viewport-mercator-project

Utility to convert map or world coordinates to screen coordinates back and forth

npm install viewport-mercator-project --save

Overview

Projection and camera utilities supporting the Web Mercator Projection. At its core this is a utility for converting to and from map (latitude, longitude) coordinates to screen coordinates and back.

FlatMercatorViewport - For 2D applications, a simple, fast utility is provided that supports the basic flat Web Mercator projection and unprojection between geo coordinates and pixels.
PerspectiveMercatorViewport - For 3D applications, a subclass of a generic Viewport class (which is essentially a 3D matrix "camera" class of the type you would find in any 3D/WebGL/OpenGL library).

The constructor of this "advanced" perspective-enabled viewport also takes the same typical map view parameters as the FlatMercatorViewport, however it offers perspective enabled/project unproject functions, and generates general 4x4 view matrices that correspond to the parameters.

Who is this for?

Specifically built for use with deck.gl and react-map-gl, but could be useful for any web mapping application that wants to support Web Mercator Projection with floating point zoom levels.

Notes on Coordinates

For the project/unproject functions, the default pixel coordinate system of the viewport is defined with the origin in the top left, where the positive x-axis goes right, and the positive y-axis goes down. That is, the top left corner is [0, 0] and the bottom right corner is [width - 1, height - 1]. The functions have a flag that can reverse this convention.
Non-pixel projection matrices are obviously bottom left.
Mercator coordinates are specified in "lng-lat" format [lng, lat, z] format (which naturally corresponds to [x, y, z]).
Per cartographic tradition, all angles including latitude, longitude, pitch and bearing are specified in degrees, not radians.
Longitude and latitude are specified in degrees from Greenwich meridian and the equator respectively, and altitude is specified in meters above sea level.
It is possible to query the PerspectiveMercatorViewport for a meters per pixel scale. Note that that distance scales are latitude dependent under web mercator projection see, so scaling will depend on the viewport center and any linear scale factor should only be expected to be locally correct.

API Documentation

FlatMercatorViewport

Note: The FlatMercatorViewport is completely independent of the other classes in this module and is intended as a fast, simple solution for applications that only use 2D map projections.

Example usage

import {FlatMercatorViewport} from 'viewport-mercator-project';

// NOTE: The `viewport` object returned from `FlatMercatorViewport` is immutable.
const viewport = FlatMercatorViewport({
  longitude: 0,
  latitude: 0,
  zoom: 0,
  width: 600,
  height: 500
});

// A longitude, latitude pair as an array.
const lnglat = [0, 0];
const pixels = viewport.project(lnglat); // returns [300, 250]

// A width, height pair as an array.
viewport.unproject(pixels); // returns [0, 0]

// Test if a lnglat is within the viewport
viewport.contains(lnglat); // true

PerspectiveMercatorViewport

The main purpose of the PerspectiveMercatorViewport is to enable 3D rendering to seamlessly overlay on top of map components that take web mercator style map coordinates (lat, lon, zoom, pitch, bearing etc), and to facilite the necessary mercator projections by breaking them into a minimal non-linear piece followed by a standard projection chain.

Remarks:

Because PerspectiveMercatorViewport a subclass of Viewport, an application can implement support for generic 3D Viewports and automatically get the ability to accept web mercator style map coordinates (lat, lon, zoom, pitch, bearing etc).
A limitation at the moment is that there is no way to extract web mercator parameters from a "generic" viewport, so for map synchronization applications (rendering on top of a typical map component that only accepts web mercator parameters) the PerspectiveMercatorViewport is necessary.

Constructor

Parameter	Type	Default	Description

Parameter	Type	Default	Description
`latitude`	`Number`	37	Center of viewport on map (alternative to center)
`longitude`	`Number`	-122	Center of viewport on map (alternative to center)
`zoom`	`Number`	11	Scale = Math.pow(2,zoom) on map (alternative to opt.scale)
`width`	`Number`	1	Width of "viewport" or window
`height`	`Number`	1	Height of "viewport" or window
`center`	`Array`	[0, 0]	Center of viewport [longitude, latitude] or [x, y]
`scale`	`Number`	1	Either use scale or zoom
`pitch`	`Number`	0	Camera angle in degrees (0 is straight down)
`bearing`	`Number`	0	Map rotation in degrees (0 means north is up)
`altitude`	`Number`	1.5	Altitude of camera in screen units

Remarks:

Only one of center or [latitude, longitude] can be specified
[latitude, longitude] can only be specified when "mercator" is true
Only one of center or [latitude, longitude] can be specified.
[latitude, longitude] can only be specified when mercator is true
Altitude has a default value that matches assumptions in mapbox-gl
width and height are forced to 1 if supplied as 0, to avoid division by zero. This is intended to reduce the burden of apps to to check values before instantiating a Viewport.
When using mercatorProjection, per cartographic tradition, longitudes and latitudes are specified as degrees.

`PerspectiveMercatorViewport.project`

Projects latitude and longitude to pixel coordinates in window using viewport projection parameters

Parameter	Type	Default	Description
`lnglatz`	`Array`	required	`[lng, lat]` or `[lng, lat, Z]`
`opts`	`Object`	`{}`	named options
`opts.topLeft`	`Boolean`	`true`	If true projected coords are top left

Returns: [x, y] or [x, y, z] - (depending on length of input array) in the requested coordinate system (top left or bottom left)

[longitude, latitude] to [x, y]
[longitude, latitude, Z] => [x, y, z]

Remarks:

By default, returns top-left coordinates suitable for canvas/SVG type rendering.

`PerspectiveMercatorViewport.unproject`

Unproject pixel coordinates on screen onto [lon, lat] on map.

Parameter	Type	Default	Description
`xyz`	`Array`	required	pixel coordinates in viewport

Returns: Unprojected coordinates in array from, depending on input:

[x, y] => [lng, lat]
[x, y, z] => [lng, lat, Z]

`PerspectiveMercatorViewport.projectFlat([lng, lat], scale = this.scale)`

Project [lng, lat] on sphere onto "screen pixel" coordinates [x, y] without considering any perspective (effectively ignoring pitch, bearing and altitude).

Parameters:

coordinates {Array} - [lng, lat] or [xmap, ymap] coordinates.

Returns:

[x, y], representing map or world coordinates.

`PerspectiveMercatorViewport.unprojectFlat`

Unprojects a screen point [x, y] on the map or world [lng, lat] on sphere.

lnglat - Array [lng, lat] or [xmap, ymap] coordinates Specifies a point on the map (or world) to project onto the screen.
returns - [x,y] - An Array of Numbers representing map or world coordinates.

Parameters:

pixels {Array} - [x, y]

`PerspectiveMercatorViewport.unprojectFlat([x, y], scale = this.scale)`

Parameters:

[lng, lat] array xy - object with {x,y} members representing a "point on projected map plane Returns:

[lat, lon] or [x, y] of point on sphere.

`getDistanceScales()`

Returns:

An object with precalculated distance scales allowing conversion between lnglat deltas, meters and pixels.

Remarks:

The returned scales represent simple linear approximations of the local Web Mercator projection scale around the viewport center. Error increases with distance from viewport center (Very roughly 1% per 100km).
When converting numbers to 32 bit floats (e.g. for use in WebGL shaders) distance offsets can sometimes be used to gain additional computational precision, which can greatly outweigh the small linear approximation error mentioned above.

`metersToLngLatDelta(xyz)`

Converts a meter offset to a lnglat offset using linear approximation. For information on numerical precision, see remarks on getDistanceScales.

xyz ([Number,Number]|[Number,Number,Number]) - array of meter deltas returns ([Number,Number]|[Number,Number,Number]) - array of [lng,lat,z] deltas

`lngLatDeltaToMeters(deltaLngLatZ)`

Converts a lnglat offset to a meter offset using linear approximation. For information on numerical precision, see remarks on getDistanceScales.

deltaLngLatZ ([Number,Number]|[Number,Number,Number]) - array of [lng,lat,z] deltas Returns ([Number,Number]|[Number,Number,Number]) - array of meter deltas

`addMetersToLngLat(lngLatZ, xyz)`

Add a meter delta to a base lnglat coordinate, returning a new lnglat array, using linear approximation. For information on numerical precision, see remarks on getDistanceScales.

lngLatZ ([Number,Number]|[Number,Number,Number]) - base coordinate
xyz ([Number,Number]|[Number,Number,Number]) - array of meter deltas Returns ([Number,Number]|[Number,Number,Number]) array of [lng,lat,z] deltas

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