[gdal-dev] The relationship between overview levels and zoom levels?

[James Sammone]

I'm not sure if this is the best channel to ask this question as it might
be beyond the scope, but I've asked it in a few others and have had no
responses aside from others also being curious.
<https://gis.stackexchange.com/posts/450396/timeline>

I am trying to understand the relationship between overviews and zoom
levels so I know how to make more efficient Cloud Optimized GeoTIFFs. Using
gdaladdo or gdal.BuildOverviews(), I can create overviews at [2,4,8,16] or
at just [16]. From my understanding, this means the size is being divided
by those values to provide downsample arrays of the original source. In the
first example [2,4,8,16], I've created 4 separate overview arrays into the
GeoTIFF that are 2x, 4x, 8x, and 16x downsampled. And in the second example
using only [16], I've built one overview array into the GeoTIFF that is 16x
downsampled.

How can I understand how these overviews are applied when it comes to zoom
levels? Does the 16x downsample appear sooner in the second example when
zooming out than for the first example due to being first in order? Or do
the 16x downsamples appear at the same zoom level for both cases but the
second example has additional 2x, 4x, and 8x downsamples that also appear
before getting there?

Thanks for any insight into this anyone can provide. Despite using
overviews all the time, I've struggled with this for a while and had
largely consigned to not understanding it.

Best,

James
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