Crime mapping is as old as modern policing. It’s part and parcel of the detection models that police use everyday, like fingerprinting and DNA processing, and over the years it has become an indispensable tool for police officers everywhere. Naturally, crime mapping has gone from an art to a science since its first appearance in the mid-19th century. Metro police offices, such as Philadelphia’s, are able to pull off some impressive feats with crime mapping, including predictive modeling, heat maps that can suss out the flow of drugs in a given neighborhood and charting what environmental factors can lead to higher crime rates.

But, curiously, up until recently, many police departments hadn’t been mapping overdoses. There were practical reasons behind this: Police aren’t often the first to respond to overdose calls, emergency services are, and data sharing between ambulatory services and police can be a little opaque.

That has now changed, thanks to a mapping app out of Baltimore that is informing the way Philly police respond to the opioid crisis, to save lives and—hopefully—staunch the flow of drugs throughout the region.

The need for this is clear. The opioid crisis has wreaked havoc on our city, and the severity of the problem has grown dramatically over a very short period of time. For example: There were fewer than 25 deaths attributed to Fentanyl overdoses in Philly in 2013; by 2016, that number had risen to just north of 400, roughly the same number of deaths attributable to heroin overdoses. Overall, Pennsylvania experienced nearly 4,650 overdose deaths in 2016—that’s about 13 per day. This doesn’t count those who survived an overdose.

A federal organization, the High Intensity Drug Trafficking Areas program (HIDTA) is one of the first lines of defense in the opioid epidemic nationally, and is vital in helping law enforcement offices create drug policy and quantify drug issues. Funded directly by the White House, and departments in most major metropolitan areas, Philadelphia included, HIDTA was established in the late 80’s to help coordinate municipal and federal anti-drug efforts.

Last year, the Baltimore HIDTA office came up with a pretty brilliant, simple tool. Jeff Beeson, program manager at Baltimore’s HIDTA, said that the notion of an overdose mapping program came up in a meeting, with little fanfare. It was okayed immediately—it was, after all, only a mapping app—and not that much was expected of it. “It really only took a month to put together,” he says.

“The problem doesn’t end at City Avenue, or the Tacony Creek or whatever,” says Jeremiah Daley, who runs the Philadelphia/Camden branch of HIDTA. “It travels. An overdose cluster, or spike, that starts in Kensington may start rolling out to the suburb. There’s huge value for all communities, and it gives a much better regional and statewide understanding of the problem.”

Instead, the app has proven to be a critical tool in tracking overdoses, and the drugs that cause them in jurisdictions around the country. In Philly, which started using the app in the spring, police on the scene report overdoses back to their headquarters, where an officer enters the information into the HIDTA app. (An option exists for officers to enter overdose data on their own through their phones, but Philly’s police don’t take advantage to make sure that information isn’t erroneously or accidentally entered.)

Now, in addition to police, the tool is being used by SEPTA police and Prevention Point Philadelphia, the drug-fighting non-profit. Together, they have reported around 3,000 fatal and non-fatal episodes through the app.

According to the PPD’s chief inspector, Dan McDonald, OD mapping has been a major boon to the department’s mission to decrease overdoses, something long overdue. This time last year, for example, McDonald says there were two mass overdose incidents that caught officials off guard. In November, 40 people in one day went to emergency rooms with nonfatal overdoses. And in December, 35 people died in five days from an OD. And those numbers don’t count the people who never went to the hospital.

With the HIDTA app, police can understand the flow of drugs through certain neighborhoods and, critically, understand where officers equipped with and trained to use the life saving drug naloxone should be stationed.

“Now, if we see a spike in overdoses, above what you’d expect on any given day, we are then immediately able to go on a higher level of alert, and put more officers on the street, and prevent it from becoming a mass overdose incident,” he says.

McDonald can’t say officially whether or not the program is leading to more arrests related to drug dealing—but says that’s not the point. A major part of the war on opioids is being able to react to overdoses, and OD mapping allows that to happen more quickly. Police don’t arrest overdose cases saved by naloxone; rather, they escort them to the hospital. And McDonald says fewer overdose fatalities have happened since OD mapping was launched in Philly.

“We don’t look at OD mapping as an arresting tool,” he says. “We look at it as a life-saving tool.”

Overdoses related to tainted drugs move like like storm systems across the map. First one appears in, say, South Philly; then another two miles away three hours later in Point Breeze. Police and ambulatory services would then likely guess that the bad batch is making its way across town to West Philly, and possibly out to the suburbs.

Knowing that overdoses are spreading, and predicting where they’re spreading to, is vital to allowing police departments to better work with one another to respond. There’s very little guesswork involved: if the mapping system senses a pattern, it alerts local police and emergency authorities. To date, the mapping system has been used by police and emergency services in nearly 20 states, and in over 80 countries.

“We don’t look at OD mapping as an arresting tool,” says Chief Inspector McDonald. “We look at it as a life-saving tool.”

“Philadelphia is bordered by four counties—Bucks, Montgomery, Chester, Delaware,” says Jeremiah Daley, who runs the Philadelphia/Camden branch of HIDTA. “The problem doesn’t end at City Avenue, or the Tacony Creek or whatever. It travels. An overdose cluster, or spike, that starts in Kensington may start rolling out to the suburb. There’s huge value for all communities, and it gives a much better regional and statewide understanding of the problem.”

The OD mapping system is part of a city and statewide push to better analyze information related to the opioid epidemic. Prior to the implementation of the OD map, Daley says, OD information wasn’t well organized. Information on overdoses came from several disparate sources, like coroners and police offices, and there was no program unifying all of them, so they could study patterns.

Thus far, HIDTA’s mapping information is not available to the public, though it might help parents know which corners to tell their kids to avoid, anti-drug nonprofits where to target and generally improve community safety. Beeson, however, says there’s a good reason for that: Drug dealers and gangs could use the information to avoid corners and areas where many overdoses are occurring, and where police, therefore, are more likely to be cruising around.

What’s more, says Beeson, publicly sourced overdose mapping would be inherently unreliable and easy to take advantage of. Dealers or pranksters could file false reports to throw off the mapping; overzealous community police types could punch in data that isn’t necessarily correct. Daley says that while he thinks that a HIDTA map could someday be released for general consumption, they’re a long way away from being ready to give the data to the public at large.

A first step in widening its use—and therefore its effectiveness—could be by sharing the app with other nonprofits that encounter overdoses frequently. Matt Tice, the clinical director at Pathways to Housing, a nonprofit that helps house the homeless, including some addicts,says his organization, along with other similar groups, has asked to be included in the HIDTA program.

There were fewer than 25 deaths attributed to Fentanyl overdoses in Philly in 2013; by 2016, that number had risen to just north of 400, roughly the same number of deaths attributable to heroin overdoses.

Tice, of course, notes that there is no “magic bullet” in the fight against heroin and opioids. In truth, he says, metro areas are so far behind that it will take years for them to catch up, regardless of new technologies. He’s right, sadly. Addiction has exploded in recent years, and advancements in crisis management, from needle exchanges to community policing, have only been minor roadblocks to the rising tide of heroin and opioid usage.

But in concert, he says, all of these things together—HIDTA mapping included—could get cities like Philly to where they need to be within a few years.

Correction: An earlier version of this story misstated Matt Tice’s first name.