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
