D3DXVec3Unproject is inaccurate in C++

I have a problem in my 3D Direct X Map Application (aka Google Earth) All coordinates are geocentric (XYZ) in meters. The maps are drawn above earth ellipsoid. Everything works nice, except one issue. When I go to very close zoom (about 1 meter per pixel), the D3DXVec3Unproject function returns inaccurate results. I have an error of 10-30 meters and cannot make map panning. A lot of investigations isolate problem as follows:

D3DXVECTOR3 vScreen, vBack;
D3DXVec3Project( &vScreen, &m_vecAt, &m_viewPort, &m_matProj, &m_matView, &m_matWorld );    
D3DXVec3Unproject( &vBack, &vScreen, &m_viewPort, &m_matProj, &m_matView, &m_matWorld );

m_vecAt is a 开发者_C百科looking point. D3DXVec3Project returns correct vScreen - exactly center of window (+/- 0.5 pixel) with vScreen.z = 0.99, but vBack differs from m_vecAt by tens meters.

Sign again, that everything else works, so I assume, that all matrices and other data are calculated correctly.

The only one reason, that I can think about, is a not enough calculation precision of single float point arithmetic.

Please help.

Thank you guys. I solved the problem. The near plane of projection matrix was too close to camera.

Precision of float might be one reason but another issue you should investigate is the accuracy of the ellipsoid or geoid model you are using. Errors at the range of 10~40 meters is expected in most usual models. More precise models are available but they are significantly more complex. Wikipedia article has some good links to follow.

It can't be the geoid given that the example code is generic 3D projection code which doesn't know anything about it. It must be floating point inaccuracy. Is it particularly bad for points towards the back of screen space (ie z ~ 1)? Since you're projecting a frustum to a cube, the back of the projected volume covers more world space, so any inaccuracy is more pronounced.