Hi,<br><br>I would like to build a pyramid of external overviews:<br>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.<br><br><span style="color: rgb(0, 0, 0);">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?</span><br style="color: rgb(0, 0, 0);">
<span style="color: rgb(0, 0, 0);">Also
at a certain resolution/scale what is the maximum number of files that
should be opened?</span><br style="color: rgb(0, 0, 0);"><span style="color: rgb(0, 0, 0);"> </span><br style="color: rgb(0, 0, 0);"><span style="color: rgb(0, 0, 0);">As an example: If you have raster data that totals 100 GB and your limit per file is 100 MB/file
</span><br style="color: rgb(0, 0, 0);"><span style="color: rgb(0, 0, 0);">which equals 1000 files at the resolution of the source data. Say the limits are:</span><br style="color: rgb(0, 0, 0);"><span style="color: rgb(0, 0, 0);">
1.) 50 files open at once; </span><span style="color: rgb(0, 0, 0);">Build external overviews at lower scales(higher actual number) to avoid having to open a large number of geotiff files.</span><br style="color: rgb(0, 0, 0);">
<span style="color: rgb(0, 0, 0);">2.) 100 MB/file<br>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.
<br><br style="color: rgb(0, 0, 0);"></span><span style="color: rgb(0, 0, 0);">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. </span><br style="color: rgb(0, 0, 0);"><span style="color: rgb(0, 0, 0);">Take advantage of the group feature so that multiple layers can appear as one virtual layer. </span><br style="color: rgb(0, 0, 0);">
<span style="color: rgb(0, 0, 0);"></span><br style="color: rgb(0, 0, 0);"><span style="color: rgb(0, 0, 0);">
Here is how I think it could be implemented:</span><br style="color: rgb(0, 0, 0);"><span style="color: rgb(0, 0, 0);">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 12.5 files 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. <br><br></span><span style="color: rgb(0, 0, 0);">How could you derive the scale to turn on the next lower resolution level that would fit the (3) limits listed above?<br>
</span><br clear="all">Thanks,<br>-- <br>John J. Mitchell