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From |
Julia Gamas <jgamas@mit.edu> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
st: spatwmat: a further thought on XY coordinates |

Date |
Fri, 4 Feb 2005 09:07:07 -0500 |

Hi again, Simon, a further thought: XY coordinates may just be referring to the zone pair: So in my case if zones 1 and 2 are neighbors then the XY="12" and "21" entries in the table are 1. If so it seems a bit "dangerous" to name it this because a geographer will interpret them as literally the XY coordinates on the globe, making the whole thing confusing. But to be sure, probably its better to check with Maurizio Pisati whom I believe wrote spatreg, spatwmat and spatgsa. Author addresses and affiliations appear at the end of the command explanation in the help window of Stata. If you manage to find out would you let me know? Julia Date: Thu, 3 Feb 2005 14:40:36 -0500 From: Julia Gamas <jgamas@mit.edu> Subject: st: more on spatial modeling: spatwmat Dear Simon, I'm not sure about the coordinates either, but let me explain how I used "spatwmat", in case it helps you. Even if different from what you are doing, it may help you understand it in the context of your own problem. I wanted to find spatial correlation. So I created an "adjacency" matrix with spatwmat. In my particular definition of adjacency, the values in the matrix were 1 if two zones were neighbors and 0 otherwise. This was first order adjacency (akin to first order serial correlation in time series). For some biological data, second or third order adjacency can be the issue (if you're interested not in neighbors, but neighbors of neighbors, for example). I created the adjacency matrix in a GIS (ArcView) and then imported it into Stata and created the W matrix using "spatwmat". My matrix was something like: zone #: 1 2 3 1 0 1 0 2 1 0 1 3 0 1 0 In this example: zones 1 and 2 are neighbors and zones 2 and 3 are neighbors but zones 1 and 3 are not (they aren't adjacent). I then looked for spatial correlation using "spatgasa" (moran's I and a bunch of other types of correlation that Stata tests for). Then, when I confirmed the presence of spatial correlation, I ran "spatreg". You have to determine what kind of model you are trying to run, and which question you are trying to answer and then create the W matrix accordingly. My knowledge about this is limited. To find out which W you need, you may want to consult Luc Anselin's books and papers if you haven't done so already, and the author of the Stata code himself. I hope this may be of some help. Julia > Hi Julia, > > Thanks a lot for your information. It is totally new to me that Stata > has commands for spatial analysis. I looked at the Stata code for > contracting the weighting matrix, but I am not sure the mechanism behind > it. Could you explain it to me, if you understand it? I mean, it seems > to me that the command "spatwmat" uses both y and x to construct the > weighting matrix, how come? It also occurs to me that each unit has only > one value, is that so? What exactly are the xcoord(x) and ycoord(y) > representing? > > > Thanks, > Simon * * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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