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Extension to allow negative zoom levels (potential concern for CDB grid negative levels) #514

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jerstlouis opened this issue Mar 10, 2020 · 6 comments · May be fixed by #523
Open

Extension to allow negative zoom levels (potential concern for CDB grid negative levels) #514

jerstlouis opened this issue Mar 10, 2020 · 6 comments · May be fixed by #523
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Tile Matrix Set effort to align to TMS standard
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@jerstlouis
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CDB LODs go down to -10 where tiles are 1x1 pixels.

@jyutzler
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jyutzler commented May 5, 2020

In today's SWG meeting, we approved this request because there is no clear benefit to this requirement, the potential interoperability risk is slight, and it is desirable to support CDB directly.

@jyutzler jyutzler linked a pull request May 6, 2020 that will close this issue
@jyutzler jyutzler added this to the 1.3.0 milestone May 6, 2020
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jyutzler commented May 6, 2020

@jerstlouis please propose alternate text for the first paragraph of http://www.geopackage.org/spec121/index.html#zoom_levels in light of this change.

@tabinfl
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tabinfl commented May 6, 2020

Are we making GeoPackage zoom levels equal to CDB LODs? They are not the same in the respective specs and we should consider carefully implications of conflating the two.

@jerstlouis
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jerstlouis commented May 6, 2020

In a GeoPackage, zoom levels are integers in sequence that identify tile matrices in a tile matrix set that contain tiles of progressively finer spatial resolution. Adjacent zoom levels immediately precede or follow each other and differ by a value of 1.

  • "from 0 to n " removed
  • Change "tile matrix layers" to "tile matrices" (layers is confusing here)
  • "decreasing spatial extent and " removed because that is not necessarily true (negative zoom levels in CDB have all the same extent, but different pixel width & height, and the extent only change from level 1+)

Also later in that paragraph it says:

Other "zoom other intervals" conventions use different factors

Is that second other there superfluous?

@jerstlouis
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@tabinfl The idea is to allow encoding content from multiple CDB LODs in a GeoPackage.
This is possible with the new TileMatrixSet extension ( https://gitlab.com/imagemattersllc/ogc-vtp2/-/blob/master/extensions/14-tile-matrix-set.adoc ) which supports varying width tile matrices, which can represent both the CDB zones and CDB LODs ( as described here: http://maps.ecere.com/geoapi/tileMatrixSets/CDBGlobalGrid ).

@jyutzler jyutzler modified the milestones: 1.3.0, post 1.3.0 May 20, 2020
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Since this change is not sufficient by itself to completely satisfy the CDB LOD requirement, the SWG decided to defer this change until after 1.3. One possibility is rolling it into a new extension as described in #504.

@jyutzler jyutzler added the Tile Matrix Set effort to align to TMS standard label Mar 2, 2021
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