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From |
"Nick Cox" <n.j.cox@durham.ac.uk> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
st: -zmap- command available on SSC |

Date |
Tue, 16 Mar 2010 13:20:59 -0000 |

Thanks to the ever efficient and energetic Kit Baum, a -zmap- command is now available from SSC. Stata 8 is required. Use -ssc- to install or inspect if interested. -zmap- is a fairly obvious convenience command that was developed for teaching use with spatial series and tested on a remotely sensed image dataset with almost 600,000 observations with distinct x and y coordinates and various responses (z) measured for each. At that scale the areas (pixels, or more precisely their ground equivalents) can be considered as point locations for graphics and indeed the only possible marker symbol is a point. Be that as it may, some people may be interested in its uses for other kinds of trivariate data. The help file carries a detailed worked example in which other marker symbols are advisable. A bit more formally: -zmap- graphs (or maps) binned values of a variable z with respect to two variables x and y treated as Cartesian coordinates. The range of z is divided into two or more bins or classes and points in each bin are shown distinctly. The resulting plot is thus a composite scatter plot. The user may specify percentile breaks or breaks on the scale of z. I'm a bit surprised to be thinking that no-one has published precisely this kind of program before, but apologies if I'm overlooking something. Several people must have done this kind of thing ad hoc, however. Maurizio Pisati's much more versatile -spmap- I understand to be based on the idea of mapping areas as polygons, which is not needed here. Sergiy Radyakin gave a talk at the DC conference last summer with what at first sight some similar (and very impressive) results. But as the underlying program and help files appear to remain private I cannot comment in detail, except that Sergiy uses what he calls "advanced Stata programming", which I don't do. Vince Wiggins made a very helpful suggestion on getting (colours of) markers to show visibly in the legend. In essence, each variable shown by a point has a bigger sibling with the same colour but a square symbol, which features in the legend but not in the plot (and vice versa). Nick n.j.cox@durham.ac.uk * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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