[Chameleon] Searches, Buffers and Choropleth Maps

[Paul Spencer]

Darren,

I think that I understand what you are looking for.  Unfortunately most 
of what you are looking for it not yet in Chameleon.  Some responses 
follow in line ...

Darren.Cope@statcan.ca wrote:

> Hello everyone,
> 
> I've been working with the CWC2 application from the MS4W package for 
> the last few weeks.  I have a demo set up which displays some of our own 
> data, and allows me to perform many of the basic functions I need.  
> However, I am researching the feasibility of some more advanced 
> functionality, and am not finding much to suit my needs.  I have 
> searched the archive, and found some snippets of information, but never 
> enough to help me pull everything together.  I am currently using 
> shapefiles as a data source.
> 
> I would like to know if the following are possible, and if so, how I 
> would go about implementing them:
> 
> - do searches based on my own data, rather than the CGDI-based (i.e.. 
> postal code, locality, nts) searches.  I would like to do searches based 
> on one field (e.g. find a street name), and multiple fields (possibly in 
> different layers) (e.g. find a city in a specific province).  Is there a 
> widget which I could modify easily in order to do such a search?  
> Someone out there must have a widget which does this?

There is no widget that does this but one has been discussed a couple of 
times.  We just haven't had a specific application to implement one for 
it yet.

The place name lookup in the locate widget actually uses a dbf file 
locally to do the lookup I think, and since a shape file has a dbf file, 
you could start there.

There is certainly support in mapscript for doing a query by attribute, 
and it shouldn't be too difficult to write a widget to use this function 
internally.  Most widgets are wrapped around a specific piece of 
internal mapscript functionality like this.  I'm guessing the tricky 
part is in defining the requirements of the widget.  It's pretty easy to 
implement something for a specific application but it gets more 
difficult to design something intended for general use.

> 
> - search based on topology (e.g. find the intersection of two streets)

nothing like this exists nor is in the works.  Unfortunately, it is 
difficult to this type of thing in a web based application.

> 
> - create a circular buffer around a user-selected point (where user 
> clicks on the map) with a user-entered radius

There are tools for creating regions of interest, and there is one that 
allows the user to define a circular region by clicking a point and 
dragging a radius ...

> 
> - select and highlight a feature when it is "identified," and retain 
> this selection until the user clears it

There is an ROIFeatureTool that allows turning a feature into a region 
of interest and it is persistent until cleared, but I doubt that is what 
you want.

> 
> - display a choropleth map (without hard-coding the ranges in the .map 
> file)

we have been working on tools to allow user-classification of layers, 
but it doesn't have a lot of automated features yet.  I suspect it would 
probably too limited for now, but it may improve over time and allow for 
different types of generation of classes and automating application of 
styles etc.

> 
> Do I need to use something like PostGIS or GeoServer do accomplish these 
> tasks?  If so, how do these work with Chameleon/MapServer?

probably not unless you need an application that can modify the data, in 
which case they may be better suited.  Except perhaps for searching 
based on topology, I think postgis would be more efficient and 
feature-full in that area.

> 
> I am very new to all of this, and have very little programming 
> knowledge, so please keep this in mind.
> 
> I realize that I am asking a lot of questions, so thanks in advance for 
> any help!

no problem.  The features that you are looking for are common in desktop 
GIS but, in my experience, not available in most or all web GIS systems 
yet.  Web GIS is getting better and better, but this is still just 
outside the envelope I think.

> 
> Darren Cope
> Geography Division
> Statistics Canada
> 

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