There is the Manhattan most people know and the one that actually exists. The one they know stops at 96th Street, parallel to the tip of the Jackie Onassis reservoir in Central Park. The one that exists extends north through Spanish and black Harlem, and then on into Washington Heights, where the traffic and the wallets are thinner.
East 100th Street, wedged between Third Avenue and the train lines that skirt Park Avenue, is perched on an incline offering a view of both. It sits just four blocks from the invisible border drawn by class, race and ethnicity. Geographically close and culturally distant.
When Bruce Davidson arrived with his old camera in 1966 and said that he wanted to record life on the block, the local citizens' committee was apprehensive. Davidson had first noticed the area 10 years earlier, when he'd been staying with his parents in upstate New York and was commuting into town to work as an assistant in Eastman Kodak's studios.
"From the window of the train, I could see into the windows of the buildings in East Harlem, catching momentary glimpses of life in those dwellings," he writes in the afterword to a collection of his photographs. "Even then, I wanted to get behind those brick walls and to encounter the invisible." He was not made immediately welcome.
"New York is made up of thousands of hamlets," he says over tea and biscuits in his Upper West Side apartment. "On 96th Street, you were in the world of white wealth, but East 100th was notorious as the worst block in the city. So they were suspicious of people coming and photographing, and then going away again without leaving any kind of record."
The committee agreed to let him take one picture of his choice, and they would then look at it and decide whether or not they wanted him around. Davidson asked for the largest family on the block. They pointed him in the direction of a family of 10. It took him two weeks to get them all together in the same place. The committee was impressed.
"They thought I would take the picture and run," recalls Davidson. "That I wouldn't stay around long enough for the whole thing to be of any use. They were worried that I wouldn't respect their dignity."
Dignity is a word never far from Davidson's lips. Talking of the photographs he took over the two years he spent chronicling life on the block, he says he tried to strike "some sort of balance between dignity and destitution". He wandered the street with an old-fashioned large format camera, complete with tripod, bellows and black focusing cloth, because he wanted depth and detail, and became part of its scenery. The children called him the "picture man".
"People had to present themselves to the camera," he says. "I would never concoct a situation, but I would pose them. I didn't feel that I was there to photograph the other. I felt I was close to them and empathised with them, and I think they learned about me, too."
Now, looking around his sparse but elegantly furnished Manhattan apartment, it is difficult to divine from where the material basis for that empathy might have come, but back then he was at the start of his career. "It taught me how much I'd taken for granted," he said at the time. "I'm not wealthy by any means, but by contrast I am. I have hot water. I don't have 10 children to support."
Davidson grew up in Oak Park, Illinois, a suburb of Chicago. As a 10-year-old, he visited a friend's "dank basement darkroom" and saw a picture take form in a tray of developing fluid. "Out came an image," he says, "and I thought that was great." He took to photography "like Heifetz to the violin. It was natural to me."
Take a look at the portfolio that developed from that time and the human basis for his interest in East 100th Street becomes very clear. His first major project was gang members in Brooklyn. "They were moody, depressed and invisible to the city. I was 25 at the time and I suppose there was something in that I connected with."
Then came several years with the civil rights movement - from the Freedom Rides of 1961 to the Bloody Sunday march from Selma to Montgomery, Alabama, in 1965. After East 100th Street came projects on the New York subway and Central Park. With 1965 bringing the passage of the Voting Rights Act and 1967 heralding major riots in Detroit, Newark, Boston and Milwaukee, 1966 would seem like the perfect year to move from the rural south to the urban north, but if there was a broader purpose to Davidson's choice of subject matter, he makes no claim to it.
His subject matter is political, but his motivations are more humanist and artistic than anything else. "I never work out a good idea of where I'm supposed to be going next. I'm a loner in that respect. I suppose I favour the underdog. I like to explore and see where I end up."
What tied together all of Davidson's work of that time was his desire to stay as long as it took to find the dignity in his subjects, rather than make a polemical argument. "Growing up in Chicago, we never said it was cold in winter, because it was obvious," he says. "So, while they never had material things, they had spiritual things. I was there for a long time. I didn't see their poverty, but I felt their possibility."
Since he took the East 100th Street photographs, he has returned over the decades to keep in touch with friends, to record family reunions, and to provide pictures for a fund-raising brochure.
The block was in a state of transformation when he first arrived, with local activists working to improve the impoverished living conditions and economic prospects of its inhabitants who were mostly Hispanic (particularly Puerto Rican) and black.
Recently Mexican immigrants have added to the mix. Now, with the building fronts renovated and some of the old tenements pulled down to make way for a lush community garden, it is not difficult to detect the progress that has taken root over the past 37 years. Gentrification has seen to it that the borders between rich and poor are now blurred. The linguistic boundaries are equally flexible. The shop fronts and "No trespassing" signs are in both English and Spanish, and the children heading for school move easily between the two languages.
"You gonna get yo' ass whupped," one angelic-looking girl in a neatly pressed uniform tells another. Her friend responds in Spanish, then a chase begins, passing the Spanish-speaking Jehovah's Witness church. Another scene for another photograph.
· East 100th Street, by Bruce Davidson, published this month by St Ann's Press, priced £50. To order a copy for £45, plus UK p&p, call the Guardian book service on 0870 066 7979.