[gdal-dev] GPKG footprint of ratser data shrinks as zoom level decreases

[Philippe Ghesquiere]

Hi all,

I have problems with the footprint of GPKG raster data, as it varies with
zoom level.

1) Data creation
I need a color-relied'ed DTED in GPKG format. I create it with the
following commands:

gdaldem  color-relief  E017N30.DT2 colormap.lut  E017N30_lut.tif
gdal_translate -of GPKG -co TILING_SCHEME=InspireCRS84Quad -co
ZOOM_LEVEL_STRATEGY=UPPER -co RASTER_TABLE=tiles -r bilinear
 E017N30_lut.tif E017N30_lut.gpkg
gdaladdo E017N30_lut.gpkg

2) Observed
UL and LR corners are given with (where zoom max is ZOOM_LEVEL=12):
gdalinfo.exe E017N30_lut.gpkg | grep -E "Upper Left|Lower Right"
Upper Left  (  16.9998611,  31.0001389) ( 16d59'59.50"E, 31d 0' 0.50"N)
Lower Right (  18.0001320,  29.9998680) ( 18d 0' 0.48"E, 29d59'59.52"N)

I noticed that the GPKG footprint depends on zoom level:
gdalinfo.exe E017N30.gpkg -oo ZOOM_LEVEL=7| grep -E "Upper Left|Lower Right"
Upper Left  (  16.9998611,  31.0001389) ( 16d59'59.50"E, 31d 0' 0.50"N)
Lower Right (  17.9996170,  30.0003830) ( 17d59'58.62"E, 30d 0' 1.38"N)


We observe:
UL corner are identical
LR corner "moves" northward and westward from Z=12 to Z=8

Is it as expected?

3) Impact:
When computing the pyramid (gdaladdo), one column (on the east side) and
one line (on the south side) are not generated.
In our system, we can see this artefact when when requesting tiles with
WMTS requests.

4) Workaround
Our generation script is currently based on gdal_translate/gdaldem/gdaladdo
(and not mapcache_seed). In order to get all the expected pixels, I
implemented a workaround:
* generate the highest zoom level layer with standard gdadem/gdal_translate
commands (see above)
* sqlite command: increase max_x and decrease min_y in table gpkg_contents
* gdaladdo: pyramid computation
* sqlite command: reset max_x and min_y to original values

Thank you for any help / explanation
Philippe
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