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Hi Wenli, I will try to answer some of your questions based on my understanding, but I am not an expert on grids, so I hope others will correct me if I am wrong. 1) 3d origin / offset vectors and 2d grid: While the grid itself is two dimensional, it spans a plane (defines a two-dimensional grid coordinate system) in a 3d coordinate reference system. Therefore the origin and the two defining axis vectors have three dimensions. 2) GRID-B: "2." is correct. The origin is the origin of the grid coordinate system, but not necessarily a corner of the grid. Choosing the origin is somewhat arbitrary and just a convention - like choosing Greenwich for the zero meridian. So, my assumption so far was that typically when you specify a grid for each coverage individually then you would likely end up with a grid envelope that has (0 0) for the low value, but when you want to use a common grid system across a series of coverages then this may be different. By the way, if you have ISO 19123, I think it has additional examples for rectified grids. Best regards, Clemens
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