## Proposal Add `Geo::Coord` class to Ruby standard library, representing `[latitude, longitude]` pair + convenience methods. Add `Geo` standard library with additional calculations and convenience methods. ## Rationale In modern applications, working with geographical coordinates is frequent. We propose to think of such coordinates (namely, `latitude, longitude` pair) as of "basic" type that should be supported by standard library - the same way as we support `Time`/`Date`/`DateTime` instead of having it defined by user/gems. This type is too "small" to be defined by separate gem, so, all of existing geo gems (GeoKit, RGeo, GeoRuby, Graticule etc.) define their own `LatLng`, or `Location`, or `Point`, whatever. On other hand, API design for this "small" type is vague enough for all those similar types to be incompatible and break small habits and conventions when you change from one geo library to another, or try to use several simultaneously. Additionaly, many gems somehow working with geo coordinates (for weather, or timezone, or another tasks) generally prefer not to introduce type, and just work with `[lat, lng]` array, which is not very convenient, faithfully. So, having "geo coordinates" functionality in standard library seems reasonable and valuable. ## Existing/reference solutions Ruby: * [GeoKit::LatLng](http://www.rubydoc.info/github/geokit/geokit/master/Geokit/LatLng); * [RGeo::Feature::Point](http://www.rubydoc.info/gems/rgeo/RGeo/Feature/Point) (with several "private" implementation classes); RGeo implements full [OGC Simple Features](https://en.wikipedia.org/wiki/Simple_Features) specification, so, its points have `z` and `m` coordinates, projection and much more; * [Graticule::Location](http://www.rubydoc.info/github/collectiveidea/graticule/Graticule/Location) (not strictly a `[lat,lng]` wrapper); * [Rosamary::Node](http://www.rubydoc.info/gems/rosemary/0.4.4/Rosemary/Node) (follows naming convention of underlying OpenStreetMap API); Other sources: * Python: [geopy.Point](http://geopy.readthedocs.org/en/latest/#geopy.point.Point); * [ElasticSearch](https://www.elastic.co/blog/geo-location-and-search) uses hash with "lat" and "lon" keys; * Google Maps [Geocoding API](https://developers.google.com/maps/documentation/geocoding/intro#GeocodingResponses) uses hash with "lat" and "lng" keys; * PostGIS: [pretty complicated](http://postgis.net/docs/manual-2.2/using_postgis_dbmanagement.html#RefObject) has _geometrical_ (projected) and _geographical_ (lat, lng) points and stuff. ## Design decisions While designing `Geo` library, our reference point was standard `Time` class (and, to lesser extent, `Date`/`DateTime`). It has this responsibilities: * stores data in simple internal form; * helps to parse and format data to and from strings; * provides easy access to logical components of data; * allows most simple and unambiguous calculations. **Main type name**: as far as we can see, there's no good singular name for `(lat, lng)` pair concept. As mentioned above, there can be seen names like `LatLng`, or `Location`, or `Point`; and in natural language just "coordinates" used frequently. We propose the name `Coord`, which is pretty short, easy to remember, demonstrates intentions (and looks like singular, so you can have "one coord object" and "array of coords", which is not 100% linguistically correct, yet convenient). Alternative `Point` name seems to be too ambigous, being used in many contexts. `Geo::Coord` object is **immutable**, there's no semantical sense in `location.latitude = ...` or something like this. **Units**: `Geo` calculations (just like `Time` calculations) provide no units options, just returning numbers measured in "default" units: metres for distances (as they are SI unit) and degrees for azimuth. Latitude and longitude are stored in degrees, but radians values accessors are provided (being widely used in geodesy math). All coordinates and calculations are thought to be in WGS 84 coordinates reference system, being current standard for maps and GPS. There's introduced **concept of globe** used internally for calculations. Only generic (sphere) and Earth globes are implemented, but for 2016 we feel like current design of basic types should take in consideration possibility of writing Ruby scripts for Mars maps analysis. Only one geodesy formula is implemented (Vincenty, generally considered one of the most precise), as for standard library class it considered unnecessary to provide user with geodesy formulae options. No **map projection** math was added into current proposal, but it may be a good direction for further work. No **elevation** data considered either. ## Proposal details ### `Geo::Coord` class Represents `[latitude, longitude]` pair. Latitude is -90 to +90 (degrees). Longitude is -180 to +180. Class methods: * `new(lat, lng)` creates instance from two Numerics (in degrees); * `new(lat:, lng:)` keyword arguments form of above; * `new(latd:, latm:, lats:, lath:, lngd: lngm:, lngs: lngh:)` creates instance from coordinates in (deg, min, sec, hemisphere) form; hemispheres are "N"/"S" for latitude and "W"/E" for longitude; any component except for degrees can be omitted; if hemisphere is omitted, it is decided by degrees sign (lat: positive is "N", lng: positive is "E"); * `from_h(hash)` creates instance from hash with `"lat"` or `"latitude"` key and `"lon"` or `"lng"` or `"longitude"` key (case-independent); * `from_radians(phi, la)` creates instance from radian values; * `strpcoord` parses string into coordinates by provided pattern (see below for pattern description); * `parse_ll` parses coordinates string in `"float, float"` form; * `parse_dms` parses coordinates string in `d m s h, d m s h` format (considering several widely used symbols for degrees, minutes and seconds); * `parse` tries to parse string into coordinates from various formats. Instance methods: * `lat` and `lng`, returning `Float`s, signed; * `latitude` and `longitude` as an aliases; `lon` as an additional aliases for longitude; * `latd`, `latm`, `lats`, `lath`: degree, minute, second, hemisphere; `latd` and `latm` are `Fixnum`, `lats` is `Float`, `lath` is "N"/"S"; all numbers are unsigned; * `lngd`, `lngm`, `lngs`, `lngh`: the same for longitude (hemisphere is "W"/"E"); * `latdms(nohemisphere = false)` returns `[latd, latm, lats, lath]` with `nohemisphere` param equal to `false`, and `[±latd, latm, lats]` with `true`; same with `lngdms` for longitude; * `phi` and `φ` is latitude in radians (helpful for math), `la` or `λ` is longitude in radians (not `lambda` to not confuse with Kernel method); * `to_s` returning string like "50.004444,36.231389" (good for map URLs construction, for example); * `to_h(lat: :lat, lng: :lng)` converts coord to hash (with desired key names); * `to_a` converts coord to simple `[lat, lng]` pair; * `strfcoord(formatstr)` for complex coordinate formatting (see below for format string description); * `distance(other)` calculates distance to another point (in metres); * `azimuth(other)` calculates direction to target (in degrees); * `endpoint(direction, azimuth)` calculates final point of the line of `distance` metres going in `azimuth` direction from current point. #### `strpcoord`/`strfcoord` Example: ~~~ruby kharkiv.strfcoord('%latdu°%latm′%lats″ %lath, %lngdu°%lngm′%lngs″ %lngh') # => "50°0′16″ N, 36°13′53″ E" ~~~ Directives: * `%lat` - full latitude, float; can be formatted with more control like `%.4lat` (four digits after point) or `%+lat` (explicit plus sign for positive latitudes); * `%latd` - latitude degrees, unsigned, integer * `%latds` - latitude degrees, signed * `%latm` - latitude minutes, unsigned, integer * `%lats` - latitude seconds, unsigned, integer, but can be formatted as float: `%.2lats` * `%lath` - latitude hemisphere, one letter ("N"/"S") * `%lng`, `%lngd`, `%lngds`, `%lngs`, `%lngh`, `%lngH` - same for longitude * `%%` literal `%` sign ### Current implementation Proposed implementation can be found at https://github.com/zverok/geo_coord. It was created with thoughts of standard library, so, all docs are in RDoc format, and tests/specs are in mspec-compatible rspec flavour.