Those spots are in fact some of the city's densest neighborhoods, its favelas, or shantytowns, that blanket entire hillsides. Though they've long provided most of the city's affordable housing, government officials have traditionally considered them eyesores and literally left them off the map, condemning millions to legal invisibility.
Now, those communities are being charted after decades of informality, each route and alley outlined and their names researched. A nonprofit organization run by current and former favela residents called Redes da Mare kick-started the first mapping program in the grouping of communities known as Mare with a simple but powerful goal: putting their homes on the map, with named streets, zip codes and official addresses.
Being left off had meant whole neighborhoods were unable to receive mail at home. It had also blocked people from giving required information on job applications, getting a bank account or telling the police or fire department where to go in an emergency call. Favela residents had to pick up their mail from their neighborhood associations, and entire slums housing a small town's worth of residents had to use the zip code of the closest officially recognized street.
Getting an official address represents a fundamental step toward real citizenship and helps break the stigma and abandonment that has marked communities, said Redes director Eliana Silva. At heart, she said, the effort erases the barriers between the formal city and favela neighborhoods, which house one in five cariocas, as Rio residents are called.
"The right to the city starts with making visible these areas that historically were not on the map," Silva said. "Putting the community on the map says, 'These people exist, they're here, with the same rights as everyone else.'"
Maps aren't simply data culled and presented objectively, said Jason Farman, a University of Maryland professor who researches mapping and digital media. They also represent the perspective of a cartographer, corporation, organization or government.
"For a community to be left off of the map is the equivalent of saying that the community doesn't matter," he said. "It removes a vital part of their identity."
Theoretical debates aside, those who live in Mare are celebrating the practical benefits of the blue-and-white ceramic street signs going up on corners.
The nonprofit used the same methodology as the government's Institute of Geography and Statistics to survey the complex of 16 favelas housing about 130,000 residents. It then produced a slick guide, distributed free to residents, that includes not only street names but the history of the original smaller favelas that make up the community. The guide offers information about the people some streets are named after, while leaving some blank, to be filled in later by residents.
Daniel Remilik, a Mare native, recently watched workers mount a sign on Jose Caetano street, noting that its namesake was a barber who helped newcomers get settled. Remilik had helped the nonprofit group interview locals to figure out street names throughout the sprawling slum, which stretches between two of Rio's main highways and is routinely crisscrossed by young men on motorcycles armed with assault weapons who openly sell drugs.
Remilik said the work taught him a lot about a community he already thought he knew well.
"I love this place, I grew up here," he said. "Seeing it recognized like this, on a map, with street signs, makes me proud. I can look up and think, I helped do this."
Doralice de Freitas, who has lived in Nova Holanda, one of the favelas that make up Mare, said her neighborhood will be like anywhere else in town with the maps and street signs, which are produced in Redes da Mare art classes.
"Before, if we went somewhere we didn't know, we'd have to go asking everyone, 'Do you know where this person lives?'" she said. "Now, we can do it like anyone else, have a street name and a number, look it up on a map, and go."
The favelas' new visibility hasn't come without controversy.
Some cariocas have complained that Google Maps exaggerated the size of favelas, giving them undue prominence while ignoring established residential districts and making the city look like an "agglomeration of shantytowns," reported Rio's biggest newspaper O Globo last year. The city's tourism secretary called the online maps "absurd" and demanded Google modify them.
The company's local spokesman responded that they had never intended to defame the city and promised to label tourist sites and forgotten districts. They also pledged to change the website's design so that the names of favelas appeared only when users zoomed in.
For decades, many were happy to simply ignore the slums.
A 1937 city ordinance decreed they shouldn't appear on maps because they were "temporary," and the middle and upper classes shrugged them off as unsightly but convenient sources of cheap labor.
That disregard was replaced in the 1980s with fear when many favelas were taken over by the drug trade and violence escalated. By the 1990s, nascent urbanization programs began mapping main streets in well-established communities, but left off vast areas.
Safety concerns, however, have taken center stage with the 2014 World Cup around the corner and 2016 Olympics not far behind, and authorities had to pay attention to the slums. State and city police have been taking over select favelas, reclaiming territory that had been forfeited to drug-dealing factions and re-establishing the state's authority.
Such mapping projects became possible with the police takeover, said Vinicius Gentil, who is leading the effort's expansion to 120 police-controlled favelas through the city's urban planning arm, the Instituto Pereira Passos.
The scope is ambitious and the territory hard to navigate, encompassing 400,000 residents in neighborhoods until only recently occupied by armed drug dealers. Winding and narrow alleyways are sometimes so dark that naked light bulbs substituting as street lights must often stay on during the day.
The institute has tackled the challenge by pairing professionals such as historians, sociologists and social workers with residents to tap both insider knowledge and outsider training, Gentil said.
"This partnership also encourages a constant exchange between residents and outsiders, and that too helps do away with this idea that the favela is a separate territory," he said.
The institute's teams, which are trained by the Mare mappers, set off on their task with basic Google Maps satellite photos of the community, and any available basic maps showing the main streets. From there they spread out into labyrinthine alleyways, dead-end roads and other passageways.
Along the way, they note neighborhood needs such as sewage systems, sports facilities and child care centers with the idea of directing city and private resources there, Gentil said.
Finally, geographers double-check it all. Street names only enter the official database if they aren't already taken, he said. Getting a zip code assigned means navigating even more bureaucracy. But it's all worth it in the end, when the street signs go up.
"It's a long process," Gentil said. "But what is important is that it is the same process the rest of the city goes through."