Hi Folks, Glad to have you on the right side of the barricade! hjenter@stress.er.usgs.gov (Harry Jenter ) writes: > - Text deleted - > I think that netCDF does not require this assumption to be made. It is > just more intuitive for storing rectangular or, more generally, > quadrilateral grids, without introducing one's own conventions. (Note: > I stick to the 2-d grid analogy for the rest of this post. The > arguments can be extended to 3 or more dimensions, but I don't know the > words for "equivalent of quadrilateral" in 3 or more dimensions.) > There seem to be no assumptions regarding grid shape. There are also > seem to be no conventions regarding grid shape. I agree to some extent. netCDF said nothing about the rectangularity of the grid because it looked obvious and intuitive. That was the assumption itself: we did not even mention that the grid can be non- rectangular, we just followed the common sense and sticked to the rectangular grid. When you say: example { dimensions: x = 4; y = 4; variables: float temp( x, y ); int x(x); int y(y); data: x = x1,x2,x3,x4; y = y1,y2,y3,y4; temp = ... ; } Variable 'temp' is automatically considered as given over a rectangular grid: | | | | | | | | y1---|-----|--------|---|----------------- | | | | | | | | | | | | y2---|-----|--------|---|----------------- | | | | y3---|-----|--------|---|----------------- y5---|-----|--------|---|----------------- x1 x2 x3 x4 I think this is a stone-hard assumption made in the definition of netCDF. Surely very intuitive, that is why it became a part of the definiton. But originally, it used to be just a restrictive assumption, purposely limiting the complexity of the grid. > > I think, however, that the issue of storing non-quadrilateral grids is > a more difficult issue to address than the regularly-spaced-coordinate > issue. In fact, it is, in a sense, the opposite problem. It's correct: it is really the opposite. I wanted to point out that rectangular grid is just a special case -however, very obvious and intuitive- of the general polygonal grid. I do think non-rectangular quadrilateral and triangular grids are very important, too. I'm just saying that much unfinished work seems to be left behind our back in the opposite direction, too. > For regularly-spaced coordinates, one can reduce the information stored in > a netCDF file relative to the "intuitive" method of storing a 1-d array for > each coordinate, and still can reproduce the grid. For non-quadrilateral > grids or even quadrilateral-but-not-rectangular grids, one must > increase the amount of information stored in the netCDF file in order > to reproduce the grid. For non-quadrilateral grids, information about > what nodes are connected must be stored. For curvilinear grids, the > coordinate arrays must have more than one dimension. > - Text deleted - Yes, it is perfectly true. At a higher conceptual level the reconstruction of the grid needs more information. But I strongly believe the isssue is not only to reduce the size of the storage but to increase its functionality, too. In this particular case: we need more complex grids than the rectangular. I do not want the user to specify his/her regular data always in the most general way; the user must be able to specify the grid at the lowest possible level in order to keep the storage size and processing time at the minimum. I just propose we streighten out more than the issue of the regularly spaced grids. Why don`t we want the whole pie: the concept for storage of grids of different complexity and mathematical generality. I could imagine something like this: general grids ----> quadrilateral ---> rectangular ---> reqularly spaced | / | \ |--> triangular / | \ | linear | other important |--> other logarithmical Do not be afraid of (mandatory?) attributes and adequate defaulting while it is designed consistently. We just need correct definition preventing future exceptions and additional assumptions. It can be done with you, folks, for sure! Cheers! Gabor Fichtinger Scientific Visualization Group, Center for High Performance Computing The University of Texas System Balcones Research Center, 1.154 CMS 10100 Burnet Road, Austin, TX, 78758-4497 Ph. : (512) 471 2409 Ph. : 1-800-262-2472/2409 (toll free) Fax : (512) 471 2445 Email: gabor@chpc.utexas.edu __o __o -\<, -\<, __________O / O ___________O / O