It's the dots that do it, on the "i's". Except that they're not dots at all, but diamonds. The adornments on an otherwise beautifully simple typeface are an identifying quirk of Johnston Sans, a font that is as synonymous with London as Big Ben. It has become known as the "handwriting" of the city, but can you identify it? Did you even know its name?

Antony Harrington is obsessed with type. It is partly the job of successful fonts to be invisible and as such they are usually overlooked. Johnston Sans, as it happens, is the font of London Underground. Harrington wants everyone to know this and is inviting fellow enthusiasts to draw a typographic map of the capital. He wants to use modern technology to record great or endangered examples of lettering to show the unique and quiet way the words around us can help shape the identity of a place and how we feel about it.

I meet Harrington, a partner at a branding and design company in north London, outside the Covent Garden Tube station, where lunchtime shoppers steer a course around two men behaving strangely. We are doing what few Londoners ever do, looking up to admire the Underground's unmistakable roundel sign, as well as the more ornate typeface used on the station's façade. After a few minutes I reach for my phone, start the app Harrington has devised and take a photo. I write a caption and upload the image to the London Typographica website, where it is added to a map of the city now dotted with examples of type.

Harrington admits to being a font geek, but says there's a reason we should all look with fresh eyes at the words in our own towns and cities. "Typefaces work well as little milestones," he says. "They anchor a building to a time and a function, whether it's commercial or social, and this is a heritage worth preserving."

Edward Johnston was commissioned in 1913 to design a unifying font for an underground network still made up of lines owned by different companies. Using a quill at his studio in Sussex, he ignored the conventions of the time to design a typeface of startling simplicity. It was among the first functional fonts to communicate only information but nothing about class or education and first appeared on posters in 1916. It has survived minor tweaks to appear as clean and as modern today as it did on the drawing board 100 years ago, diamonds included. Johnston also inspired, directly or otherwise, several other sans serif fonts (serifs are the little flourishes that appear at the end of the strokes in, for example, Tiempos, the typeface you're reading now; it's Georgia if you're online. Sans serif type does away with these marks).

Gill Sans, designed by Eric Gill, a student of Johnston's, became the font of the old British Railways and, now, the BBC logo. Helvetica, a Swiss font designed in 1957, has been embraced by corporations and cities and, in New York City, is as strongly associated with the subway as Johnston is with the Tube.

Simon Garfield, author of Just My Type, a book about fonts, says the letters on signs as well as shopfronts and buildings have a crucial and increasingly overlooked role. "You should be able to be parachuted into any city in Europe and know where you are instantly from seeing the typeface," he says. "You can land at a London airport and know you're there because you see Gill Sans everywhere." But Garfield says the power of type to characterise a place is threatened by the digital drop-down menu. "Since computers arrived, type has become internationally homogenised," he says. "In the old days someone would come up with a typeface and great metal blocks had to be carted around. Now someone designs a typeface in Hawaii and gives it to a big typographical sales bureau and it's all over the world within a day.

"If you're branding a place today you go with the font everyone else has gone with – and only a few of these have an impact."

In some cases the fonts that once marked out cities face a physical threat. From Covent Garden Tube station, Harrington guides me down Long Acre, one of the area's busiest shopping streets. We're following a route originally charted by Phil Baines, a professor of typography at Central Saint Martins College of Art and Design. "Can you see it?" Harrington asks as we stop to look up at a branch of H&M. "See what?" And then, like a camera coming into focus, my eyes begin to see the ghostly remnants of words in the stone: "London's favourite fruiterers, T Walton & Sons (London) Ltd."

The shapes are the shadows left by metal letters that once identified this handsome building, which stands a few cartwheels away from the old Covent Garden fruit and vegetable market. "It won't be long before that disappears completely," Harrington says. We take a photograph and record the ghost font on Harrington's website, before continuing our walk.

If the modern typographic landscape fails to distinguish cities as it once did, one man is trying to preserve the role of the font, casting himself as a modern-day Edward Johnston. Jeremy Dooley is a type designer who lives in Chattanooga, a small city in the southern American state of Tennessee. Once known as one of America's most polluted cities, Dooley says it's now a thriving centre for a creative industry that struggles to separate itself. In 2012, Dooley wondered, could a font help a city make a comeback?

Dooley joined forces with a fellow type designer, Robbie de Villiers, to create Chatype, unveiled this year. "By creating a custom font for the city we wanted it to immediately say, in every application, this is Chattanooga," Dooley says. "We wouldn't even need to say it – it would be subliminally communicated."

Dooley says the design of a city font is "as important as picking the colours in a flag". His font combines the city's Native American heritage with its industrial past and entrepreneurial, creative present. Chatype has already been used by a handful of Chattanoogan businesses and Dooley has had good feedback from the city's authorities.

"Chattanooga already has a vernacular language but not one that distills down into a font," Dooley says. "And we're big on distilling in Tennessee."

It was not always clear what the designers of the last font on Harrington's tour were trying to achieve. We have walked from Covent Garden down St Martin's Lane, where we nip down May's Court, an alley, to admire the vast Art Nouveau letters built in stone which mark the London Coliseum, the home of the English National Opera. At Trafalgar Square we pause at the clock installed to count the seconds until the 2012 Olympics. When London revealed its Olympic logo in 2007 it was ridiculed.