index.js

import {getCoords, collectionOf} from '@turf/invariant';
import {featureEach} from '@turf/meta';
import {isObject} from '@turf/helpers';

/**
 * Takes a {@link Point} grid and returns a correspondent matrix {Array<Array<number>>}
 * of the 'property' values
 *
 * @name gridToMatrix
 * @param {FeatureCollection<Point>} grid of points
 * @param {Object} [options={}] Optional parameters
 * @param {string} [options.zProperty='elevation'] the property name in `points` from which z-values will be pulled
 * @param {boolean} [options.flip=false] returns the matrix upside-down
 * @param {boolean} [options.flags=false] flags, adding a `matrixPosition` array field ([row, column]) to its properties,
 * the grid points with coordinates on the matrix
 * @returns {Array<Array<number>>} matrix of property values
 * @example
 *   var extent = [-70.823364, -33.553984, -70.473175, -33.302986];
 *   var cellSize = 3;
 *   var grid = turf.pointGrid(extent, cellSize);
 *   // add a random property to each point between 0 and 60
 *   for (var i = 0; i < grid.features.length; i++) {
 *     grid.features[i].properties.elevation = (Math.random() * 60);
 *   }
 *   gridToMatrix(grid);
 *   //= [
 *     [ 1, 13, 10,  9, 10, 13, 18],
 *     [34,  8,  5,  4,  5,  8, 13],
 *     [10,  5,  2,  1,  2,  5,  4],
 *     [ 0,  4, 56, 19,  1,  4,  9],
 *     [10,  5,  2,  1,  2,  5, 10],
 *     [57,  8,  5,  4,  5,  0, 57],
 *     [ 3, 13, 10,  9,  5, 13, 18],
 *     [18, 13, 10,  9, 78, 13, 18]
 *   ]
 */
export default function gridToMatrix(grid, options) {
    // Optional parameters
    options = options || {};
    if (!isObject(options)) throw new Error('options is invalid');
    var zProperty = options.zProperty || 'elevation';
    var flip = options.flip;
    var flags = options.flags;

    // validation
    collectionOf(grid, 'Point', 'input must contain Points');

    var pointsMatrix = sortPointsByLatLng(grid, flip);

    var matrix = [];
    // create property matrix from sorted points
    // looping order matters here
    for (var r = 0; r < pointsMatrix.length; r++) {
        var pointRow = pointsMatrix[r];
        var row = [];
        for (var c = 0; c < pointRow.length; c++) {
            var point = pointRow[c];
            // Check if zProperty exist
            if (point.properties[zProperty]) row.push(point.properties[zProperty]);
            else row.push(0);
            // add flags
            if (flags === true) point.properties.matrixPosition = [r, c];
        }
        matrix.push(row);
    }

    return matrix;
}

/**
 * Sorts points by latitude and longitude, creating a 2-dimensional array of points
 *
 * @private
 * @param {FeatureCollection<Point>} points GeoJSON Point features
 * @param {boolean} [flip=false] returns the matrix upside-down
 * @returns {Array<Array<Point>>} points ordered by latitude and longitude
 */
function sortPointsByLatLng(points, flip) {
    var pointsByLatitude = {};

    // divide points by rows with the same latitude
    featureEach(points, function (point) {
        var lat = getCoords(point)[1];
        if (!pointsByLatitude[lat]) pointsByLatitude[lat] = [];
        pointsByLatitude[lat].push(point);
    });

    // sort points (with the same latitude) by longitude
    var orderedRowsByLatitude = Object.keys(pointsByLatitude).map(function (lat) {
        var row = pointsByLatitude[lat];
        var rowOrderedByLongitude = row.sort(function (a, b) {
            return getCoords(a)[0] - getCoords(b)[0];
        });
        return rowOrderedByLongitude;
    });

    // sort rows (of points with the same latitude) by latitude
    var pointMatrix = orderedRowsByLatitude.sort(function (a, b) {
        if (flip) return getCoords(a[0])[1] - getCoords(b[0])[1];
        else return getCoords(b[0])[1] - getCoords(a[0])[1];
    });

    return pointMatrix;
}