Hi Bill, if you are using MapScript you could request the frequency of indiviuals at the concerning positions from data-base insteed of plotting symbols for each. Then generate star-glyphs with a jag for each concerning individual or bins of them. Star-glyphs are originially used to visualize multi-dimensional data, but why not using it for one-dimension.
Just an idea. Regards, Oliver Am Freitag, 3. Februar 2006 08:06 schrieb Bill Binko: > Hi everyone, > > I posted a fairly specific question to the GDAL-dev list (and more on IRC) > the other day, asking for input on how to do Kernel Density mapping using > an OGR data source and sending the raster back through GDAL to be > displayed through Mapserver. > > I got some good responses, one of which was: are you sure that's what you > want? Well, to be honest, I don't know. So I'm posting a more general > question here, hoping that someone in this larger audience has solved the > problem I'm facing. > > What I'm trying to do is visualize point data (stored in PostGIS, but that > can change) so that users can determine where the points are concentrated, > even while zoomed out. One caveat is that the points can be coincident > (identical Lat/Long). > > For example, I'd use this with include recent real estate sales data - > condos in the same building have the same shape, but both sales should > count. Similarly, in a customer counting scenario, two customers with the > same address (even only to the building level) will geocode to the same > lat/long. > > When Mapserver serves up Point layers directly, the results can be quite > disappointing (and misleading). In Mapserver, the last Point to be drawn > on any spot will just hide the previous points, often hiding multiple data > points. > > Similarly, when you zoom out on a point diagram, there are two options: > either you set the symbol to have a max size (so that eventually, they > become "less than clear"), or you keep the symbol size constant, > eventually obliterating the background with huge blobs of overlapping > points. > > When I tried to tackle this problem, I turned to a Real Estate Geography > book I have (written by professor Thrall at my alma mater) > http://www.amazon.com/gp/product/0195076362/sr=1-2/qid=1138949940/ref=sr_1_ >2/104-4827941-6490312?%5Fencoding=UTF8 > > There, he is discussing Site Selection and Customer Spotting methods, > and he discusses the Kernel Density method of generating a surface map > showing where the customers live. The discussion is brief, but the GDAL > folks pointed me to two tools (parts of R and GRASS respectively) that can > take points and generate kernel surfaces. With some magic, contour maps, > and other visualizations could be pushed through GDAL and into Mapserver. > > However, with further inspection, these methods all work on distance > between customers, which causes a problem with coincident points. > > I am thinking that this problem must have been solved by someone out in > this audience, so at this point, I'm very open to suggestions. > > I am very willing (and fairly able) to implement any custom software the > solution my require. I have considered filters between GDAL and OGR, or > stand alone processing that generates the needed data. I'm also very > happy to feed it back to the community, if there's interest. > > (So, basically, I will trade working code for modeling/GIS/statistics > knowledge if anyone's interested :) > > Thanks, > > Bill -- Junior Research Group IMPULSE ICBM * Carl-v.-Ossietzky University of Oldenburg PO-Box 2503 * 26111 Oldenburg - Germany - http://www.icbm.de/impulse room: W15-2-239 * fon:+49-441-798-3622 * fax:+49-441-798-3404
