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[求助]
ArcGlobe二次开发，鼠标拾取点转换成地理坐标错误
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第一次发帖，不知道有什么要注意的，有什么不足请大家多多指教。 C#+ArcEngine Globe二次开发，通过鼠标拾取点转换得到地理坐标。看帮助文档和别人基本都是这么做的，直接利用WindowToGeographic函数就能从二维屏幕坐标转换到三维空间大地坐标，我的代码如下： IGlobeViewUtil globeViewUtil = axGlobeControl1.GlobeCamera as IGlobeViewUtil; IGlobeDisplay pGlobeDisplay = axGlobeControl1.GlobeDisplay; double dLon = 0; double dLat = 0; double dAlt = 0; globeViewUtil.WindowToGeographic(pGlobeDisplay, pGlobeDisplay.ActiveViewer, e.x, e.y, true, out dLon, out dLat, out dAlt);//由屏幕获取经纬度和高程 这里我有点儿疑问。鼠标在屏幕上点击可获取一个二维坐标，映射到三维场景中其实就从观察点到屏幕坐标的一条射线，转换到三维空间不可能是一个特定的点，除非给定深度值或者说能严格限定视线长度的条件。我不知道这个函数是怎么确定的，猜想可能是得到视线和三维地形的交点，也就是说拾取的这个点在地面上。 但是我现在做这一步的时候，周围的地形高度都在100米左右，鼠标拾取的点地理坐标的高度有七百多米，使得相应的经纬度也成了错误的。如下图所示：  e是鼠标点取的位置（跟踪了下应该没问题），转换出来的应该是P的位置，但是实际上得到了P'的位置，由于高度不对，使得经纬度也有很大的偏差。直观的感受就是视线长度不对，还没有延伸到和地面的交点上。中间只用了一个函数，一直不知道为什么会出现这个问题。是不是有什么地方需要设置？菜鸟求指导，先谢谢了。 |
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2楼2013-11-12 21:58:55
【答案】应助回帖
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(1) WindowToGeographic() function was introduced for a fast and rough estimate of a geographic location that can then be used, for example, for accumulating the geographic extent of the 3D view as defined by a window; or as a faster (and rougher) substitute for standard Locate() when doing picking. (2) It appears that a safe mode from within an AfterDraw() handler, is 'useZBufferValue = true' -- because otherwise Locate() is used and will draw the whole frame again in our 'internal' transformation and scaling for precision. (3) Regarding precision: (a) when useZBufferValue is true, we directly read from the Z-Buffer the depth component and then use gluUnProject() to get x, y, z coordinates. The precision of Z-Buffer depth values varies with "how far you zoom in or out" and is not that precise anyway. (b) when useZBufferValue is false, we use Locate() which uses a 'Pick-Render' pass to calculate the intersection with the mesh at the current LOD as defined by the view -- so again the precision varies with the current view parameters. - Note that the precision of the altitude (i.e. elevation) component can be improved by additionally calling IGlobeDisplay::GetSurfaceElevation(); with bGetMaxResolution argument set to true. (4) Currently we are not using a "mesh-ray intersection algorithm " which would work for us only when no elevation data is present -- the case that can probably be easily calculated on the client side anyway. |
3楼2013-11-12 22:02:11
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