerously like a party town somewhere south of Tijuana.

And the fact that the entire world is shown - in the familiar unpeeling of the globe that places the Americas to the left of the frame - tells us one more thing about the producers' nervousness. They couldn't be absolutely confident that everyone in the audience would know where Africa was. That map is a chart of presumed punter ignorance just as much as it is a scene-setting device.

Still, it had a certain nostalgic charm to it, redolent of a pre-globalised world in which virtually everything was exotic, not to mention of a movie culture in which the promise of exoticism was a big part of the sales pitch. Movies used to begin with maps much more often than they do now - a trend which I might as well admit I can't back up with hard facts and figures (map openings down 3.7 per cent year on year from 2005), but which would seem to go with the flow of changing cinema fashion. You very rarely see turning-page titles any more either - another familiar cliché of Forties and Fifties Hollywood that has fallen victim to the impatience of audiences ("Cut to the chase!") and, I fear, to their assumed hostility to books. What both tropes did was to suggest, subliminally if not openly, that older forms of information (or "content", as we might say now) were being superseded by a newer one.

Perhaps the most vivid example of this comes not from film culture but from television - that dependably delicious moment in the titles for Bonanza when the flame burns through the old parchment map of Virginia City, allowing you to see through to the Cartwrights galloping towards the camera across a Nevada meadow. It's as if the moving image is so hot that it scorches through the paper itself, but the map isn't simply there to be displaced. It evokes a time when the map was still mostly blank spaces - and implicitly promises that television will fill out those voids with something vividly real.

And here, as elsewhere, the plain functional purpose of a map opening - to let you know where the action will take place - bleeds into a more poetic effect. From the very beginning, cinema has always been about transport, taking the viewer out of a darkened room and into the world. Hardly surprising, really, that both RKO and Universal Pictures should have used the globe itself as a studio logo - suggesting to the audience that the entire world would be their oyster.

The question of whether it was there to be consumed or understood is another matter entirely. That can be another implication of maps, of course; that they introduce a film that is not an escape from the real world of the audience, but a guide to its fractious borderlines. The opening of Casablanca, with its animated line drawn from Paris, through Marseilles and Algeria to the city of the title, claims for a romantic melodrama some of the contemporary urgency of a documentary (in fact, this sequence also uses real documentary footage of wartime refugees to press the point home).

I take it that the map at the beginning of Blood Diamond aims at something similar, locating the action of the film in a particular place and locating the film itself in the genre of instructive entertainment. Just as news reports begin with maps, so too does this - doing its bit to piece together the jigsaw of geopolitical relationships. In practice, though, Blood Diamond turns out to have a slightly uneasy relationship with what Conrad's Marlow called "the blank spaces on the earth" - unquestionably well-intentioned in its attempt to shine a light on a dark story, but also unable to resist the dangerous frisson of a place that will effectively remain off the map for most of its domestic viewers. Sometimes maps aren't an encouragement to travel, just a reminder of how much better you will be if you stay at home.