Calculate great circles routes as lines in GeoJSON or WKT format.
Algorithms from http://williams.best.vwh.net/avform.htm#Intermediate
Includes basic support for splitting lines that cross the dateline, based on a partial port of code from OGR.
$ npm install --save arc
$ bower install --save arc.js
Require the library in node.js like:
var arc = ;
Use in the browser like:
1) Create start and end coordinates
First we need to declare where the arc should start and end
var start = x: -122 y: 48 ;var end = x: -77 y: 39 ;
x here is longitude in degrees and
y is latitude in degrees.
2) Create GreatCircle object
Next we pass the start/end to the
GreatCircle constructor, along with an optional object representing the properties for this future line.
var generator = start end 'name': 'Seattle to DC';
3) Generate a line arc
Then call the
Arc function on the
GreatCircle object to generate a line:
var line = generator;
line will be a raw sequence of the start and end coordinates plus an arc of
intermediate coordinate pairs.
> lineproperties: name: 'Seattle to DC'coords:-122 4800000000000001-11206161978373486 477241672604096-10238404317022653 4660813199882492-9322718895342909 44716217302635705-8474823988299501 4214415510795357-77 3899999999999999length: 6
The first argument to
Arc specifies the number of intermediate vertices you want in the resulting line. The higher the number the more dense and accurate the line will be.
The second argument is an optional object to declare options. The
offset option controls the likelyhood that lines will be split which cross the dateline. The higher the number the more likely. The default value is 10, which means lines within 10 degress of the dateline will be split. For lines that cross and dateline and are also near the poles you will likely need a higher value to trigger splitting. It is unclear to me (@springmeyer) what the drawbacks are of high offsets. I simply ported the code from OGR's
gdal/ogr/ogrgeometryfactory.cpp and have not taken the time to fully comprehend how it works.
4) Convert line to GeoJSON geometry
To serialize to a GeoJSON geometry:
> line;geometry:type: 'LineString'coordinates: Object Object Object Object Object Objecttype: 'Feature'properties: name: 'Seattle to DC'
Or to WKT (Well known text):
> line;'LINESTRING(-122 48.00000000000001,-112.06161978373486 47.7241672604096,-102.38404317022653 46.60813199882492,-93.22718895342909 44.716217302635705,-84.74823988299501 42.14415510795357,-77 38.99999999999999)'
It is then up to you to add up these features to create fully fledged geodata. See the examples/ directory for sample code to create GeoJSON feature collection from multiple routes.