Quickstart: Determining current heading with the compass (HTML)

You can use the compass to determine the current heading with an app written in JavaScript.

Navigation apps use the compass to determine the direction a device is facing and then orient the map accordingly.

Objective: After completing this quickstart you will understand how to use the compass to detect changes in heading.

Prerequisites

You should be familiar with HTML, JavaScript, and events.

The device or emulator that you're using must support an compass.

Time to complete: 15 minutes.

Instructions

1. Open Microsoft Visual Studio

Open an instance of Microsoft Visual Studio

2. Create a new project

Create a new project, choosing a Blank App from the JavaScript/Store Apps project types.

3. Replace the JavaScript code

Open your project's default.js file and replace the existing code with the following.

// For an introduction to the Blank template, see the following documentation:
// https://go.microsoft.com/fwlink/p/?linkid=232509
(function () {
    "use strict";
    var compass;

    var app = WinJS.Application;

    // This function is invoked within onDataChanged to
    // retrieve the given identifier from the HTML document.
    function id(elementId) {
        return document.getElementById(elementId);
    }

    // This function is called each time a compass event
    // is fired by the driver.
    function onDataChanged(e) {
        var reading = e.reading;
        id('txtMagNorth').innerHTML = reading.headingMagneticNorth.toFixed(2);
        if (reading.headingTrueNorth != null) {
            id('txtTrueNorth').innerHTML = reading.headingTrueNorth.toFixed(2);
        }
    }

    // This function responds to all app activations.
    app.onactivated = function (eventObject) {
        if (eventObject.detail.kind === Windows.ApplicationModel.Activation.ActivationKind.launch) {
            // Retrieve the default compass and
            compass = Windows.Devices.Sensors.Compass.getDefault();

            // Choose a report interval supported by the sensor
            var minimumReportInterval = compass.minimumReportInterval;
            var reportInterval = minimumReportInterval > 16 ? minimumReportInterval : 16;
            compass.reportInterval = reportInterval;

            // Establish the event handler
            compass.addEventListener("readingchanged", onDataChanged);
            WinJS.UI.processAll();
        }
    };

    app.start();
})();

4. Add the HTML for the apps

Open the default.html file for the Windows and Windows Phone projects, and copy the following HTML into inside the BODY tags of the file.

    <div class="item" id="scenario1Output">
        Magnetic North: <a id="txtMagNorth">no data</a>
        <br />
        True North: <a id="txtTrueNorth">no data</a>
    </div>

5. Build, deploy and run the app

Press F5 or select Debug > Start Debugging to build, deploy, and run the app.

Once the app is running, you can change the accelerometer values by moving the device or using the emulator tools.

6. Stop the app

1. Press ALT+Tab to return to Visual Studio.
2. Press Shift+F5 or select Debug > Stop Debugging to stop the app.

Summary and next steps

The previous example demonstrates how little code you'll need to write in order to integrate compass input in your app.

The app establishes a connection with the default compass in the onactivated function. This occurs on the following line.

compass = Windows.Devices.Sensors.Compass.getDefault();

The new compass data is captured in the onDataChanged function. Each time the sensor driver receives new data from the sensor, it passes the values to your app by using this function (or event handler). The app registers this event handler on the following line.

compass.addEventListener("readingchanged", onDataChanged);

These new values are written to the screen via updates to the DOM elements shown below.

    <div class="item" id="scenario1Output">
        Magnetic North: <a id="txtMagNorth">no data</a>
        <br />
        True North: <a id="txtTrueNorth">no data</a>
    </div>

The app establishes the report interval within the onactivated function. This code retrieves the minimum interval supported by the device and compares it to a requested interval of 16 milliseconds (which approximates a 60-Hz refresh rate). If the minimum supported interval is greater than the requested interval, the code sets the value to the minimum. Otherwise, it sets the value to the requested interval.

var minimumReportInterval = accelerometer.minimumReportInterval;
var reportInterval = minimumReportInterval > 16 ? minimumReportInterval : 16;
accelerometer.reportInterval = reportInterval;

If you're writing a navigation app, the next steps will involve using compass input to control map orientation.

Related topics

Compass class

Compass Sample