[Geopackage] Query on gpkg_tile_matrix - matrix_width and matrix_height

Jeff Yutzler jeffy at imagemattersllc.com
Thu Jun 4 00:30:02 EDT 2015


All,
First of all, I apologize for my delay in responding here. I had my
thoughts on the matter but I wanted to talk to one of the developers of
WMTS (from which the tiling model derives) before responding. We are at the
OGC TC this week and I wasn't able to have this conversation until this
afternoon.

My conversation with Joan Masó has confirmed what I and a few others on
this thread have said. The matrix_width and matrix_height elements in Table
9 of 2.2.7.1.1 describe the size of the tile matrix. It seems to be there
for convenience purposes but I wonder whether it is actually useful. Since
it is anchored to the top left, it can't reasonably describe an MBR. (As an
aside, the reference to MBR in 2.2.6 is wrong [1] - a similar mistake was
made in WMTS.)

We could have a way to describe the tiles that are present in a particular
tile matrix but this capability is completely missing in GPKG today (short
of doing an SQL query and counting). We are currently analyzing potential
approaches. WMTS has TileSetLimits (Table 12 in 07-057r7[2]) but they
didn't get it right either - you can use it as an MBR but you can't have a
set of them to describe a sparse tile set. The WMS SWG has agreed to look
into this.

> BTW: Why is the ERDC sample in 3857 when the NGA prohibited that as a
valid use?
That was done for the GeoPackage Plugfest which was an OGC initiative, not
an NGA/NSG one. EPSG 3857 is still the most commonly supported projection
at this point.

> Interestingly the link in footnote 18 for JFIF is 404.

Noted. [3]

I hope this clears things up.
-Jeff

[1] Added #105
[2] http://portal.opengeospatial.org/files/?artifact_id=35326
[3] Added #104
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