# Density Altitude API
> Aviation atmosphere maths as an API, computed locally and deterministically using the exact International Standard Atmosphere relations — the numbers a pilot, dispatcher or flight-planning tool needs before take-off, not a rough rule of thumb. The density-altitude endpoint turns the field elevation, altimeter setting and outside air temperature into the pressure altitude (elevation + (29.92 − setting) × 1000) and then the density altitude — the altitude the air actually feels like to the wings and engine — computed from the true ISA density ratio rather than the approximate 120-foot-per-degree rule, with the ISA temperature deviation: on a hot, high day the density altitude soars, robbing lift and thrust and lengthening the take-off roll, the classic mountain-airport hazard. The true-airspeed endpoint gives TAS from calibrated airspeed as CAS ÷ √(density ratio), so the navigator gets the real speed through the air that climbs above the indicated reading with altitude and temperature. The isa endpoint returns the standard-atmosphere temperature, pressure, pressure and density ratios and the speed of sound at any altitude in the troposphere — the reference every altimeter, performance chart and engine rating is built on. Everything is computed locally and deterministically, so it is instant and private. Ideal for flight-planning and EFB apps, drone and UAV tools, aviation weather dashboards, and aerospace-engineering utilities. Pure local computation — no key, no third-party service, instant. Troposphere (≤ 36,089 ft); incompressible TAS. 3 compute endpoints. For the speed of sound and Mach use a Mach-number API; for runway wind components a crosswind API.

## Authentication
All requests require your oanor API key in the `x-oanor-key` header. Get one at https://www.oanor.com/developer/keys.

```bash
curl -H "x-oanor-key: oanor_live_…" "https://api.oanor.com/densityaltitude-api/..."
```

## Pricing
- **Free** (Free) — 6,200 calls/Mo, 2 req/s
- **Starter** ($11/Mo) — 68,000 calls/Mo, 6 req/s
- **Pro** ($35/Mo) — 284,000 calls/Mo, 15 req/s
- **Mega** ($106/Mo) — 1,390,000 calls/Mo, 40 req/s

## Endpoints

### Atmosphere

#### `GET /v1/density-altitude` — Pressure & density altitude

**Parameters:**
- `field_elevation_ft` (query, required, string) — Field elevation (ft) Example: `5000`
- `altimeter_inhg` (query, optional, string) — Altimeter setting (inHg, default 29.92) Example: `29.92`
- `oat_c` (query, required, string) — Outside air temperature (°C) Example: `25`

**Example:**
```bash
curl -H "x-oanor-key: $KEY" \
  "https://api.oanor.com/densityaltitude-api/v1/density-altitude?field_elevation_ft=5000&altimeter_inhg=29.92&oat_c=25"
```

**Response:**
```json
{
    "data": {
        "note": "Pressure altitude = field elevation + (29.92 − altimeter setting) × 1000. Density altitude is the altitude the air 'feels like' to the wings and engine — computed here exactly from the ISA density ratio, not the rough 120-ft-per-degree rule. On a hot, high day the density altitude soars: less lift, less thrust and a longer take-off roll, the classic mountain-airport hazard. Above the ISA temperature density altitude exceeds pressure altitude.",
        "inputs": {
            "oat_c": 25,
            "altimeter_inhg": 29.92,
            "field_elevation_ft": 5000
        },
        "isa_temp_c": 5.1,
        "density_ratio": 0.8041,
        "isa_deviation_c": 19.9,
        "density_altitude_ft": 7263,
        "pressure_altitude_ft": 5001
    },
    "meta": {
        "timestamp": "2026-06-06T23:53:54.746Z",
        "request_id": "3dd1f7b8-bf3c-4b0c-a9c3-0be6c2e05f71"
    },
    "status": "ok",
    "message": "Density altitude",
    "success": true
}
```

#### `GET /v1/isa` — ISA standard atmosphere at an altitude

**Parameters:**
- `altitude_ft` (query, required, string) — Altitude (ft, ≤ 36089) Example: `10000`

**Example:**
```bash
curl -H "x-oanor-key: $KEY" \
  "https://api.oanor.com/densityaltitude-api/v1/isa?altitude_ft=10000"
```

**Response:**
```json
{
    "data": {
        "note": "The International Standard Atmosphere: from 15 °C and 1013.25 hPa at sea level the temperature falls 1.98 °C per 1,000 ft up to the tropopause (36,089 ft), and pressure and density follow from it. It is the reference every altimeter, performance chart and engine rating is built on — the real atmosphere differs, which is exactly why pressure and density altitude exist. This endpoint covers the troposphere.",
        "inputs": {
            "altitude_ft": 10000
        },
        "temp_c": -4.81,
        "pressure_hpa": 696.82,
        "density_ratio": 0.7385,
        "pressure_ratio": 0.6877,
        "speed_of_sound_kt": 638.3
    },
    "meta": {
        "timestamp": "2026-06-06T23:53:54.833Z",
        "request_id": "e1ab2ead-150b-43d8-a1ba-0fa2b390b52c"
    },
    "status": "ok",
    "message": "ISA atmosphere",
    "success": true
}
```

#### `GET /v1/true-airspeed` — True airspeed from CAS

**Parameters:**
- `cas_kt` (query, required, string) — Calibrated airspeed (knots) Example: `100`
- `pressure_altitude_ft` (query, required, string) — Pressure altitude (ft) Example: `5000`
- `oat_c` (query, required, string) — Outside air temperature (°C) Example: `25`

**Example:**
```bash
curl -H "x-oanor-key: $KEY" \
  "https://api.oanor.com/densityaltitude-api/v1/true-airspeed?cas_kt=100&pressure_altitude_ft=5000&oat_c=25"
```

**Response:**
```json
{
    "data": {
        "note": "True airspeed = calibrated airspeed ÷ √(density ratio): thinner air reads low on the airspeed indicator, so TAS climbs above CAS with altitude and temperature (a rough 2 % per 1,000 ft). This is the incompressible form — fine for light aircraft; near and above the transonic range compressibility adds a further correction. Use TAS for navigation and ground-speed work; the wings still only feel CAS.",
        "inputs": {
            "oat_c": 25,
            "cas_kt": 100,
            "pressure_altitude_ft": 5000
        },
        "density_ratio": 0.8041,
        "true_airspeed_kt": 111.5
    },
    "meta": {
        "timestamp": "2026-06-06T23:53:54.917Z",
        "request_id": "eb3fee5f-9711-4b9a-b2b3-c0f9d296bed2"
    },
    "status": "ok",
    "message": "True airspeed",
    "success": true
}
```

### Meta

#### `GET /v1/meta` — Spec

**Example:**
```bash
curl -H "x-oanor-key: $KEY" \
  "https://api.oanor.com/densityaltitude-api/v1/meta"
```

**Response:**
```json
{
    "data": {
        "notes": "ft, inHg, °C, knots. PA = elev + (29.92 − altimeter)×1000; density altitude from the exact ISA density ratio; TAS = CAS/√σ. Troposphere (≤ 36,089 ft). For the speed of sound / Mach use a Mach-number API; for runway wind components a crosswind API.",
        "service": "densityaltitude-api",
        "endpoints": {
            "GET /v1/isa": "ISA standard-atmosphere properties at an altitude.",
            "GET /v1/meta": "This document.",
            "GET /v1/true-airspeed": "True airspeed from calibrated airspeed, pressure altitude and temperature.",
            "GET /v1/density-altitude": "Pressure altitude, ISA deviation and density altitude."
        },
        "description": "Aviation atmosphere maths: pressure & density altitude from altimeter and temperature, true airspeed from CAS, and the ISA standard atmosphere."
    },
    "meta": {
        "timestamp": "2026-06-06T23:53:54.996Z",
        "request_id": "114b02a7-c2aa-4b9f-8a6c-29644b6e88ca"
    },
    "status": "ok",
    "message": "Meta",
    "success": true
}
```


---
Marketplace page: https://www.oanor.com/api/densityaltitude-api
OpenAPI spec: https://www.oanor.com/api/densityaltitude-api/openapi.json