"The problem is that rent is expensive, and then you have to pay for a model. So I thought, why not go outside and use the outdoors as our subject? It's a low-cost hobby and the tools aren't expensive either," he says.

There is another advantage. Since Hong Kong's urban landscape is constantly changing, Cheung targets neighbourhoods that are forgotten or ignored and worth remembering. So the group is helping to capture on paper parts of the city that may soon fall prey to redevelopment.

"We're in Sai Ying Pun today because it will have the MTR soon, and shops and buildings will slowly disappear. We want to let people know about these places," he says.

Sketching shiny corporate landmarks such as the IFC and Pacific Place doesn't appeal to Cheung's group because these buildings can be found in any big city. Older neighbourhoods, with their and local shops, are the attraction.

"We want to draw places that are very Hong Kong," says Cheung. The group has even gone out sketching in the evenings, with the aid of small LED lights attached to their sketchbooks. "The night scenes with neon lights are also very Hong Kong."

Not all members of the group have a background in art; some have only recently picked it up. Cheung says anyone is welcome to join. Members are encouraged to look at each others' work at the end of a session, which helps to give them insight into different techniques and styles.

"The most important thing is to have the guts to actually draw. You are not making a masterpiece, so just enjoy the process," he says.

Queenie Chan Kwan-lei flips through her sketchbook and remembers places she's drawn.

"You sit for a while and draw what you see. I can close my eyes and remember every little detail. It's like touching the building because you feel a connection. Lots of places I've drawn are now gone. This one is Pok Fu Lam village," she says, pointing to a drawing. "A resident invited me to eat some turnip cake."

Another of Chan's drawings depicts an elderly man in front of his tiny stall in Sham Shui Po.

"I asked if I could draw him and he was very eager, pulling up a chair to pose for me, because no one had ever drawn him before," she says. "I promised to scan this drawing and send it to him."

Cheung says residents passing by sometimes stop and look, while the chattier ones offer a historical context or anecdotes about the place. This kind of interaction makes the sketching experience more meaningful for the group.

Such is the popularity of the pastime that another group, Urban Sketchers Hong Kong, was formed in April last year. It held its first exhibition last month at Boom cafe and gallery in Tai Ping Shan. The works included drawings and watercolours of places such as PMQ, Wan Chai and City Hall.

The group has attracted more than 1,000 Facebook friends in the past few months after absorbing another, smaller sketching group and being featured in . This doubled the average number of attendants to 20 or more, ranging from full-time artists and illustrators to lawyers and pilots.

Sketching the city is a pastime that is also popular in other places in the region.

"Singapore has several thousand urban sketchers, while Taiwan has 6,000," says Urban Sketchers co-founder Gary Yeung Fuk-chi, a primary school art, English and music teacher in Tai Wai.

The group not only draws historical sites, old villages or buildings that will be torn down, but also attends events such as Tin Hau's birthday, Classic Car Club shows and Chinese opera performances in West Kowloon.

The artists also record changes in the skyline, as can be seen in one of Yeung's sketches of To Kwa Wan with Lion's Rock in the distance.

"Before, when we had Kai Tak airport, the buildings had to be low, but now the landscape is changing with new buildings that are much taller."

The original Urban Sketchers group was founded in the US in 2007 by Gabriel Campanario, a journalist and illustrator at

The group has a manifesto that includes rules such as: drawings are made on location, capturing what the sketcher sees; they are truthful to the scenes they witness and can be drawn using any kind of media. The sketchers support each other and share their drawings online. Drawing on location is an essential part of the experience.

"Some take pictures and then draw it out, but that's too easy," says Yeung. "When you use pencil and paper, you have to think about what you see in front of you.

"If you take a picture you don't remember, but you remember what you draw."

Co-founder Ben Luk Kin-bong points out that three sketchers can be in the same location and draw three different pictures.

"You see so many things so you need to think about your composition. I have to walk around and see what I find eye-catching, or have a feeling for a particular spot - it could even be something as mundane as a broken drainpipe."

Another co-founder of Urban Sketchers, Alvin Wong Kai-chung, says when he travels, his sketchbook is his travelogue, a collection of sketches of places he has visited with ticket stubs attached to them. "I don't remember what I ate, but I can remember what I wore, who I was with, and I have a better understanding of the place having drawn it."

Wong says his technique has improved greatly in the past few years and that has helped his career as an architect. "It's a skill everyone in my profession should have," he says. "Now, when I am at meetings, I can make quick sketches for clients or colleagues to explain my ideas. I'm polishing my draughting skills."

Drawing with a pen or pencil and paper also helps people express themselves, he says. "When we write nowadays, we use a keyboard and computer, not a pen and paper. It's not often that we use our hands to create something."

Sketching can be done anywhere, at any time, group members say, from attending a classical music concert to waiting at the airport for a flight.

However, not everyone appreciates the sketchers. "Some building security staff don't welcome us," says Luk. "They tell us we can't be there even though we are on public land. Maybe they think we are too close to their building."

Yeung agrees. "Even in Nan Lian Garden [across from the Chi Lin Nunnery in Kowloon] the security have told us we can't draw there because it's for people to appreciate the landscape."

Located adjacent to City Hall, the gallery is an exhibition space for planning proposals and infrastructure projects. It also documents changes in Hong Kong's coastline and skyline, and looks at sustainable development, travel and communication, and urban renewal.

"I saw Urban Sketchers Hong Kong's exhibition and wanted to collaborate with them," says Jane Kwan Wai-ling, a senior town planner at the department. "Planning is not just about new developments but also preservation and protecting our heritage."

Wong hopes Urban Sketchers Hong Kong can eventually compile a book of their work. "We want it to be like a sketchbook, but also a tourist guide, too, showing the different districts and with a brief introduction of the areas. We wouldn't be like other tour guides, where you have to get the most updated one. For us, it's about remembering what used to be there, because 10 years later many buildings will be gone."