Hi John,
I think you need to decide if you want to do the classification on
server or in the client or both.
1. in the client
This implies that you have to send all the polygons and attributes to
the client and have code in the client for classifing the polygons and
and assigning colors and drawing them
2. on the server
You need to decide what tool is best for classifying you polygons and if
it can output data in a format you need or work with a tools that can
output the format you need at the client.
Now you have to decide what you want the format output to be then pick
the best tool for that. Mapserver generally renders images, but it is
possible to get it to render MKL, GeoJson and other formats other than
images. I often write simple PHP Ajax handlers that talk to
PostgreSQL/PostGIS to issues queries and then reformat the query output
into the form I want.
3. some combined approach
Here you need to decide what is best done in the client vs the server
and build that out as needed.
There are trade offs for each approach. Maybe you can extend your Django
project. There are lots of articles and books written on choropleth
generation there is no "standard" choropleth. The best choropleth for a
given data set is highly dependent on the data and what you need to show
about the data.
HTH,
-Steve W
On 4/2/2012 12:48 PM, John Abraham wrote:
I've been looking for an open-source and easily deployable solution
to allow users to view choropleth maps of PostGIS layers. The layers
are straightforward enough, just a geometry column, a GID, and then a
bunch of value columns. I simply want the user to be able to use his
own computer resources to select the value columns, select the ranges
of the classes for the colors, and perhaps select the color ramp. I
already have a Django project which allows the user to generate the
layers (select the appropriate geometry and value columns and
crosstab them into a single PostGIS view.)
We have a partially implemented system using Mapserver and
Openlayers, but the maps are delivered as bitmaps by the server,
which means sending full bitmaps every time the user changes the
ranges or selects a different value column. Here are the
technologies I think I can use to do it in the browser alongside my
existing Django code:
1) Javascript, probably GeoEXT http://geoext.org/ using open layers,
so the browser draws the map, it seems this should be dead-easy and
there should be a cut-n-paste example out there but I've been
struggling a bit trying to find one,
2) Java applet, there must be one out there I can customize but I
haven't found one yet (admittedly haven't looked too hard yet for a
Java applet, because I've been preferring the idea of Javascript)
3) Adobe flash/flex, I have no experience with this and the
development environment is not open-source, but it looks pretty
flexible. I can probably grab code from a colleague of mine who does
have something similar already working, but when I looked his code it
looked pretty complex internally and it used a few non-open-source
libraries.
Any suggestions or thoughts? I have a feeling that there is A Very
Easy Solution out there that I'm not finding.
Alternatively, I could use
4) a QGIS plugin, abandoning the web interface all together, moving
my existing python code for generating the layer out of Django and
into QGIS, then using QGIs to view the layer. This would require me
to deliver and support QGIS.
-- John Abraham _______________________________________________
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