8 replies to this topic

[benbakelaar]

Hey all, I am looking for an algorithm that generates a density map. I want to use it to generate density maps "on the fly" or at least from the command line. I can handle anything after that, such as creating a tile-set from the output raster, or before that, such as formatting the data to be input/received in a certain structure. From my research, this algorithm has a few names:

- kernel density map
- percent volume contour
- isopleth
- contour lines

I've found leads at the following places:

Hawth's Tools (http://www.spatialec...tvolcontour.php)
talked to author, it uses the ESRI Spatial Analyst library for this function.
GeoIQ "Heat Maps" (http://www.programma...b.com/api/GeoIQ)
it is web-based and is currently limited to a small number of data points. Public Beta will be coming out in Q3 2007. It is not open-source.
Grass GIS (http://grass.itc.it/...ol4-usermap.pdf)
not sure if I can extract the source code, or locate it. Also not sure if it will help me in the end.
Javascript contour mapping (http://blog.case.edu..._seems_feasible)
talked to the author of this blog, he hasn't come up with anything that fully works yet. But it does talk about the techniques necessary to do it.

Anything would be helpful at this point, some pseudo-code, a textbook reference, even if it doesn't have any actual equations in it.

[Polaris]

I believe that Golden sotware's Surfer oughta work.

e

[benbakelaar]

Hey, nice find! I've actually known about Golden Software for quite some time, but I always forget to check them out.

Surfer does have the ability to be script-controlled, so that's great! I just wonder if it does *exactly* the right kind of contour mapping that is needed for this project. I know Hawth's Tools - Percent Volume Contour is the tool currently used to generate the static maps.

[mdsumner]

R is good at this. Check out the functions

contour
contourLines
quantile
image
kde2d ## package MASS

There are other specialist packages such as KernSmooth for more sophisticated KDE (for example), and plenty of GIS-interop via rgdal.

www.r-project.org

[frax]

Are you trying to create a density map from points? If so, in ArcGIS, search for 'kriging' in the help, you probably need some extensions for this - such as spatial and/or geostatistical analyst.

[frax]

Ben - what tools do you have at hand? This could be done using ArcInfo workstation, ArcServer (maybe?) and possibly through scripting ArcGIS through python as well.

And when you say on the fly - referring to your post about ArcIMS - would this be for a web application, where they are created for every request - or do you mean - as it is submitted, and then grids are automatically generated - to be cached and reused, until the point data gets resubmitted?

[benbakelaar]

On the fly = Created for every request, not because the data changes, but because the range of years to incorporate into the density map changes.

I did not know you could possibly use ArcGIS through Python scripting! It's the Windows equivalent of command-line execution, correct?

[frax]

yep it is the equivalent. It is the sucessor to AML and ArcInfo workstation, I don't think it has taken off at the same degree yet though. I think it came in 9.1. VBA was held as the main scripting engine before that, since the launch of the whole COM rewrite and the launch of ArcGIS (version 9, I think). But there are a lot of things you can't do with VBA, and it is mostly useful for scripting things that are happening within the ArcGIS interface.

It is kind of cool that they picked Python, which I have meant to learn anyway, and that they expose a lot of the processing functionality in this way, but I haven't really gotten in to it yet - I am still partly stuck in AML-land (which is still incredibly powerful and easy to use).

I think there are loads of info in the ArcGIS help, on the support site (certainly whitepapers) and there are also some free tutorials on the ESRI virtual campus, and of course courses that you can buy there. And of course some kind of community on the ESRI forums (not as big as the AML community was at it's peak?).

I am not sure at what license level the scripting is available (is it in ArcView?) but the functionality that you script (e.g. the geoprocessing objects) certainly are limited by the same licensing as when you access them from the various toolboxes.

For your on-the-fly idea - I think you need to look at how fast the algorithms are for the different software, and what quality you are after - I did some experimentation quite recently using kriging using the GeoProcessing tools in ArcGIS - and it seemed to take forever. I was just playing with it, and I was overwhelmed by the different choices of options and algorithms there...

[gregsd]

- kernel density map

CRIMESTAT 3 will do this.

Greg