HTML5地理定位技术详解

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"HTML5精品书籍，专注于HTML5的地理位置定位功能，适合英语水平较高的读者阅读" HTML5是现代网页开发中的核心标准之一，其中的一项重要特性就是地理位置定位（Geolocation）。这个特性允许网页应用获取用户的地理位置信息，从而提供更个性化的服务，比如基于位置的搜索、导航、社交网络分享等。在"HTML5 Geolocation"这本书中，作者Anthony T. Holdener III深入探讨了这一功能的使用和实现。 HTML5的地理位置定位API是跨平台的，可以在各种设备上运行，包括桌面电脑、智能手机和平板电脑。通过这个API，开发者可以请求访问用户的经纬度坐标，甚至高度、速度等附加信息。这些信息通常是通过GPS、Wi-Fi信号强度、移动通信基站等多种方式综合计算得出的。书中的内容可能涵盖了以下几个方面： 1. **API基础**：介绍如何在JavaScript中调用Geolocation API，包括`navigator.geolocation`对象的使用，以及`watchPosition()`和`getCurrentPosition()`方法的详细解释。 2. **权限管理**：讨论用户隐私和安全问题，解释如何处理位置信息的获取权限，以及如何在用户拒绝访问时优雅地处理。 3. **误差处理**：讲解如何处理定位数据的误差，包括精度、时间和垂直误差，以及如何使用`position.coords`属性获取这些信息。 4. **实时定位更新**：介绍`watchPosition()`方法如何持续监控用户的位置变化，并在位置更新时触发回调函数。 5. **最佳实践**：提供优化定位体验的建议，如在何时请求位置信息、如何减少电池消耗等方面。 6. **示例与应用**：书中可能包含多个实际应用示例，展示如何将地理位置信息集成到网页应用中，例如创建一个简单的地图应用或者一个基于位置的社交网络功能。 7. **兼容性与测试**：讨论不同浏览器和设备对HTML5 Geolocation的支持情况，以及如何进行跨平台测试。 8. **错误处理**：介绍如何处理定位失败的情况，以及如何设置`position.error`对象来捕获和处理错误。 9. **隐私策略和法规**：提醒开发者遵守相关的隐私政策和法律法规，确保用户数据的安全。 10. **未来展望**：可能还会探讨HTML5 Geolocation的未来发展，包括新特性和技术趋势。通过阅读这本书，读者不仅可以掌握HTML5地理位置定位的基本使用，还能了解到与之相关的最佳实践和潜在挑战，从而在开发中更好地利用这一功能，提升用户体验。同时，由于是外语版，这本书也提供了提升专业英语阅读能力的机会。

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Acknowledgments

First, a special thanks to my wife, Sarah, for not only taking care of things while I was

busy writing (especially managing the kids), but also for putting on your editing hat

and taking a red pen to the first draft of the book. I know you made this book more

readable with your amazing writing skills. I am glad to have written something you

were interested in reading!

I want to thank the reviewers who gave me suggestions, comments, and corrections;

you made this a better book and I really appreciate it. Brian Dunn and John Jackson—

thank you.

Also a big thank you to my editor, Simon St.Laurent, who not only continues to give

me opportunities to write on topics I care about, but is also a great editor and a pleasure

to work with. Thank you for having the confidence in me to allow me to put pen to

paper once again for O’Reilly Media.

Finally, I want to thank everyone else who helped make this book happen. Thanks to

O’Reilly Production Services for proofreading this work and to Adam Zaremba for all

of the last minute production edits. To Karen, thank you for my cover animal. Thank

you David for getting the book layout the way it needed to be. And Robert, thank you

for interpreting my hand-drawn figures and creating the great illustrations that you did.

I am honored to have created a book about geolocation for the Web, and found it a

pleasure to write, difficult though it was at times. I hope you enjoy it!

xiv | Preface

CHAPTER 1

Finding Our Way

As long as people have been traveling from one place to another on Earth, they have

used a variety of methods, with varying degrees of accuracy, to calculate approximately

where they are located at any given time. As our technologies have improved, so has

our ability to detect our position accurately. The term geolocation is best described as

the determination of the geographic position of a person, place, or thing. In our modern

era this involves the use of Internet-enabled devices (computers, routers, tablets, etc.),

smartphones, or GPS-based systems.

Geolocation in the Past

While now it is easy and convenient to identify our position using devices with built-

in GPS capabilities, it has not always been the case. Over the millennia people have

come up with many inventive solutions to calculate where they were, which is the

essence of geolocation—figuring out your real-world geographic location with availa-

ble technology.

Location B.C.E.

Thousands of years ago, people relied on visual forms of geolocation to help in orienting

where they were in a given area. One of the earliest forms of visual location documented

in history was the smoke signal. Recorded evidence indicates that ancient Chinese,

Greeks, and Native Americans used smoke signals to aid navigation and to communi-

cate over great distances (as far as the eye can see). Smoke signals assisted the navigator

by providing a better frame of reference to similarly shaped terrain, and would also give

a rough approximation of distance from the signal. These indications would be helpful

to hunting parties and other expeditions in finding their way back home.

As human cultures advanced, so did our understanding of mathematics and nature.

Ancient seafarers discovered the position of the sun and stars in relation to the Earth,

and calculated how to use the angles of certain “fixed” stars, like the Pole Star (otherwise

known as the North Star) in their navigational calculations. Civilizations like the

Greeks, Phoenicians, Norse, Persians, and Chinese all used the stars to assist in navi-

gating the seas, developing tools, like those discussed further later, which allowed them

to venture out past the sight of land. The ability to venture farther and farther from

their homes led to the discovery of new lands and the spread of their respective

civilizations.

Technology with Exploration

Navigation of the oceans and seas during the Middle Ages onward was primarily for

trade with other parts of the world, though exploration began to play a larger and larger

role starting in the 15

century. Sailing farther distances across vast bodies of water

was made possible by better tools to help in the location of ships as they moved through

trackless waters.

The Arab Empire made vast contributions to navigation early in the Middle Ages while

it was one of the major economic powers for over six centuries. This was accomplished

in large part because of those people’s ability to travel not only the river trade routes,

but the oceans as well. The primary tools employed by the Arabs on their ships were a

magnetic compass and an instrument known as a kamal, pictured in Figure 1-1. The

kamal was a navigation device that aided in the determination of latitude. Used first by

Arab sailors, this technology eventually spread to Indian and Chinese navigators as

well. The kamal consisted of a rectangular piece of wood to which a string with several

equally spaced knots would be attached through a hole in the middle.

The angle of the

wooden card, slid along the string until aligned to a fixed star, such as the North Star,

could then be measured by counting the number of knots from the end of the string to

the card.

The magnetic compass used on ships during this time period used the same basic prin-

ciples as a compass does today. A magnetized pointer swiveled on a pin to align itself

with Earth’s magnetic field. The combination of the compass and kamal allowed sea-

farers to calculate headings and rough latitudes in seas nearer to the equator.

As the centuries passed, the sailors of Europe began to venture farther out to sea. They

had been introduced to the navigational tools of the Arab sailors, but found that the

kamal, in particular, did not work very well at the higher latitudes where the Europeans

sailed. For these latitudes they needed more complex devices to calculate the angles of

the sun and stars. The first device developed was the cross-staff, also known as a Jacob’s

staff at that time. It functioned using the same basic principles as the kamal, except it

was made of two longer pieces of wood shaped like a cross. It was eventually replaced

by more precise instruments, namely the sea astrolabe and the quadrant.

The astrolabe was a graduated circle used to measure vertical angles at the sun’s dec-

lination, or the declination of a fixed star. These astrolabes were built specifically for

boats and meant to withstand rough seas and wind. At about this same time, navigators

* Launer, Donald. Navigation Through the Ages. Sheridan House; First American Edition, 2009.

2 | Chapter 1: Finding Our Way

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