[mapserver-users] Real dynamic legends

[Steve Lime]

> Hello everybody,

Hi Armin:

> I was thinking about the possibility to have 'real' dynamic legends.
That is,
> legends that show only the items (of active layers) that are visible
in the
> current map extent. For complex layers definitions with plenty of
classes one
> loses a bit the overview if all classes are always drawn in the legend
even
> if only 10 % or so of them are visible in the map. Real dynamic
legends could
> make it much easier to identify the map contents via legend.

Yup, I agree.

> One possibility I see for such legends could be using MapScript with
> 'queryByRect' on all active layers and the current map extent. The
results
> are written in an array with unique result values for the classitem.
Then one
> had to compare these results with the class definitions of the layer
and draw
> the legend icons.

Seems like a lot of work when the information needed for this is already
available as a map is drawn.

> One restriction of this approach I found is that it seems not to be
possible
> to get class expressions via MapScript. A question to the PHP
MapScript
> developers: Would it be possible to add this possibility to MapScript?

I'm not sure what you mean by "get class expressions". I believe the
perl/python version has a classify method that will allow a shape to be
evaluated for a given layer. I'll have to check though.

> Assumed one had this expression it can be a bit difficult to match
results
> found via the query with class definitions. There are quite a lot of
> different class definitions possible. And at the end, the speed of
such a
> query will decrease very much if a lot of features are visible in the
map (it
> could be much faster, however, using PostGIS instead of shapefiles and
a
> 'select distinct' query).


> Therefore a question to the MapServer developers: Is it conceivable to
> implement such a dynamic legend function directly in the main
MapServer
> functionality?

It's possible. We would just need to add a bit of book keeping to the
map file structure (i.e. like keeping a count of drawn features by
class). This might make the mapfile structures less thread safe
though....

> Reading shapefiles and map file with complex class definitions and
rendering
> a map will take only very few seconds even for relatively large
datasets. So
> I think producing lists of affected layer classes in a similar step or
even
> simultaneous should be also quite fast. Maybe I'm wrong...

> Regarding MapScript functionality I'm thinking of a function like
> $map->drawDynLegend() that instead of an image returns a multiple
array of
> active layers with a sub-array of affected classes (classes in the
current
> extent) for each layer. One could then use these classes and draw the
legend
> icons for them.

I would propose extending the legend object by adding a "type"
parameter.

> Any ideas?

> Armin

Steve


Stephen Lime
Data & Applications Manager

Minnesota DNR
500 Lafayette Road
St. Paul, MN 55155
651-297-2937