设置瓦片PagedLOD节点最小可视距离因子，默认是6.0
minTileRangeFactor()
Map *map = new Map();
WWOptions wwImgGlobe;
map->addImageLayer( new ImageLayer( "GlobeImg", wwImgGlobe ) );
WWOptions wwElevationGlobe;
map->addElevationLayer( new ElevationLayer( "GlobeElv", wwElevationGlobe ) );
osgEarth::MapNodeOptions mapNodeOption;
osgEarth::TerrainOptions terrainOption;
osgEarth::LoadingPolicy lp;
lp.mode()=osgEarth::LoadingPolicy::MODE_PARALLEL;
lp.numCompileThreads()=4;
terrainOption.loadingPolicy()=lp;
terrainOption.minTileRangeFactor()=3;
mapNodeOption.setTerrainOptions(terrainOption);
MapNode* mapNode= new MapNode( map, mapNodeOption );
osgearth 配置mapNode TerrainOptions
原文：http://www.cnblogs.com/coolbear/p/4126505.html
