# `Crs`

A coordinate reference system, by its EPSG code: `EPSG:27700`.

Use it to record which map system a set of coordinates is in, as [`EastingNorthing`](/docs/types/type-easting-northing.md) does. [`Point`](/docs/types/type-point.md) is always in EPSG:4326, so it needs no `Crs`.

## Shape

One value, held as text in one set form, with no parts: `EPSG:3857`.

## Example

```jadpo
Crs("EPSG:27700")
```

`Crs(raw)` checks the text, and text that isn't one breaks the `format` rule.

`text.parse(raw, as: Crs)` reads one from text that comes from outside. Text that isn't one at all fails `malformed`, such as `epsg:4326`, `EPSG:` or `EPSG:04326`.

## Good to know

- `EPSG` is in capitals, with no space.
- `==` tells you whether two are the same. `<` and `list.sort_by` don't take it.

## In JSON

A string, in a request, a response or `json.write`:

```json
"EPSG:4326"
"EPSG:27700"
"EPSG:3857"
```

## Details

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

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

### Text form

`text.from` writes one standard text form, the same form `text.parse` reads back: `EPSG:4326`, `EPSG:27700` and `EPSG:3857`.

### How it is stored

It is stored in one column named for the field, of type `text`. `EPSG:27700` is stored as `'EPSG:27700'`.

### How it is shown to people

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

With `locale: Locale.en_gb`:

| Value | Shown |
|---|---|
| `EPSG:4326` | `EPSG:4326` |
| `EPSG:27700` | `EPSG:27700` |
| `EPSG:3857` | `EPSG:3857` |

### Operations

It also takes `text.parse`, `text.from`, `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>