On a Google map, the long stretch of Florida coastline from deep South Miami-Dade County to Sebastian Inlet appears a seamless mass of urban development jammed between a thin border of sand on one side and wetlands and farmland on the other.

But zoom in and it’s soon sliced up by lines both real and imaginary: roadways, highways, railways, waterways and the boundaries of numerous, and often overlapping, governmental jurisdictions.

Now this vast area, at once connected and disconnected, is the subject of one of the most ambitious planning efforts ever undertaken in Florida. Called Seven50, it aims to chart a coordinated, integrated future for the development of Southeast Florida’s seven counties for a couple of generations, through the year 2060.

On Thursday, the big moveable feast of thinkers, planners, economists, government officials and business leaders that is Seven50 convened in downtown Miami for the effort’s second public summit since its launch in Delray Beach last June.

The seven counties of southeast Florida, already more populated than 35 states, will cram 3 million people into the region in the next 50 years, planners told several hundred attendees Thursday morning.

But transportation networks, public infrastructure and land use rules that encourage sprawl mean the region could face gridlock, environmental degradation, and dropping quality of life if current policies continue, they said.

The free public meeting is to take all day.

It may sound like “wonky stuff,’’ said Seven50 lead consultant Victor Dover, a Coral Gables-based planner. But he said Seven50’s scores of participants are convinced that agreeing to coordinated plans across jurisdictional lines is critical if the region is to prosper and meet a long list of common challenges. They range from transportation logjams to the prospect of rising seas and U.S. and international competitors trying to grab our share of international investment, tourism, cargo and trade.

And that competition is serious and well-organized, Dover said. In Texas, for instance, 13 counties and 100 cities between Houston and Galveston have banded together in a similar planning alliance, and so have cities and states along the Great Lakes.

The advantage Southeast Florida has, Seven50 planners say, is that old real-estate cliché: location, location, location.

But the region risks throwing its advantage away unless it better links up its airports and seaports, installs more and better-connected mass transit, and develops strategies to improve education and retain and attract the kind of skilled, educated young people considered key to economic prosperity in today’s economy.

“Planning at this scale is profoundly American, from Jefferson to the creation of Washington, D.C., and if we don’t do it, we’re going to get blown away by the competition,’’ said Andres Duany, a renowned Miami-based planner who will give the keynote address at the downtown gathering. “They’re gunning for us.’’

The sessions at Miami Dade College’s Wolfson campus are designed to gather public input and share a still-in-development snapshot of the region as planners build what they describe as a massive data warehouse covering everything from demographics to housing, economics and transportation networks. Key discussion areas will be transportation, education and the daunting implications of climate change.