# `Crs`

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

[Geospatial](types.md#geospatial); not ordered; since Jadpo 0.1.0.

## Make one

Build one from its text, `Crs("EPSG:4326")`, which checks it: text that isn't one breaks `format`.

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

## Text, JSON and storage

`text.from` writes its canonical text, the one form `text.parse` reads back. JSON sends its canonical text as a string.

| Text | JSON |
|---|---|
| `EPSG:4326` | `"EPSG:4326"` |
| `EPSG:27700` | `"EPSG:27700"` |
| `EPSG:3857` | `"EPSG:3857"` |

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

## 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 has the contract's own operations: `text.parse`, `text.from`, `text.format`, `json.read`, `json.write` and `==`. It isn't ordered, so `<` and sorting don't take it. [Every standard type meets one contract](../topics/every-standard-type-meets-one-contract.md) says how they work.