# Coordinate Systems

### ENU

* a local projection from sphere to cylinder (base point is not fixed)
* z axis is always the opposite direction of gravity (z can represent altitude)
* the far from base point, the greater distortion of the space

### GENU

* a global projection from sphere to cylinder (base point is fixed)
* the far from base point, the greater difference between z axis and gravity (z cannot represent altitude anymore)
* no distortion of the space, no matter how far from the base point

### ECEF

* the earth-centered coordinate system
* no distortion, but not intuitive for objects moving on earth surface

### References

* <https://zhuanlan.zhihu.com/p/33295151>
* <https://en.wikipedia.org/wiki/Mercator_projection>
* <https://en.wikipedia.org/wiki/Local_tangent_plane_coordinates>
* <https://en.wikipedia.org/wiki/Geographic_coordinate_conversion#From_ECEF_to_ENU>

![](/files/2hWFwwo2DM1ibaaZbcWx)

![](/files/fAYaXPbZNY5OtcROuBP4)


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