[Geopackage] Query on gpkg_tile_matrix - matrix_width and matrix_height
Brad Hards
bradh at frogmouth.net
Fri May 29 02:15:36 EDT 2015
On Fri, 29 May 2015 12:54:17 AM you wrote:
> Hi Brad,
>
> > Geopackage spec (2.2.7.1.1) describes the gpkg_tile_matrix table.
> >
> > matrix_width and matrix_height are described as >=1.
>
> Well, Table 9 is a bit more verbose :
> matrix_width Number of columns (>= 1) in tile matrix at this zoom level
> matrix_height Number of rows (>= 1) in tile matrix at this zoom level
>
> and then Table 10 and req 55 and 56 also give a hint of the meaning.
OK, so matrix_width / matrix_height doesn't represent the actual number of
tiles present, but some notional upper bound on the number of tiles that might
be present?
> > The note below Requirement 44 says that there can be records for zoom
> > levels that do not have any tiles populated.
> >
> > However I can't find an authoritative reference that says what
> > matrix_height and matrix_width are intended to describe. I assume that
> > they're meant to be a describe the "framework" for the tiles. So if we're
> > power of two, and the next higher matrix_width was 2, then this
> > matrix_width will be 4 (irrespective of what tiles are actually
> > populated).
>
> Not being an authoritative voice, my understanding is that it is not
> necessary.
> For example if you're interested in North America, in GoogleMaps tile matrix
> set, you could have in theory :
> level 0 : matrix_width = matrix_height = 1 (world coverage)
> level 1 : matrix_width = matrix_height = 1 (top-left quadrant coverage)
> since you will never populate other quadrants.
So at some lower zoom level where there are a lot of tiles, if you add tiles
to the left / upper of what is already present, you don't have to change
matrix_width / matrix_height; but if you add tiles in the lower/right
direction, you do have to change matrix_width / matrix_height. That seems
pretty inconsistent.
> As this was inspired from WMTS, the meaning of matrix_width and
> matrix_height should be the same as WMTS MatrixWidth and MatrixHeight
OK, maybe some easier questions:
1. What is always true about matrix_width / matrix_height, except for it
always being at least one?
2. What is the intent of having them, for a reader?
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