A minor flap arose on social media recently over how much area of the city of Detroit is vacant and abandoned.

For those who don't follow this question, estimates of Detroit's vacant land range from a low of about 24 square miles to a high of about 40 square miles of abandoned land in the 139-square-mile city.

By any measure it's a lot of vacancy. And while it may be an esoteric question to some, it matters a lot.

"The amount of vacant land and its location in the city drives what a realistic vision could be for the city," said John Mogk, a longtime law professor of development law at Wayne State University Law School. "This, in turn, shapes the details of an achievable master plan to coordinate future public and private investment in the city."

And Dan Pitera, dean of architecture at the University of Detroit Mercy, echoed that. He said the volume of vacant land give Detroiters the potential to plan and implement an equitable, ecological and inspirational city. But it's essential to understand the amount and location of the undeveloped and vacant open space.

"This open space is truly the land of opportunity for all Detroiters and our environment,” Pitera told me.

Different estimates

The latest kerfuffle arose after Ken Buckfire, a New York financial consultant who had worked on Detroit's municipal bankruptcy a few years ago, told an audience at Crain's Detroit's recent Homecoming event that 40 square miles of Detroit consisted of vacant land. That estimate has been around for many years. I've written that myself on occasion.

That prompted some push back from critics on social media who said the actual figure was a lot smaller, no more than 24 square miles of vacancy. That's a figure used by the Detroit Future City office, a nonprofit consultancy that works on issues of land use in the city.

But as Sarah Hayosh, director of Land Use & Sustainability for the DFC office, told me, that 24-square-mile estimate includes only "structure-free" parcels, or lots where buildings have been removed. It doesn't count abandoned schoolyards (of which there are so many in Detroit) or parkland that's no long maintained. Nor does it count abandoned right-of-ways like the old rail lines now being turned into greenway trails.

In other words, the 24-square-mile estimate of vacancy leaves out a lot of, well, vacancy. Throw in the tens of thousands of abandoned structures still standing in the city, including blighted houses and derelict industrial buildings that are likely to come down one day, and the volume of abandonment swells considerably.

"My seat-of-the-pants estimate would be closer to 40 square miles in the aggregate including sites with vacant structures," Mogk said.

What to do with it all

OK, so it's a big number. But as Alan Mallach, author of the book "The Divided City" and an urban expect who has worked in Detroit, says, the exact number matters less than what to do with our vacant land.

"It strikes me that it’s not that important in the abstract to have a total number — does it really matter if it’s 26 rather than 23 square miles?" Mallach told me. "But what really matters is knowing what’s out there so that one can plan accordingly."

Detroit, he said, needs land for many reasons: for redevelopment projects, for recreational uses, for storm water retention ponds and wetlands, and for much more.

Paradoxically, even in a city with perhaps more vacant and abandoned parcels than any other, large tracts of land can still be hard to come by.

Recently, Mayor Mike Duggan's team scrambled to assemble more than 200 acres of land for Fiat Chrysler's new Jeep assembly plant near St. Jean and Mack on the east side. To get the needed acreage, the city had to swap considerably more land from its inventory to give to private owners including the family of Manuel (Matty) Moroun to get the land needed for the Jeep plant.

That wasn't the only example. Duggan recently suggested closing the shorter of two runways at the Coleman A. Young International Airport (the former City Airport) and using the roughly 80 acres there for a new industrial park. Aviation interests oppose the plan and it's unclear if Duggan will ever get to close the runway.

But those two examples show that assembling large swaths of land free and clear for redevelopment can prove difficult even in a city with as much vacant land as Detroit.

Where did it all come from?

Detroit, of course, was considerably more crowded in the 1940s and '50s than it is now. With a population peaking at nearly 2 million in the 1950 Census count, Detroit was a city packed with single-family houses, vast factories and thousands of smaller commercial structures.

But as the population fled to the suburbs in the post-war years, tens of thousands of houses and commercial structures were left empty. Blight claimed many of them. Over the decades the city demolished numerous structures, leaving entire blocks as ghost streets in some neighborhoods, with sidewalks and driveways but no more houses.

Urban farms and other uses

The vacancy was so widespread that then-Mayor Dave Bing suggested in 2010 that Detroit consider relocating the final few residents from sparsely populated districts to save money on city services. The idea went nowhere amid much criticism. But Bing's idea did spark the effort that became Detroit Future City report.

Released in 2013, the report laid out a bold vision to reuse much of Detroit's vacant land that has once been residential or commercial as "productive landscapes" — urban gardens, tree farms and similar uses. Urban farming was already a popular idea with the city; thousands of residents had been planting neighborhood gardens for years.

While still mostly an idea in the embryo stage, greening strategies have grown more common in Detroit in recent years. Recreational corridors like the Dequindre Cut and the RiverWalk now run where little or nothing stood before their creation. And a 22-acre parcel of vacant former industrial land on the west riverfront is in line to become the Ralph C. Wilson Jr. Centennial Park in a few years.

Making progress, but ...

New development has filled in some of the vacancy, as in the city's I-94 Industrial Park on the east side near Mt. Elliott and Georgia. For many years the vast site was nothing more than almost 200 acres of vacant and overgrown land from which houses had been removed. But new industrial users including the Moroun family's Crown Enterprises and the new Flex-N-Gate plant opened there, reclaiming much of the property.

And the city has made progress in recent years in getting a better handle on its vacancy problem. The creation of the Detroit Land Bank Authority several years ago gave the city a legal mechanism to clear titles of tax-foreclosed parcels and assemble bigger patches of land for projects.

The Land Bank has sold more than 10,000 side lots to residents who use them for playscapes, parking, or just bigger yards.

And efforts like Motor City Mapping, which surveyed every parcel in the city, makes data about land more readily available for research and redevelopment.

Yet until Detroit's economic revival spreads more generally into the neighborhoods, the problem of vacant and abandoned land in the city will remain unsolved. Indeed, it may get worse before it gets better.

Detroit's vacant land may be, as Pitera and others say, an opportunity. But taking advantage of that opportunity requires years of work ahead of us.

Contact John Gallagher at313-222-5173 or gallagher@freepress.com.Follow him on Twitter@jgallagherfreep. Read more on business and sign up for our business newsletter.