Find length and area

Learn how to calculate the length and area of geometries.

You can calculate the length of a line and determine the area of a polygon using the geometryEngine. The measurement depends on the coordinate system (or spatial reference) defined for the geometry. If the geometry's spatial reference is Web Mercator (3857) or WGS84 (4326), you would use geodesic calculations to take into account the curvature of the Earth. If the spatial reference is something different from Web Mercator (3857) or WGS84 (4326), you would use planar measurements based on Euclidean distances.

In this tutorial, you will use the Sketch component to draw graphics on the view and the geometryEngine to calculate both geodesic and planar lengths and areas to see the difference between the two measurements.

Prerequisites

Steps

Create a new pen

1. To get started, either complete the Display a map tutorial or use this pen.

Get an access token

You need an access token with the correct privileges to access the location services used in this tutorial.

1. Go to the Create an API key tutorial and create an API key with the following privilege(s):
   
   • Privileges
     • Location services > Basemaps
2. In CodePen, set esriConfig.apiKey to your access token.
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     var esriConfig = {
       apiKey: "YOUR_ACCESS_TOKEN"
     };
   

To learn about other ways to get an access token, go to Types of authentication.

Recenter the map and add a scalebar

The ScaleBar component will display a scale bar on the map. You can choose either metric or imperial values. For example, if you set the unit attribute to metric, it will show kilometers or meters, depending on the scale.

1. Change the center and zoom attributes on the arcgis-map.

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       <arcgis-map basemap="arcgis/topographic" center="-10, 30" zoom="3">
   
         <arcgis-zoom position="top-left"></arcgis-zoom>
   
       </arcgis-map>
   

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2. Add an arcgis-scalebar component to the bottom left of the map and set its unit attribute to be "metric".

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       <arcgis-map basemap="arcgis/topographic" center="-10, 30" zoom="3">
   
         <arcgis-zoom position="top-left"></arcgis-zoom>
   
         <!-- Scale bar component -->
         <arcgis-scale-bar position="bottom-left" unit="metric"></arcgis-scale-bar>
   
       </arcgis-map>
   

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3. Run the app to verify the change in the center and zoom level. The scalebar should appear at the bottom left of the map.

Add the Sketch component

The Sketch component provides UI that allows you to create and update graphics.

1. Add an arcgis-sketch component to the top-right corner of the map. Any geometry drawn using the component will be added to a GraphicsLayer that the component will create for us as its layer property. To simplify the user interface, limit the tools available on the component by setting the appropriate attributes.

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       <arcgis-map basemap="arcgis/topographic" center="-10, 30" zoom="3">
   
         <arcgis-zoom position="top-left"></arcgis-zoom>
   
         <!-- Scale bar component -->
         <arcgis-scale-bar position="bottom-left" unit="metric"></arcgis-scale-bar>
   
         <!-- Sketch component -->
         <arcgis-sketch
           creation-mode="update"
           hide-selection-tools-lasso-selection
           hide-selection-tools-rectangle-selection
           hideSnappingControls
           hide-undo-redo-menu
           hide-settings-menu
           position="top-right"
         ></arcgis-sketch>
   
       </arcgis-map>
   

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2. Run the app to verify that the component appears in the view and that you can draw graphics.

Add HTML to display measurements

1. Add an arcgis-placement component and inside of the element tags place a div to display the results of a calculation in the bottom-right corner of the map.

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         <!-- Sketch component -->
         <arcgis-sketch
           creation-mode="update"
           hide-selection-tools-lasso-selection
           hide-selection-tools-rectangle-selection
           hideSnappingControls
           hide-undo-redo-menu
           hide-settings-menu
           position="top-right"
         ></arcgis-sketch>
   
         <!-- Placement component -->
         <arcgis-placement position="bottom-right">
           <div id="measurementsDiv"></div>
         </arcgis-placement>
   

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2. Add some CSS styling to set the background-color, border-radius and padding.

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       <style>
         html,
         body {
           margin: 0;
         }
   
         arcgis-map {
           display: block;
           height: 100vh;
         }
   
         #measurementsDiv {
           background-color: white;
           border-radius: 6px;
           padding: 6px;
         }
   
       </style>
   

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Add modules

1. Use an AMD require statement to add the necessary modules in a new <script> at the bottom of the <body>.

   More info

   The ArcGIS Maps SDK for JavaScript is available as AMD modules and ES modules, but this tutorial is based on AMD. The AMD require function uses references to determine which modules will be loaded – for example, you can specify "esri/layers/FeatureLayer" for loading the FeatureLayer module. After the modules are loaded, they are passed as parameters (e.g. FeatureLayer) to the callback function where they can be used in your application. It is important to keep the module references and callback parameters in the same order. To learn more about the API's different modules visit the Overview Guide page.

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       <script>
   
         require([
           "esri/Graphic",
           "esri/geometry/geometryEngine",
           "esri/geometry/Polygon",
           "esri/geometry/Polyline",
           "esri/symbols/SimpleFillSymbol"
         ], (Graphic, geometryEngine, Polygon, Polyline, SimpleFillSymbol) => {
   
         });
   
       </script>
   

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Get references to the sketch and the measurements elements

1. Use the document.querySelector method to get a reference to the arcgis-sketch component and the #measurementsDiv.

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       <script>
   
         require([
           "esri/Graphic",
           "esri/geometry/geometryEngine",
           "esri/geometry/Polygon",
           "esri/geometry/Polyline",
           "esri/symbols/SimpleFillSymbol"
         ], (Graphic, geometryEngine, Polygon, Polyline, SimpleFillSymbol) => {
   
           // Get references to the arcgis-sketch component and measurementsDiv elements
           const arcgisSketch = document.querySelector("arcgis-sketch");
           const measurementsDiv = document.querySelector("#measurementsDiv");
   
         });
   
       </script>
   

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Set properties on the sketch component

1. Set the availableCreateTools property on the sketch component to further refine the available tools.

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       <script>
   
         require([
           "esri/Graphic",
           "esri/geometry/geometryEngine",
           "esri/geometry/Polygon",
           "esri/geometry/Polyline",
           "esri/symbols/SimpleFillSymbol"
         ], (Graphic, geometryEngine, Polygon, Polyline, SimpleFillSymbol) => {
   
           // Get references to the arcgis-sketch component and measurementsDiv elements
           const arcgisSketch = document.querySelector("arcgis-sketch");
           const measurementsDiv = document.querySelector("#measurementsDiv");
   
           // Set the available create tools for the arcgis-sketch component
           arcgisSketch.availableCreateTools = ["polyline", "polygon", "rectangle"];
   
         });
   
       </script>
   

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Create function to calculate length and area

The geometryEngine allows you to calculate a geometry's planar length/area or geodesic length/area. Because the geometries in this application are projected in Web Mercator, it is best practice to use geodesic measurements. In this tutorial, we calculate both when a graphic is drawn to visualize the difference between geodesic and planar calculations.

1. Create a getArea function that takes a polygon as its parameter. Call the geodesicArea method and the planarArea methods to calculate the area of the polygon in square-kilometers. Append the calculation results to the innerHTML of measurementsDiv.

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           // A function to get the area of a polygon
           function getArea(polygon) {
             const geodesicArea = geometryEngine.geodesicArea(polygon, "square-kilometers");
             const planarArea = geometryEngine.planarArea(polygon, "square-kilometers");
   
             measurementsDiv.innerHTML =
               "<b>Geodesic area</b>:  " +
               geodesicArea.toFixed(2) +
               " km\xB2" +
               " |   <b>Planar area</b>: " +
               planarArea.toFixed(2) +
               "  km\xB2";
           }
   
   

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2. Create a getLength function that takes a line parameter. Call the geodesicLength and planarLength methods to calculate the length of the line in kilometers. Append the results of the geodesic length calculation to the innerHTML of the measurementsDiv.

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           // A function to get the area of a polygon
           function getArea(polygon) {
             const geodesicArea = geometryEngine.geodesicArea(polygon, "square-kilometers");
             const planarArea = geometryEngine.planarArea(polygon, "square-kilometers");
   
             measurementsDiv.innerHTML =
               "<b>Geodesic area</b>:  " +
               geodesicArea.toFixed(2) +
               " km\xB2" +
               " |   <b>Planar area</b>: " +
               planarArea.toFixed(2) +
               "  km\xB2";
           }
   
           // A function to get the length of a polyline
           function getLength(line) {
             const geodesicLength = geometryEngine.geodesicLength(line, "kilometers");
             const planarLength = geometryEngine.planarLength(line, "kilometers");
   
             measurementsDiv.innerHTML =
               "<b>Geodesic length</b>:  " +
               geodesicLength.toFixed(2) +
               " km" +
               " |   <b>Planar length</b>: " +
               planarLength.toFixed(2) +
               "  km";
           }
   
   

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3. Create a getMeasurements function that will either call the getArea function or the getLength function depending on whether the geometry is a polygon or a polyline.

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           // A function to get the length of a polyline
           function getLength(line) {
             const geodesicLength = geometryEngine.geodesicLength(line, "kilometers");
             const planarLength = geometryEngine.planarLength(line, "kilometers");
   
             measurementsDiv.innerHTML =
               "<b>Geodesic length</b>:  " +
               geodesicLength.toFixed(2) +
               " km" +
               " |   <b>Planar length</b>: " +
               planarLength.toFixed(2) +
               "  km";
           }
   
           // A function to get the measurements of a geometry
           function getMeasurements(geometry) {
             switch (geometry.type) {
               case "polygon":
                 getArea(geometry);
                 break;
               case "polyline":
                 getLength(geometry);
                 break;
               default:
                 console.log("No value found");
             }
           }
   

   Expand

Create a graphic to measure

To show the user how to interact with the application, create a polygon and display its area when the app starts.

1. Create a new Polygon. Set the the wkid in the spatialReference property to 3857 (Web Mercator). To style the graphic, create a SimpleFillSymbol.

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           // Create a polygon geometry
           const polygon = new Polygon({
             spatialReference: {
               wkid: 3857
             },
             rings: [
               [
                 [-4508069.082189632, 3599936.936171892],
                 [-4508069.082189632, 5478453.343307884],
                 [-2629552.6750536393, 5478453.343307884],
                 [-2629552.6750536393, 3599936.936171892],
                 [-4508069.082189632, 3599936.936171892]
               ]
             ]
           });
   
           // Create a simple fill symbol
           const simpleFillSymbol = new SimpleFillSymbol({
             outline: {
               color: [200, 0, 0],
               width: 2
             }
           });
   
   

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2. Create an instance of the Graphic class. Set the geometry and symbol property values to the polygon and simpleFillSymbol.

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           // Create a polygon geometry
           const polygon = new Polygon({
             spatialReference: {
               wkid: 3857
             },
             rings: [
               [
                 [-4508069.082189632, 3599936.936171892],
                 [-4508069.082189632, 5478453.343307884],
                 [-2629552.6750536393, 5478453.343307884],
                 [-2629552.6750536393, 3599936.936171892],
                 [-4508069.082189632, 3599936.936171892]
               ]
             ]
           });
   
           // Create a simple fill symbol
           const simpleFillSymbol = new SimpleFillSymbol({
             outline: {
               color: [200, 0, 0],
               width: 2
             }
           });
   
           // Create a polygon graphic
           const polygonGraphic = new Graphic({
             geometry: polygon,
             symbol: simpleFillSymbol
           });
   

   Expand

Add event listeners

Add event listeners to the Sketch component to listen for when the user adds a polygon graphic to the map. Create another event listener to calculate the length and area of the graphic when it's updated.

1. Add an event listener to the sketch component to wait for it to be ready, then add the polygon graphic to the component's graphics layer. Call the component's update method on the polygonGraphic and call the getArea function based on the geometry of the polygonGraphic.

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           // Add the polygon graphic to the arcgis-sketch layer when the arcgis-sketch is ready
           // and update the the measurementsDiv with the area of the polygon
           arcgisSketch?.addEventListener("arcgisReady", () => {
             arcgisSketch?.layer.add(polygonGraphic);
             arcgisSketch.update(polygonGraphic);
             getArea(polygonGraphic.geometry);
           });
   
   

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2. Create an event listener on the sketch component that listens to the arcgisUpdate event. This event is emitted when you create, resize, or move a graphic. Save the geometry of the first graphic from the event.detail.graphics collection as a constant.

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           // Add the polygon graphic to the arcgis-sketch layer when the arcgis-sketch is ready
           // and update the the measurementsDiv with the area of the polygon
           arcgisSketch?.addEventListener("arcgisReady", () => {
             arcgisSketch?.layer.add(polygonGraphic);
             arcgisSketch.update(polygonGraphic);
             getArea(polygonGraphic.geometry);
           });
   
           // Listen for the arcgisUpdate event and update the measurementsDiv
           // based on the new geometry
           arcgisSketch?.addEventListener("arcgisUpdate", (event) => {
             const geometry = event.detail.graphics[0].geometry;
   
           });
   

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3. Create conditional statements based on whether the sketch event's state is: start, complete, or in a state of change. Except for complete, each state calls the getMeasurements function with the geometry as its parameter. When the event is complete, remove the graphic from the sketch component's layer and clear the innerHTML.

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           // Listen for the arcgisUpdate event and update the measurementsDiv
           // based on the new geometry
           arcgisSketch?.addEventListener("arcgisUpdate", (event) => {
             const geometry = event.detail.graphics[0].geometry;
   
             if (event.detail.state === "start") {
               getMeasurements(geometry);
             }
             if (event.detail.state === "complete") {
               arcgisSketch.layer.remove(arcgisSketch.layer.graphics.getItemAt(0));
               measurementsDiv.innerHTML = "";
             }
             if (
               event.detail.toolEventInfo &&
               (event.detail.toolEventInfo.type === "scale-stop" ||
                 event.detail.toolEventInfo.type === "reshape-stop" ||
                 event.detail.toolEventInfo.type === "move-stop")
             ) {
               getMeasurements(geometry);
             }
   
           });
   

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Run the app

In CodePen, run your code to display the map.

When you run the app, you will see a polygon with its calculated geodesic and planar area. You can use the sketch component to draw other geometries and display their measurements. If you move the geometry, the geodesic measurements will change but the planar measurements will not.

What's next?

Learn how to use additional API features and ArcGIS services in these tutorials:

• Display projected geometries
• Calculate geometries
• Find spatial relationships