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Public Safety great at finding buildings. They’re less great at finding people inside them. This post unpacks the messy world of “Z” (altitude): why your phone’s barometer lies without context, why “sea level” isn’t your friend, and how a simple lobby placard could shave minutes off response times for Public Safety.
Today we’ll be talking about vertical location accuracy (the “Z‑axis”) for enterprise 9‑1‑1: what it is, how devices estimate it, why “where is zero?” is the most important (and overlooked) question, and practical steps enterprises can take so Public Safety gets the right floor, right now.
Most callers today have a smart device
Good news first: nearly every caller is carrying a multi‑sensor location powerhouse. Phones blend GNSS (GPS, Galileo, GLONASS, BeiDou), Wi‑Fi, cell timing, barometric pressure, and sometimes UWB/BLE (Ultra Wide Band and Bluetooth Low Energy) to estimate position. Horizontal (X/Y) on a map is typically excellent—often within tens of meters in urban canyons and much tighter with good sky view. Vertical (Z) is the goblin in the basement—and sometimes on floor 37.
XY is great…until you stack floors
Inside a high‑rise, the same XY repeats for each story. Take Chicago’s Willis Tower (formerly the Sears Tower): it’s widely described as a 110‑story skyscraper (the owner and Skydeck both use 110; some “standard” counts say 108—yes, counting floors is also messy). That’s up to 110 stacked points sharing the same XY but differing in Z. Public Safety needs the right floor to turn a good map pin into a good outcome.
Before measuring anything: where is zero?
You can’t measure ANYTHING without an agreed starting point, and altitude is no exception. Here are the usual suspects considered ‘valid’ – be all have lots of baggage:
- Mean Sea Level (MSL): Sounds authoritative, but seas…move. Tides, barometric pressure, and geoid variations mean “sea level” is a statistical average, not a stable zero. Useful for sailors; problematic for floor numbers.
- Height Above Ellipsoid (HAE): Measures height relative to a mathematical model of the Earth (e.g., WGS84). It’s stable and globally consistent—phones and satellites love it. Think of it as the measuring tape baked into GNSS.
BTW: WGS84 is the World Geodetic System 1984. It’s a spatial reference system of GPS satellites with an error of less than 2 cm to the center of Earth. A pretty stable “0”
- Height Above Average Terrain (HAAT): A broadcast‑engineering metric the FCC uses for antennas: antenna height above averaged terrain over multiple radials. Great for estimating FM coverage for Hot 97 the Metro’s Hit Monster. But, terrible for telling you which floor someone’s on—especially when the back of the building is on a slope.
Bottom line: if you don’t define zero, your Z is just an irrelevant number.
Floors are not a uniform 10 feet
If only. Lobbies can be two or three stories tall, mechanical floors vary, and “creative” mezzanines blur the math. Dividing a raw altitude by “10 feet per floor” is a guess, not a standard. That’s how responders end up checking 24, then 25, then 23, while time slips away.
And mezzanines make it weirder
Sky lobbies, split levels, and open balconies mean floor numbering and physical elevation don’t always match one‑to‑one. This is why we need both a precise Z (in meters) and a building‑specific mapping from Z to floor labels (“26,” “26M,” “Sky Lobby,” etc.).
So what is Z, really? A quick tour of how devices estimate altitude
- GNSS‑derived HAE: Satellites give a height above the WGS84 ellipsoid. It’s consistent but can drift indoors or in urban canyons unless aided by other local signals. This is the preferred reference for emergency Z because it’s globally standard.
- Barometric pressure: Your phone’s barometer senses pressure changes that correlate with altitude. It’s very sensitive (great!), but weather swings pressure too (not great!). You need a reference—like a nearby calibrated station or “ground‑truth” at the building—to turn pressure into accurate height.
- Wi‑Fi RTT / multilateration: With known access point locations (including elevation), multilateration can estimate 3D position indoors. Harder to deploy consistently but promising in enterprise‑managed spaces. IF MAINTAINED.
- UWB/BLE beacons: Ultra‑wideband and dense Bluetooth beacons can produce accurate relative positioning; BUT you anchor them to an absolute Z reference (HAE). If you do – they can help infer floor.
- 5G/ Observed Time Difference Of Arrival (OTDOA) & network‑based methods: Carrier‑side techniques are improving, but floor‑level reliability still varies by building density and handset capability.
- LiDAR & 3D building models: Not in your phone (usually), but agencies can use LiDAR‑derived models to map a measured Z to an actual floor plan. The public‑safety community is already exploring this for “dispatchable floor” intelligence.
“Explain it like I’m new”: HAE (Height Above Ellipsoid)
Imagine Earth wrapped in a smooth, slightly squashed beach ball—the ellipsoid. HAE tells you how far above that surface you are. It’s not “sea level,” but it’s consistent everywhere, which is why NENA standards and FCC z‑axis rules use it for reporting altitude to 9‑1‑1. Or, put snarkily: we picked the one zero that doesn’t slosh around twice a day, despite it covering 2/3 of the planet.
What do the standards say?
- FCC: Providers must deliver a z‑axis within ±3 meters (80% of calls) and report it as Height Above Ellipsoid (HAE). That’s floor‑useful accuracy when paired with a building map.
- NENA: In Requirements for 3D Location Data for E9‑1‑1 and NG9‑1‑1 (NENA‑REQ‑003.1‑2022), NENA specifies conveying altitude as HAE (meters) referenced to WGS84—i.e., the same, consistent zero used by GNSS.
- NENA Knowledge Base reinforces HAE as the device’s orthogonal distance from WGS84 (often called the “Z coordinate”).
Turning good Z into a reachable floor: define your building’s “ground truth”
Now that we agree on HAE as zero, the enterprise can do its part:
- Post the building’s TRUE ground‑floor HAE (in meters) where it matters—think lobby security, fire command room, and digital signage.
- Provide a Z→Floor mapping table: “HAE 192.3–195.7 m = Floor 1 (Lobby)”, “…195.8–199.1 m = Floor 2,” “Sky Lobby = 232.0–236.5 m,” etc.
- Publish it via QR in the lobby and on an easy URL. Keep the page simple HTML so responders can load it fast on any device.
- Version it (v1.2, updated 2025‑08‑23). When construction changes floor heights, update the ranges.
Why QR? It’s universal, offline‑friendly (once scanned), and gives responders a single source of truth that facilities can update without touching carrier networks or PSAP CAD integrations.
“Can we standardize that, please?”
Yes—NENA could (and should) spin up a short‑fuse workgroup to define a standard form for this data: building HAE baseline, Z→Floor ranges, notable anomalies (mezzanines), stairwell labels, AED and FACP locations, and a recommended URL/QR placement. If that spec’s HTML/JSON is standard, CAD vendors can auto‑ingest it, and responders know exactly what to expect—every time, every building.
Practical pitfalls & pro tips (a friendly reality check)
- Barometers drift: Pair barometric readings with a local reference (e.g., lobby HAE) or a network calibration source.
- Mixed floor numbering: Write down the weirdness. If your ground entrance on the river is “Floor 1,” and the rear entrance on the hill is “B2,” document it—and map Z accordingly.
- Renovations: A two‑story lobby is gorgeous. It also breaks simplistic “10 feet per floor” math. Measure once, record forever.
- Data custody: The enterprise should own and maintain this mapping. Put facilities and security—not marketing—in charge of the QR page.
The payoff
Better Z means fewer wrong doors, fewer wrong floors, and faster patient contact. With HAE as the reference, enterprise‑maintained Z→Floor mapping, and a QR link responders can trust, we turn “pin on building” into “hand on doorknob.”
The World According to Fletch . . . 