osgearth3.2平面模式，加载多块本地地形

问题描述：osgearth3.2平面模式，同时加载多个具有相同srs的地形和影像文件

解决思路：

1.首先给map指定一个srs信息，指定一个bound信息，即要显示的范围，加载多块地形时，bound的范围为所有影像的minx,miny,maxx,maxy;

2.使用map->addlayer()依次添加所有的影像层和高程层。

具体实现见代码，此代码中拥有完整的实现流程。

#include 
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#include
#include 
#include 
#include

#define DEBUG
#ifdef DEBUG
#pragma comment(lib,"osgd.lib")
#pragma comment(lib,"osgGAd.lib")
#pragma comment(lib,"osgViewerd.lib")
#pragma comment(lib,"osgEarthd.lib")
#pragma comment(lib,"osgDBd.lib")
#else
#pragma comment(lib,"osg.lib")
#pragma comment(lib,"osgGA.lib")
#pragma comment(lib,"osgViewer.lib")
#pragma comment(lib,"osgEarth.lib")
#pragma comment(lib,"osgDB.lib")
#endif // DEBUG

struct Extents
{
	double minx, miny, maxx, maxy;
	Extents() { minx = 0.0, miny = 0.0, maxx = 0.0, maxy = 0.0; };
};
Extents getExtents(std::string inputfilepath)
{
	GDALDataset* poDataset;
	GDALAllRegister();  //注册所有的驱动
	CPLSetConfigOption("GDAL_FILENAME_IS_UTF8", "NO");
	poDataset = (GDALDataset*)GDALOpen(inputfilepath.c_str(), GA_ReadOnly);//打开数据集

	if (poDataset == NULL)
	{
		cout << "fail in open files!!!" << endl;
		exit(1);
	}

	//获取图像的尺寸
	int nImgSizeX = poDataset->GetRasterXSize();
	int nImgSizeY = poDataset->GetRasterYSize();

	//获取坐标变换系数
	double trans[6];
	CPLErr aaa = poDataset->GetGeoTransform(trans);
	//计算范围
	Extents extents;
	extents.minx = trans[0];
	extents.maxx = trans[0] + nImgSizeX * trans[1];
	extents.miny = trans[3] + nImgSizeY * trans[5];
	extents.maxy = trans[3];
	return extents;
}
std::string getProjectInformation(std::string inputfilepath)
int main()
{
	osgEarth::initialize();
	osgViewer::Viewer viewer;
	osgEarth::GLUtils::setGlobalDefaults(viewer.getCamera()->getOrCreateStateSet());
	osg::ref_ptr