# `Point`

A position on Earth: its latitude, its longitude and, if you know it, its height.

Use it for anywhere you'd drop a pin, such as a shop or a delivery drop-off. For a path, use [`LineString`](/docs/types/type-line-string.md). For an area, use [`Polygon`](/docs/types/type-polygon.md). For an address someone can post to, use [`PostalAddress`](/docs/types/type-postal-address.md).

## Shape

| Field | Type | Required? |
|---|---|---|
| `latitude` | [`Latitude`](/docs/types/type-latitude.md) | Yes |
| `longitude` | [`Longitude`](/docs/types/type-longitude.md) | Yes |
| `altitude` | [`Length`](/docs/types/type-length.md) | No |

Write every field when you build one, and give a field that isn't required as `none`.

## Example

```jadpo
Point { latitude: 51.5072 longitude: -0.1276 altitude: none }
```

These operations give one: `geo.centroid`, `geo.from_lat_lng` and `geo.from_lng_lat`.

## Good to know

- Fields are named, so latitude and longitude can't be swapped by accident. In JSON they follow GeoJSON's order, longitude first.
- `geo.distance` gives the distance between two, and a route takes and returns one as GeoJSON.
- `==` tells you whether two are the same. `<` and `list.sort_by` don't take it, so sort by one of its parts instead.

## In JSON

A GeoJSON geometry, with each position's longitude first, in a request, a response or `json.write`:

```json
{"type":"Point","coordinates":[-0.1276,51.5072]}
{"type":"Point","coordinates":[151.2093,-33.8688]}
{"type":"Point","coordinates":[0,0]}
```

When you read one, a field that isn't required may be left out, or sent as `null`.

## Details

<details>
<summary>Text form, storage, how it is shown, and operations</summary>

`Point` is in the group Geospatial, and has been in Jadpo since 0.1.0.

### Text form

It has no single text form, so `text.from` and `text.parse` don't take it.

### How it is stored

It is stored in one column named for the field, of type `geography`. `{"type":"Point","coordinates":[0,0]}` is stored as `'SRID=4326;POINT (0 0)'::geography`.

### How it is shown to people

`text.format(value, locale:)` shows it to people in their locale.

With `locale: Locale.en_gb`:

| Value | Shown |
|---|---|
| `{"type":"Point","coordinates":[-0.1276,51.5072]}` | `51.5072° N, 0.1276° W` |
| `{"type":"Point","coordinates":[151.2093,-33.8688]}` | `33.8688° S, 151.2093° E` |
| `{"type":"Point","coordinates":[0,0]}` | `0°, 0°` |
| `{"type":"Point","coordinates":[-180,-90]}` | `90° S, 180° W` |
| `{"type":"Point","coordinates":[86.925,27.9881,8848.86]}` | `27.9881° N, 86.925° E, 8,848.86 m` |

### Operations

- `geo.bearing(from: Point, to: Point) -> Bearing`: The direction the geodesic leaves `from` in, in degrees clockwise from true north, at least 0 and below 360, to 9 places. Two equal points have none: the call fails with `same_point`, which its `attempt` maps.
- `geo.bounding_box(shape: T) -> BoundingBox`: The smallest box of parallels and meridians holding the shape, an edge's highest or lowest point counted, across the antimeridian when that box is narrower; a shape holding a pole runs from -180 to 180.
- `geo.centroid(shape: T) -> Point`: The shape's centre on the sphere, as PostGIS's `ST_Centroid(x, false)` finds it: each point a unit vector, weighted by area, length or count by the shape's kind, summed, and back to latitude and longitude. It may lie outside a concave polygon.
- `geo.contains(a: T, b: U) -> Bool`: Whether no point of `b` lies outside `a`, edges included. `a` may be a `BoundingBox`, whose sides are compared directly, or a `Circle`, holding the points within its radius of its centre.
- `geo.distance(from: Point, to: Point) -> Length`: The geodesic distance between two points on the ellipsoid, rounded half to even to the micrometre.
- `geo.from_lat_lng(numbers: List<Number>) -> Point`: The `Point` two numbers give, latitude first, as `[51.5072, -0.1276]`, or three, the third its altitude in metres. A latitude past 90 or a longitude past 180 fails with `out_of_range`, and any other count of numbers with `malformed`, which the call's `attempt` maps.
- `geo.from_lng_lat(numbers: List<Number>) -> Point`: The `Point` two numbers give, longitude first, as GeoJSON writes a position, `[-0.1276, 51.5072]`, or three, the third its altitude in metres. A longitude past 180 or a latitude past 90 fails with `out_of_range`, and any other count of numbers with `malformed`, which the call's `attempt` maps.
- `geo.intersects(a: T, b: U) -> Bool`: Whether the two share at least one point, edges included.
- `geo.to_wkt(shape: T) -> Text`: The shape as WKT, longitude first, as PostGIS's `ST_AsText` writes it: `POINT(-0.1276 51.5072)`, `POINT Z (-0.1276 51.5072 35)` with an altitude, each ring in the order it holds its points.
- `geo.within(a: T, b: U, distance: Length?) -> Bool`: `geo.contains(b, a)`; or, with `distance:`, whether two points are at most that far apart, as `geo.distance` measures them.

It also takes `text.format`, `json.read`, `json.write` and `==`. [How every standard type behaves](/docs/every-standard-type-meets-one-contract.md) says how they work.

</details>