Building External Overviews

[John Mitchell]

Hi,

I would like to build a pyramid of external overviews:
Which is stacks of layers going from many files at the high resolution to
smaller numbers of files as the resolution gets smaller(lower scale/higher
actual scale number).  The map file needs to have these layers
appear/disappear in a mutually exclusive fashion depending on the
resolution.  So only one appears at any one time.

Is their a formula for pyramiding as far as how the levels are spaced out,
how many levels are implemented, and at what scale levels are turn on and
off?
Also at a certain resolution/scale what is the maximum number of files that
should be opened?

As an example: If you have raster data that totals 100 GB and your limit per
file is 100 MB/file
which equals 1000 files at the resolution of the source data.  Say the
limits are:
1.) 50 files open at once; Build external overviews at lower scales(higher
actual number) to avoid having to open a large number of geotiff files.
2.) 100 MB/file
3.) Each overview level is 50% (x) and 50% (y) of the previous higher
resolution overview.  I believe that if resolution is reduced 50% by 50%,
then can increase each footprint size by double for (x) and (y) dimension in
order to keep the same file size.

Then would adjust resolution to keep at 100 MB/file and adjust maximum zoom
layer(higher number means smaller scale), so that no more than 50 files are
open at any one time.
Take advantage of the group feature so that multiple layers can appear as
one virtual layer.

Here is how I think it could be implemented:
Start at the highest resolution and make the scale small enough to include
50 files, and then set the maximum scale(note this means higher scale
number) for this layer at this scale.  Create a new layer that is at lower
resolution and a larger footprint per file such that at this scale
only 12.5files display at 100 MB/file and set the minimum scale at the
same value as
the previous layers maximum scale.  Continue adding lower resolution layers
as the scale gets smaller and smaller until you get to the initial display
scale.

How could you derive the scale to turn on the next lower resolution level
that would fit the (3) limits listed above?

Thanks,
-- 
John J. Mitchell
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