How to find the midpoint of a polyline?

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04-26-2011 07:52 AM
RenField
New Contributor
Can anyone share with me the easiest way to calculate the midpoint of a polyline on a graphics layer?

TIA
9 Replies
JenniferNery
Esri Regular Contributor
polyline.Extent.GetCenter();
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deleted-user-0W0-oLHxDjCX
New Contributor III

I think you will not get a midpoint if the line is irregular.

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deleted-user-0W0-oLHxDjCX
New Contributor III

Actually the truecentroid property will give you a gravity center of an irregular polyline but if you want to reach the middle point of a feature you have to do something else.

The picture above is wrong. The centroid property will give you the nearest point from centroid (actually, within the polyline).

Sincerely

Ezequias.   

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deleted-user-0W0-oLHxDjCX
New Contributor III

You can do it in Arcpy but I did not find any reference in Javascript.

// Arcpy

Midpoint = geometry.positionAlongLine(0.50,True).firstPoint
... print Midpoint.X
... print Midpoint.Y

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JenniferNery
Esri Regular Contributor

I think this will be the runtime equivalent method then: GeometryEngine.CreatePointAlong Method 

deleted-user-0W0-oLHxDjCX
New Contributor III

Nice to know. Unfortunately I am in a Javascript challenge  

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ScottDavis3
New Contributor II

Try the code below. The get_length is another function that just uses the geometryEngine length function. This works for a single part polyline.

 

function midPointPolyline(line) {
var path = line.paths[0];
var seglen = get_length(line, "feet");
var midLen = seglen / 2;
var currentDistance = 0;
var beforeIndex = 0;
var startPoint = line.getPoint(0, 0);
for (i = 1; i < path.length - 1; i++) {
var nextPoint = line.getPoint(0, i);
var d = geometryEngine.distance(startPoint, nextPoint, "feet");
if (currentDistance + d < midLen) {
currentDistance += d;
startPoint = nextPoint;
} else {
beforeIndex = (i === 1) ? 0 : i;
break;
}
}
startPoint = line.getPoint(0, beforeIndex);
var endPoint = line.getPoint(0, beforeIndex + 1);
var x = (startPoint.x + endPoint.x) / 2;
var y = (startPoint.y + endPoint.y) / 2;
var midpt = new esri.geometry.Point({ x, y, spatialReference: { wkid: line.spatialReference.wkid } });
return midpnt;
}

AndyWhitaker1
New Contributor III

(Deleting double post)

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AndyWhitaker1
New Contributor III

Hi @ScottDavis3.  With your help, I was able to get a working solution.  Thank you!  Seems this would be built into the ArcGIS API for JavaScript?!

I have made a couple of changes to Scott's code.  Most notably, I had to change get_length() to geodesicLength() in the geometryEngine class because I couldn't find get_length().

The code below utilizes the ArcGIS for JavaScript 3.x version.

function getPolylineMidPoint(polyline) {
	const path = polyline.paths[0];
	const length = GeometryEngine.geodesicLength(polyline, "feet"); // esri.geometry.geometryEngine
	const middleLength = length / 2;
	var currentDistance = 0;
	var beforeIndex = 0;
	var startPoint = polyline.getPoint(0, 0);
	for (let i = 1; i < path.length - 1; i++) {
		var nextPoint = polyline.getPoint(0, i);
		var d = GeometryEngine.distance(startPoint, nextPoint, "feet");
		if (currentDistance + d < middleLength) {
			currentDistance += d;
			startPoint = nextPoint;
		} else {
			beforeIndex = (i === 1) ? 0 : i;
			break;
		}
	}
	startPoint = polyline.getPoint(0, beforeIndex);
	const endPoint = polyline.getPoint(0, beforeIndex + 1);
	const x = (startPoint.x + endPoint.x) / 2;
	const y = (startPoint.y + endPoint.y) / 2;
	const midPoint = new Point({ x, y, spatialReference: { wkid: polyline.spatialReference.wkid } }); // esri.geometry.Point
	return midPoint;
};

function doTest () {
	const query = new Query(); // esri.tasks.Query
	query.where = "Type='Tunnel'";
	self._someLayer.queryFeatures(query,
		(fs) => {
			const features = fs.features;
			const midPoint = getPolylineMidPoint(features[0].geometry); // utilizing the midpoint function
			const location = webMercatorUtils.webMercatorToGeographic(midPoint); // esri.geometry.webMercatorUtils
			const symbol = new SimpleMarkerSymbol( // "esri.symbols.SimpleMarkerSymbol"
				SimpleMarkerSymbol.STYLE_CIRCLE,
				15,
				new SimpleLineSymbol( // "esri.symbols.SimpleLineSymbol"
					SimpleLineSymbol.STYLE_SOLID,
					new Color([0, 0, 255, 0.5]),
					8
				),
				new Color([0, 0, 255]) // esri.Color
			);
			const graphic = new Graphic(location, symbol); // esri.Graphic
			self.map.graphics.add(graphic);
		},
		(err) => {
			console.log("Error: " + err);
		});
};

 

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