Question

I have a very odd problem with javascript. I made a map with OpenLayers and mapdata from OSM.
Everytime I draw a feature (point or line) I fire a function when the sketch is completed called redrawFeatures mainly to split the lines and to give every line a start and an endpoint. Everything works fine so far, but I also want to transform the drawn points (and lines) form the mapprojection (EPSG:900913) to the displayprojection (EPSG:4326) and there is the problem. The following piece of code is the function:

var pointsOnMap = [];
var linesOnMap = [];

function redrawFeatures(e) {
    var vert = e.feature.geometry.getVertices();
    var points = [];
    var pointFeatures = [];
    var lineFeatures = [];

    for (var i = 0; i < vert.length; i++) {
        var point       = new OpenLayers.Geometry.Point(vert[i].x, vert[i].y);

        points.push(point);
        alert(points[i].x + ', ' + points[i].y);
        pointFeatures.push(new OpenLayers.Feature.Vector(point));
        alert(points[i].x + ', ' + points[i].y);
        pointsOnMap.push(point.transform(new OpenLayers.Projection("EPSG:900913"),
                                         new OpenLayers.Projection("EPSG:4326")));
        alert(points[i].x + ', ' + points[i].y);
        if (i >= 1) {
            var line = new OpenLayers.Geometry.LineString([points[i-1], points[i]]);

            lineFeatures.push(new OpenLayers.Feature.Vector(line));
            linesOnMap.push(line.transform(new OpenLayers.Projection("EPSG:900913"),
                                           new OpenLayers.Projection("EPSG:4326")));
        }
    }

    draw.destroyFeatures([e.feature]);
    draw.addFeatures(lineFeatures);
    draw.addFeatures(pointFeatures);

}

The variable named draw is the layer I draw on.

I determine the coordinates from the event, create a new point (line 11) and save them to an array (line 13). Turn the same point into a feature and save it to another array, as well (line 15) The Problem is, when I later transform this point to the displayprojection and save this point in a third array (line 17) the coordinates are also changed in the previously saved value of the first array called points. I alert the values of the saved coordinates trice and this is what I get and don’t understand:

Alert 1 (line 14) That’s right I get the coordinates of the point

Alert 2 (line 16) That’s still right (Same as the first one)

Alert 3 (line 19) Now it turns wrong! I alert the same variable like before but after the transformation of point the value of points[i] also changed without any reason.

Is there any mistake I don’t realize?

Answer 1

It seems that the transform function operates in-place, and modifies the existing point data. You should clone the point and then operate on that instead if you want to preserve the previous value

clone = point.clone();