[mapserver-users] Heatmap / choropleth from points attributes

Wed Feb 23 22:37:25 EST 2011

So much good information.  Thank you all so much for these responses.  I
will take the later part of this week digging through all of the suggestions
and trying the methods.  I guess what I am trying to do seems to be more
choropleth than heatmap, due to the fact that I don't want any areas without
color, I want it to be interpolated off of the nearest point. I am sure I
will be back on here asking more about this, but for the time being I have
been given some great code and directions.  Thanks again Tamas, Brent, and
all of the others.  What a great bunch of help.

Thanks,

Daniel

On Wed, Feb 23, 2011 at 12:41 PM, <pcreso at pcreso.com> wrote:

> Hi Daniel,
>
> Points to voronoi for mapping is surely a chloropleth more than a heatmap?
> Colouring polygons by value? A heat map is generally done by rendering a
> surface generated from point data.
>
> From the original question I'm not sure if simply plotting coloured points
> on the map is required, or some sort of aggregation of the point data by
> polygon.
>
> I _think_ the intent is to take sets of points, each within a polygon
> (field), generate a surface within each polygon based on the point values,
> then render each surface to form a heatmap (the rendered map thus comprising
> a set of polygons, each of which is represented by its own heat map) ???
>
>
> For a more elegant GIS based approach, I'd use GMT to render a static map
> for a web page, which was the intent behind original question. Mapserver is
> more for dynamic/interactive web mapping.
>
> See: http://gmt.soest.hawaii.edu/gmt/doc/gmt/pdf/GMT_Docs.pdf
>
> For different ways of showing the values of points on a map see the
> examples on pages 120, 121, 138, 141, 144, etc. And there are many others
> with a tool this powerful (& complex) including generating & plotting
> surfaces derived from point data (for heat maps)
>
> You write a script to build the map, much as in the p141 example, which
> shows how GMT can create a complex legend automatically as well, and run the
> script whenever a new or updated map is required.
>
> The GMT list (IMHO) is a helpful & supportive as any mailing list I've
> encountered, even this one!!
>
> A script which iterates through each field (set of points) generating each
> constrained surface and inserting it to a grid, then rendering the entire
> grid for your map output would do this pretty easily.
>
> See the GMT docs for the commands surface for one way to generate each
> field of points as a grid, grdmath for how to merge these into one grid, &
> grdimage for how to generate the map from these gridded data. Or you could
> just render each field separately with grid points outside the field polygon
> set to NAN & rendered transparent.
>
> Or throw all the points into a surface, render this as a single heat map &
> overlay the fields as transparent polygons (optionally labelled) with
> perhaps the regions between polygons masked if appropriate?
>
> Lots of ways of generating such maps :-)
>
> If you'd like some help going down this path, contact me off list.
>
> Cheers,
>
>   Brent Wood
>
>
>
>
> --- On *Thu, 2/24/11, Puneet Kishor <punk.kish at gmail.com>* wrote:
>
>
> From: Puneet Kishor <punk.kish at gmail.com>
> Subject: Re: [mapserver-users] Heatmap / choropleth from points attributes
> To: "Milo van der Linden" <milo at dogodigi.net>
> Cc: mapserver-users at lists.osgeo.org
> Date: Thursday, February 24, 2011, 6:23 AM
>
>
> On Wed, Feb 23, 2011 at 04:28:31PM +0100, Milo van der Linden wrote:
> > Turning points into polygons (or voronois) is the "thinking like a GIS
> > professional" approach. This is valid, but not as good as it can get.
> > Basically, using imaging techniques would be a better approach.
>
> Very well put.
>
> >
> > Perhaps you can take this: http://www.sethoscope.net/heatmap/ as a
> starting
> > point? The author has created a nice python script to generate a heatmap.
>
> And here is a Perl-based approach
> [http://blog.imtrevor.com/2009/07/16/generating-heat-maps-using-perl/]
>
>
> >
> > 2011/2/23 Josh Jordan <outerspaceman81 at yahoo.com<http://mc/compose?to=outerspaceman81@yahoo.com>
> >
> >
> > > First, you have to turn your points into shapes, theres some algorithm
> that
> > > will turn points into polygons with borders midway between each point.
> > >  Then, you have to add classes to the mapfile.  Add one class per
> color.
> > >  You have to calculate the bounds and color for each class like this:
> > >
> > > CLASS
> > >     EXPRESSION(([POPULATION] gt 90457) AND ([POPULATION] le 108397))
> > >     STYLE
> > >       COLOR 10 20 50
> > >     END
> > > END
> > >
> > > If your CSV data isnt joined to the mapserver data, you have to
> calculate
> > > what shapes are in each bucket like this-
> > >
> > > CLASS
> > >     EXPRESSION(([NAME] = "Shape1") OR ([NAME] = "Shape2"))
> > >     STYLE
> > >       COLOR 10 20 50
> > >     END
> > > END
> > >
> > > --- On *Tue, 2/22/11, Daniel Cole <daniel at southernsolutionsms.com<http://mc/compose?to=daniel@southernsolutionsms.com>>*
> wrote:
> > >
> > >
> > > From: Daniel Cole <daniel at southernsolutionsms.com<http://mc/compose?to=daniel@southernsolutionsms.com>
> >
> > > Subject: [mapserver-users] Heatmap / choropleth from points attributes
> > > To: mapserver-users at lists.osgeo.org<http://mc/compose?to=mapserver-users@lists.osgeo.org>
> > > Date: Tuesday, February 22, 2011, 10:06 PM
> > >
> > > I am a new mapserver user and have spent countless hours over the last
> > > month reading, trying code, etc. before posting here for help.  I am
> trying
> > > to generate an image (non-interactive) on a webpage based off of
> attribute
> > > data in a points file.  For example, 20 different points equally
> distributed
> > > in a field contain data about the moister level of the soil.  I want to
> make
> > > the lowest levels blue and the highest levels yellow, or something like
> that
> > > with some gradients in between.  If possible I want it to have
> interpolated
> > > data between them to show the gradual change, but that isn't a
> must.   I
> > > also want to wrap it in a polygon that trims the edges.
> > >
> > > I found some python code that I thought might do the trick , but it
> seems
> > > lots of heat maps are based off of how many points exist in a certain
> area,
> > > and thats now what I am doing.
> > >
> > > My points contain numbers in the attributes, 5, 25, 92, 71, etc.   I
> feel
> > > like I am missing some easy way to do this right in front of me with
> gdal or
> > > something.  If someone has any direction or even demo code that would
> be
> > > greatly appreciated.
> > >
> > > Thanks,
> > >
> > > Daniel
> > >
> > > -----Inline Attachment Follows-----
> > >
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