overviews internal and macro tiling

[Frank Warmerdam]

On 11/3/05, Travis Kirstine GIS Tech <tkirstine at jdbarnes.com> wrote:
> Hi,
> I have some general questions regarding overviews and performance in
> mapserver.  I've been going through the past user lists and it seems the
> general consensus it that creating sets of images (small tiles to large
> merged tiles) at different cell resolutions (reduced resolution as you zoom
> out) that are activated by the MIN MAX Scale definitions in the mapfile is
> the optimal why to go.  I was wondering is it still beneficial to create
> internal overviews for these images using tool like gdaladdo.

Travis,

My advice is to depend on internal overviews as long for as "far out"
as the user can zoom before they start getting a significant number of
files rendered per map view (say 8+).  At that point you should consider
having a whole new layer based in MINSCALE/MAXSCALE instead
of depending on internal overviews in the files.

> I've been
> doing some testing and don't see much of a change between aggregate images
> and aggregate images with overview.  I was also hoping someone could  shed
> some light around at what scale internal overviews are activated. For
> instance our data is 0.20m pixel size 1 km square tiles and I run gdaladdo
> on it
>
> gdaladdo -r average image_1.tif 2 4 8 16 32
>
> Right now its my understanding that the first level creates a resampled
> internal image that is half the resolution 0.4m and second level would
> resample the image to 0.8m resolution and so on.

That is correct.

> If I'm zoomed all the way
> in 1:1 and begin zooming out at what mapscale is the first overview level
> activated.

The 0.4m overview should kick in roughly at the point where the
map view is for 0.4m pixels.  There is a wee bit of rounding
logic that might cause it to kick in at around 0.39 or so but you
can't really depend on it till you are a wee bit past 0.4m.

Best regards,
--
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent