You may question what possible relevance this information has for you.
I would reply that I found myself agreeing to make not just one, but two scratch-and-sniff maps of New York City almost entirely by accident earlier this year. And even if you are more sensible than I am, and would never agree to such ill-informed adventures, it turns out that the magic of scratch-and-sniff map-making opens an entirely new perspective on the world -- an unexpected and invisible cartography of emotion, confusion and memory.
The first step, of course, is to decide what smells you are mapping. (The rest of the how-to comes a bit further down the page). Perhaps the earliest attempt to make an urban smell map dates back to Paris in the 1790s, when new ideas about both political equality and hygiene combined to send physician Jean-Noël Hallé on a six-mile odor-recording expedition along the banks of the Seine. His map-making technology consisted of nothing more than a notebook and pencil -- and, of course, his nose.
Today, artist Sissel Tolaas is the world's pre-eminent olfactory cartographer. She is midway through a survey of Kansas City, having already smell-mapped Paris, Vienna and Mexico City, with Nuuk and Calcutta next on her list. Although Tolaas still relies heavily on her nose, which she has spent years training, she also has a couple of important tools at her disposal, as she described her urban smell mapping process to me:
I try to go back to sites a couple of times to identify which smells remain over time and which change. If I have the time, I go back in different seasons and in different weather, so I can see which smells are important, underlying smells and how they change in different conditions. And I focus on finding the source for smells. If the source is definable and tangible, I can grab a sample and do a chemical analysis using a GCMS [a gas chromatograph/mass spectrometer] in my lab. If the source is not something that's portable, and if the budget allows, I will bring headspace technology into the field.