osgEarth 2.4 官方文档：地理空间SDK与地形引擎

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"osgearth2.10官方帮助文档" 这篇文档是osgEarth的Release 2.4版，尽管标题显示的是Release 2.4，但内部包含的内容是最新的，例如如何加载ImageLayer来添加基础地图数据。osgEarth是一个用于OpenSceneGraph应用的地理空间SDK和地形引擎。它旨在简化3D地理空间应用的开发，直接从源数据快速可视化地形模型和图像，并与开放的地图标准、技术和数据进行交互。 osgEarth项目简介： 1.1.1 Introduction osgEarth是一个开源项目，它为基于OpenSceneGraph的应用提供了一个强大的框架，用于构建具有地理空间功能的3D应用程序。这个框架使得开发者能够更轻松地处理地形数据和影像数据，而无需复杂的离线地形数据库创建工具。构建osgEarth：在文档的“Building osgEarth”部分，会详细指导用户如何构建和安装osgEarth库。这通常涉及获取源代码、配置构建环境、设置依赖项以及编译和链接过程。对于开发者来说，这部分内容是必不可少的，因为它允许自定义和优化集成到自己的项目中的osgEarth版本。用户指南：在“User Guide”章节中，文档将向用户介绍如何使用osgEarth的基本功能，包括初始化场景、加载地形模型、添加图像层（如示例中的blue marble数据）和其他地理数据。此外，可能还会涵盖如何设置视图、交互控制以及如何与其他OpenSceneGraph对象集成。开发者主题：深入到“Developer Topics”，这部分将详细介绍osgEarth的高级功能和技术细节，如自定义图层类型、插件开发、扩展渲染管道等。对于希望对osgEarth进行定制和扩展的开发者来说，这部分内容非常宝贵。处理数据：在“Working with Data”章节，文档将涵盖如何加载和管理各种地理空间数据格式，如GDAL支持的TIFF文件（如示例中的bluemarble.tif）。这部分可能会涉及数据投影、坐标转换、数据裁剪和压缩等。参考指南： “Reference Guides”提供了详细的API参考，帮助开发者查找和理解osgEarth中的类、方法和函数，以便在代码中正确地调用它们。常见问题解答： “FAQ”部分列出了开发者和用户可能遇到的问题及其解决方案，有助于快速解决遇到的难题。发行说明： “Release Notes”记录了该版本的更新、改进和已知问题，对于升级或初次使用新版本的用户来说，这部分内容至关重要。 osgEarth优先支持：最后，“osgEarth Priority Support”可能提供了关于官方支持政策的信息，包括哪些功能受到优先级支持，以及如何获取技术支持。 osgEarth是一个强大的工具，能够帮助开发者利用OpenSceneGraph构建功能丰富的3D地理空间应用。通过这个详尽的文档，用户和开发者可以学习如何充分利用osgEarth的各项特性，从而创建出引人入胜且实用的地理空间应用。

osgEarth Documentation, Release 2.4

Argument Description

--out file_name output ﬁle for boundary geometry (default is boundary.txt)

--no-geocentric Skip geocentric reprojection (for ﬂat databases)

--convex-hull calculate a convex hull instead of a full boundary

--verbose print progress to console

--view show result in 3D window

--tolerance N vertices less than this distance apart will be coalesced (0.005)

--precision N output coordinates will have this many signiﬁcant digits (12)

osgearth_overlayviewer

osgearth_overlayviewer is a utility for debugging the overlay decorator capability in osgEarth. It shows two windows,

one with the normal view of the map and another that shows the bounding frustums that are used for the overlay

computations.

1.3.2 Using Earth Files

An Earth File is an XML description of a map. Creating an earth ﬁle is the easiest way to conﬁgure a map and get

up and running quickly. In the osgEarth repository you will ﬁnd dozens of sample earth ﬁles in the tests folder,

covering various topics and demonstrating various features. We encourage you to explore and try them out!

Also see: Earth File Reference

Contents of an Earth File

osgEarth uses an XML based ﬁle format called an Earth File to specify exactly how source data turns into an OSG

scene graph. An Earth File has a .earth extension, but it is XML.

Fundamentally the Earth File allows you to specify:

• The type of map to create (geocentric or projected)

• The image, elevation, vector and model sources to use

• Where the data will be cached

A Simple Earth File

Here is a very simple example that reads data from a GeoTIFF ﬁle on the local ﬁle system and renders it as a geocentric

round Earth scene:

<url>world.tif</url>

</image>

</map>

This Earth File creates a geocentric Map named MyMap with a single GeoTIFF image source called bluemarble.

The driver attribute tells osgEarth which of its plugins to use to use to load the image. (osgEarth uses a plug-in

framework to load different types of data from different sources.)

Some of the sub-elements (under image) are particular to the selected driver. To learn more about drivers and how to

conﬁgure each one, please refer to the Driver Reference Guide.

1.3. User Guide 11

osgEarth Documentation, Release 2.4

Multiple Image Layers

osgEarth supports maps with multiple image sources. This allows you to create maps such as base layer with a

transportation overlay or provide high resolution insets for speciﬁc areas that sit atop a lower resolution base map.

To add multiple images to a Earth File, simply add multiple “image” blocks to your Earth File:

<url>c:/data/bluemarble.tif</url>

</image>

</image>

</map>

The above map provides two images from local data sources using the GDAL driver. Order is important when deﬁning

multiple image sources: osgEarth renders them in the order in which they appear in the Earth File.

Tip: relative paths within an Earth File are interpreted as being relative to the Earth File itself.

Adding Elevation Data

Adding elevation data (sometimes called “terrain data”) to an Earth File is very similar to adding images. Use an

elevation block like so:

<url>c:/data/bluemarble.tif</url>

</image>

</elevation>

</map>

This Earth File has a base bluemarble image as well as a elevation grid that is loaded from a local GeoTIFF ﬁle.

You can add as many elevation layers as you like; osgEarth will combine them into a single mesh.

As with images, order is important - For example, if you have a base elevation data source with low-resolution coverage

of the entire world and a high-resolution inset of a city, you need specify the base data FIRST, followed by the high-

resolution inset.

Some osgEarth drivers can generate elevation grids as well as imagery.

Note: osgEarth only supports single-channel 16-bit integer or 32-bit ﬂoating point data for use in eleva-

tion layers.

12 Chapter 1. Table of Contents

osgEarth Documentation, Release 2.4

Caching

Since osgEarth renders data on demand, it sometimes needs to do some work in order to prepare a tile for display. The

cache exists so that osgEarth can save the results of this work for next time, instead of processing the tile anew each

time. This increases performance and avoids multiple downloads of the same data.

Here’s an example cache setup:

<url>http://readymap.org/readymap/tiles/1.0.0/7/</url>

</image>

<path>c:/osgearth_cache</path>

</cache>

</options>

</map>

This Earth File shows the most basic way to specify a cache for osgEarth. This tells osgEarth to enable caching and to

cache to the folder c:/osgearth_cache. The cache path can be relative or absolute; relative paths are relative to

the Earth File itself.

There are many ways to conﬁgure caching; please refer to the section on Caching for more details.

1.3.3 Caching

Depending on the nature of the source data, osgEarth may have to perform some processing on it before it becomes a

terrain tile. This may include downloading, reprojection, cropping, mosacing, or compositing, to name a few. These

operations can become expensive. By setting up a cache, you can direct osgEarth to store the result of the processing

so that it doesn’t need to do it again the next time the same tile is needed.

Note! osgEarth’s cache uses an internal data storage representation that is not intended to be accessed

through any public API. It’s intended for use ONLY as a transient cache and not at a data publication

format. The structure is subject to change at any time. If you want to publish a data repository, consider

the osgearth_package utility instead!

Setting up a Cache

You can set up a cache in your earth ﬁle. The following setup will automatically activate caching on all your imagery

and elevation layers:

<map>

<path>folder_name</path>

</cache>

In code this would look like this:

1.3. User Guide 13

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