2D geometry calculation - on flat surface

Question

I am trying to do some geometry calculation (e.g distance from point to line) on 2D planar planes.

Currently investigating turfjs to calculate distance from point to line.

---

Code

distance.js:

// hello
var turf = require('@turf/turf')

init();

// init,
function init() {
    distance();
}

function distance() {
    // line
    let line = turf.lineString([
    [0, 0],
    [3, 4]
    ]);
    console.log(line);

    // point
    let point = turf.point([3, 0]);
    console.log(point);

    // calculate distance from point to line, using planar,
    let dis = turf.pointToLineDistance(point, line, {
    units: 'degrees',
    method: 'planar'
    });
    console.log("distance: %f", dis);
}

---

Output

distance: 2.398995395932417

If I change point to [30, 0] and line to [0, 0], [30, 40], then the output is:

distance: 25.741472914575986

The program is using degrees as unit, and planar as method.

The expected results are 2.4 and 24, but it's not.

---

Question

• So, does that means those points are on a curved surface, instead of a flat plane?
• Is it possible to define points & lines & polygons in flat plane, instead of the curved surface?
• If not, then it there another similar tool to perform this task?

Answer 1

If you look at turf.js source code for point to line distance, you'll see that it calculates planar distances along a rhumb line, and that's not what you would expect in planar geometry.

To achieve what you want, you could implement the equation yourself, something like

function pointToLineDistance(line, p0) {
    var p1 = line[0],
        p2 = line[1];

    var x0 = p0[0], y0 = p0[1],
        x1 = p1[0], y1 = p1[1],
        x2 = p2[0], y2 = p2[1]


    return Math.abs(
        (y2 - y1) * x0
        - (x2 - x1) * y0
        + x2 * y1
        - y2 * x1
    ) 
    /
    Math.sqrt(
        (y2 - y1) * (y2 - y1)
        +
        (x2 - x1) * (x2 - x1)
    );
}

console.log(
    pointToLineDistance([[0, 0], [3, 4]], [3, 0]),
    pointToLineDistance([[0, 0], [30, 40]], [30, 0]),
);

Or if you prefer a library, something like flatten.js could help you :

let Flatten = require('flatten-js');

let {point, line} = Flatten;

let l = line(point(0, 0), point(3, 4));
let p = point(3, 0)
let d = p.distanceTo(l);

console.log(d[0]);