[Tilesathome] Lowzoom and blank tiles

[Andrew McCarthy]

Hi folks,

I've noticed that if running lowzoom for zoom levels 8-11, blank tiles
are only uploaded at zoom 11. For cases where tiles at z8-z10 were
generated in error (such as the "unknown type" problem from
earlier), no zero-byte tile is uploaded to ensure their deletion.

Should the deleteBlankSubtiles function generate zero-byte tiles for all
zoom levels to ensure that old tiles are removed? Also, should it upload
empty-sea or empty-land tiles (67 or 69 bytes) at particular levels? The
current code has a strange combination of zoom-level checks for this
(line 220 of lowzoom_composite.pl):

	return if $Z <= 9;  # Not lowzoom's problem
	# This keeps real blank tiles at zooms 3,6 and 9
	return if ($Z+1)%3 == 0;

Cheers,

Andrew
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