[ka-Map-users] selecting multiple layers

David Bitner osgis.lists at gmail.com
Fri Jul 29 09:46:22 EDT 2005


The problem that you are going to run into with your approach is with
cacheing your tiles.  Basically creating one map call to generate all
your layers means that you will have to cache absolutely every
combination of layers that someone could possibly chose.  Right now
the ka-map cacheing structure is not set up to do that.  You would
need to modify ka-map so that it created a different directory in the
cacheing setup for each permutation of what is selected.

The approach used in the CVS version of ka-map is to limit that number
of choices by creating groups of layers that are then drawn using
transparency over the top of eachother (for this kind of thing where
you are using vector layers with no raster backdrop, png is actually
quite small  - if you had a raster background, you could perhaps
modify ka-map to use jpeg for the base layers and png for everything
else).  The problem with this approach is that it does not scale very
well to many different groups because the browser will start to choke
when you have so many layers being shown.

Ka-map is a compromise application that allows extreme interactivity
with moving around a map in a very fast way by forcing the user into
using tiles and layers that you can cache and make available very
fast.  The drawback is that all of the arbitrary decisions that a user
can make in a traditional mapserver application go out the window for
the sake of speed.  These include having lots of layers that you can
arbitrarily combine and being able to zoom to absolutely any scale
(within the min and max set in the mapfile).

If having the ka-map experience is very important to you, I would say
that you should group your layers into coherent groups of data that a
user would usually look at together.  Otherwise stick with an
interface where cacheing is not integral.


On 7/29/05, Lorenzo Becchi <lorenzo at ominiverdi.com> wrote:
> Hi David,
>   thanks for your answer,
> I have version 1.1, the stable one I guess. I did some changes on it,
> like you can see.
> I can try CVS version like you suggest but I don't think it is the
> solution I'm looking for.
> Let me know if I'm wrong:
> 1) I'm not using groups, than every layers goes in the DEFAULT one.
> 2) I'm using JPG, i guess they are smaller file and faster to download.
> 3) thinking in faster system, I don't want layers to be shown
> immediately when they are clicked on. Soon I'll add many layers and I
> want people to decide which layers to see before starting the
> download/reload.
> what do you think?
> regards
> Lorenzo
> Il giorno 28/lug/05, alle 15:51, David Bitner ha scritto:
> > What version of ka-map are you using?  In the cvs version there is the
> > option to display your layers as vertically stacked.  What this does
> > is if you have your mapfile set to provide transparent pngs is it
> > renders every group that you have defined in your mapfile to be
> > rendered separately.  The end effect is instant on and off when you
> > check or uncheck your groups.  Do note that only those layers that are
> > not marked as a group are dragged around by the mouse, the other
> > layers then fill in once you've stopped.

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