A couple of years ago I acquired a guidebook to Lombardy and Piedmont published in 1914 in which I was especially delighted to discover a map of my native Milan drawn in the distinctive and attractive style of the Touring Club Italiano publications. Looking for familiar places, I came across the road in which my father spent all of his working life and found that a river ran through it.

I knew about this: the blue line at the top left of the picture is a tract of the canal known as the fossa interna (‘inner ditch’) that was paved over in 1930 to aid the circulation of cars. What gives this cartographic detail some personal significance is that my father, when giving directions to unsuspecting motorists, always referred to that intersection as il ponte (‘the bridge’) in spite of the fact that by the time he was born it had already been removed, and the canal covered. But there they are.

I was reminded of this map last September when I visited Christchurch and heard landscape architect Di Lucas talk about the many waterways that had been paved over and canalised to make way for the growing city, some of which burst out of underground pipes and through levies following the devastating earthquakes that struck the city in September of 2010 and then again in February of 2011. The event at which Lucas spoke was entitled The City as Memory and had been planned since before the first earthquake, when that phrase would have had a different and altogether less resonant meaning. On this day we were shown amongst other images a map of central Christchurch in 1850 with those waterways still in place. The memory of a city also consists of these layers, of the things that are hidden from view but haven’t gone away.

Detail from Edward Jollie’s 1850 map of Christchurch. The full map is available at Te Ara

Then there are the things that are no longer there. It was Christchurch blogger Andrew Dean who first alerted me to CityViewAR, an application developed by Canterbury University’s HITLab that allows users to view pre-earthquake images of the city by pointing their smartphone to a building or the empty space previously occupied by a building.

It is not for me to feel one way or another about this picture. I never even visited Christchurch before the earthquakes. However there is something about the power of the app, its being – if it makes sense to use this word – so utterly persuasive, that gives me a little pause. It’s not that I can’t see the use for this, and not just in terms of the experience of using the app on the ground, which for some of us would amount to a questionable form of high-tech tourism but for others would be altogether more layered and meaningful. There is also, beside this immediate and private use, the promise of future public uses: the adding and the sharing of data that will hopefully make CityViewAR a useful social tool for rebuilding the city, as well as a repository of crowdsourced historical information on what was lost (for which there is at least a known working model in hypercities). It remains to be seen whether and to what extent these aspects of the project will eventuate, but they do at least offer a glimpse into how cyberspace could become a shared discursive environment in which to collectively remodel regular space.

But there is another side to this: for that intensely private act of seeing the past through a digital lens – in what pointedly goes by the name of ‘augmented reality’ – also creates a privileged space of memory in which the initiated is able to apprehend the past directly, seemingly without mediation. What makes this experience limited and exclusive is not that it requires ownership or use of a smartphone per se (although as a class and generational barrier it is not insignificant) but rather that ownership and use of a smartphone construct a subject that is predisposed to experiencing the world through that particular lens. It is the most banal observation imaginable – that technology isn’t neutral – and of course it applies just as much to the latest Android app as it does to a map from 1914 or 1850. But older technologies have had more time to sediment. They too have created their layers, like the city itself. And we are much more capable of identifying their products as representations, as opposed to virtually indistinguishable simulacra that blend in with the reality that surrounds us.

I am of the firm opinion that the greater and more urgent problem – and not just for Christchurch – isn’t to devise ever-more sophisticated ways to read and write the city, but to share them along with the ones that already exist. That is to say, to invest in democracy and participation to radically alter the way in which our communities are (re)constructed. However the resonance of title of those talks, ‘the city as memory’, lies in a vital challenge: to recognise that the past is inscribed in the city itself, in the highly complex system formed by the land, the buildings and the people, and that this complexity resists straightforward readings, no matter how persuasive. Imagine if you could walk though the streets of your own or any other city, point a smartphone at a particular space or structure and generate an image of what it looked like at any time in the recorded past: think of how exhilarating it would be, in the most genuine sense, but also of how much context would be missing from those images. And think of the layers that would still be hidden from view, all the things that no standalone piece of technology could ever show you but memory is also made of. This too we must carry forward, somehow.

On the city and memory, see this exceptional post written by Lara Strongman two weeks after the February 2011 earthquake. Simon Sellars has discussed the role of art and architecture in the regeneration of Christchurch for the Australian Design Review.