[Chameleon] Contexts

Paul Spencer pagameba at magma.ca
Mon Sep 27 20:04:23 EDT 2004


Ken Sanderson wrote:
> I have been goofing around with contexts today and it would do what I
> need if I was only using WMS layers, as the context saves visibility
> status as well as extents. I had a couple other questions about
> contexts:
> 1) Whats the real difference between Download a Context and Extract a
> Context? Seems one puts it right to your hard drive while the other just
> provides a URL for it, but the URL is to a file in a temp folder that
> isnt likely to be around that long. Is there a reason to use Extract vs
> Download?

Download Context builds a context document from the current state of the 
map file and allows the user to download the file to their computer. 
Extract Context builds a context document from the current state of the 
map file and sends it to a (application-defined) extraction service that 
can extract data based on the layer visibility and extents provided in 
the context.

> 2) Is it feasible to combine a context with your current map? Right now
> if you upload a context it replaces the map you have with the new one.
> Is it feasible that you could add the layers in a context to your
> existing map so that you would get a combination of the two in your map
> window instead of just the newly loaded context?

yes it is feasible.  The low level functionality is there.  MapScript 
loads a context by replacing all the layers, but it is possible to load 
a context into a new mapobj and then copy all the layers across.  Note 
that while this is possible, it may not provide the best results because 
of layer ordering (polygons on top of points for instance).  When 
dealing with WMS, it is not normally possible to discover the feature 
type of the layer, which is a disappointing limitation for client apps 
that want to be somewhat intelligent about this sort of thing.



> Thanks,
> Ken
> -----Original Message-----
> From: chameleon-bounces at lists.maptools.org
> [mailto:chameleon-bounces at lists.maptools.org] On Behalf Of Paul Spencer
> Sent: September 26, 2004 1:31 PM
> To: Ken Sanderson
> Cc: chameleon at lists.maptools.org
> Subject: Re: [Chameleon] Contexts and SLD's Widgets
> Ken,
> the Context widgets are to work with Web Map Contexts, an OGC spec for 
> portable map files ... it only works with WMS layers.  SLD is also an 
> OGC spec, this one defines styling information for WMS layers.
> Neither set of widgets will help you with that.  There are no other 
> widgets that do that, either.  You will likely need to build something 
> to do this.
> Cheers,
> Paul
> Ken Sanderson wrote:
>>What are the Context and SLD widget/tools for, ie what do they do?
>>I am looking for a way to save a current extent, visible layers, etc 
>>so that a user can come back later and start where he left off, would 
>>either of these be good for that? Is there any other widgets that 
>>would work for this type of thing?
>>Chameleon mailing list
>>Chameleon at lists.maptools.org 

|Paul Spencer                           pspencer at dmsolutions.ca   |
|Applications & Software Development                              |
|DM Solutions Group Inc                 http://www.dmsolutions.ca/|

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