# cheap-ruler  Fast approximations for common geographic measurements

2 years after

# cheap-ruler  A collection of very fast approximations to common geodesic measurements. Useful for performance-sensitive code that measures things on a city scale.

The approximations are based on an FCC-approved formula of ellipsoidal Earth projection. For distances under 500 kilometers and not on the poles, the results are very precise — within 0.1% margin of error compared to Vincenti formulas, and usually much less for shorter distances.

## Performance

Compared to corresponding Turf methods (using Node v6):

• `distance`: ~31x faster
• `bearing`: ~3.6x faster
• `destination`: ~7.2x faster
• `lineDistance`: ~31x faster
• `area`: ~3.4x faster
• `along`: ~31x faster
• `pointOnLine`: ~78x faster
• `lineSlice`: ~60x faster

• `lineSliceAlong`: ~285x faster than `turf.lineSlice(turf.along(...`
• `bufferPoint`: ~260x faster than creating a bounding box with two diagonal `turf.destination` calls
• `bufferBBox`: ~260x faster (likewise)
• `insideBBox`: ~19x faster than `turf.inside(turf.point(p), turf.bboxPolygon(bbox))`

## Usage

``````var ruler = cheapRuler(35.05, 'miles');
// ...
var distance = ruler.distance([30.51, 50.32], [30.52, 50.312]);
var lineLength = ruler.lineDistance(line.geometry.coordinates);
var bbox = ruler.bufferPoint([30.5, 50.5], 0.01);``````

Note: to get the full performance benefit, create a ruler object only once per a general area of calculation, and then reuse it as much as possible. Don't create a new ruler for every calculation.

### Creating a ruler object

#### cheapRuler(latitude[, units])

Creates a ruler object that will approximate measurements around the given latitude. Units are one of: `kilometers` (default), `miles`, `nauticalmiles`, `meters`, `yards`, `feet`, `inches`.

#### cheapRuler.fromTile(y, z[, units])

Creates a ruler object from tile coordinates (`y` and `z`). Convenient in `tile-reduce` scripts.

``var ruler = cheapRuler.fromTile(1567, 12);``

### Ruler methods

#### distance(a, b)

Given two points of the form `[longitude, latitude]`, returns the distance.

``var distance = ruler.distance([30.5, 50.5], [30.51, 50.49]);``

#### bearing(a, b)

Returns the bearing between two points in angles.

``var bearing = ruler.bearing([30.5, 50.5], [30.51, 50.49]);``

#### destination(p, dist, bearing)

Returns a new point given distance and bearing from the starting point.

``var point = ruler.destination([30.5, 50.5], 0.1, 90);``

#### lineDistance(line)

Given a line (an array of points), returns the total line distance.

``````var length = ruler.lineDistance([
[-67.031, 50.458], [-67.031, 50.534],
[-66.929, 50.534], [-66.929, 50.458]
]);``````

#### area(polygon)

Given a polygon (an array of rings, where each ring is an array of points), returns the area. Note that it returns the value in the specified units (square kilometers by default) rather than square meters as in `turf.area`.

``````var area = ruler.area([[
[-67.031, 50.458], [-67.031, 50.534], [-66.929, 50.534],
[-66.929, 50.458], [-67.031, 50.458]
]]);``````

#### along(line, dist)

Returns the point at a specified distance along the line.

``var point = ruler.along(line, 2.5);``

#### pointOnLine(line, p)

Returns an object of the form `{point, index}` where `point` is closest point on the line from the given point, and `index` is the start index of the segment with the closest point.

``var point = ruler.pointOnLine(line, [-67.04, 50.5]).point;``

#### lineSlice(start, stop, line)

Returns a part of the given line between the start and the stop points (or their closest points on the line).

``ruler.lineSlice([-67.04, 50.5], [-67.05, 50.56], line);``

#### lineSliceAlong(startDist, stopDist, line)

Returns a part of the given line between the start and the stop points indicated by distance along the line.

``ruler.lineSliceAlong(10, 20, line);``

#### bufferPoint(p, buffer)

Given a point, returns a bounding box object (`[w, s, e, n]`) created from the given point buffered by a given distance.

``var bbox = ruler.bufferPoint([30.5, 50.5], 0.01);``

#### bufferBBox(bbox, buffer)

Given a bounding box, returns the box buffered by a given distance.

``var bbox = ruler.bufferBBox([30.5, 50.5, 31, 51], 0.2);``

#### insideBBox(p, bbox)

Returns true if the given point is inside in the given bounding box, otherwise false.

``var inside = ruler.insideBBox([30.5, 50.5], [30, 50, 31, 51]);``

### Units conversion

Multipliers for converting between units are also exposed in `cheapRuler.units`:

``````// convert 50 meters to yards
50 * cheapRuler.units.yards / cheapRuler.units.meters;``````

If you don't specify units when creating a ruler object, you can use these constants to convert return values (using multiplication) and input arguments (using division) to any units:

``````// get distance between points in feet
var distanceInFeet = ruler.distance(a, b) * cheapRuler.units.feet;

// make a bbox from a point with a 200 inch buffer
var box = ruler.bufferPoint(p, 200 / cheapRuler.units.inches);``````

## Precision

A table that shows the margin of error for `ruler.distance` compared to `node-vincenty` (a state of the art distance formula):

lat 10° 20° 30° 40° 50° 60° 70° 80°
1km 0% 0% 0% 0% 0% 0% 0% 0% 0%
100km 0% 0% 0% 0% 0% 0% 0.01% 0.01% 0.04%
500km 0% 0% 0% 0.01% 0.02% 0.04% 0.08% 0.2% 0.83%
1000km 0% 0% 0.02% 0.04% 0.07% 0.15% 0.31% 0.78% 3.36%

Errors for all other methods are similar.

### Related Repositories

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#### cheapyRuler

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#### CheapRulerIOS

Swift port of https://github.com/mapbox/cheap-ruler ...

### Releases

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