Find centroids after mergepoly

[Carolin Thol]

Dear Statalisters

I'm using Robert Picard's mergepoly command to combine small areas on a map into larger areas. Is there a way to get the centroids of the new polygons? I tried D.H. Judson's centroid command, but that runs into an error, unfortunately, and only seems to output the distance between points, but not the coordinates of the centroids.

Any help appreciated!

Carolin

[Robert Picard]

There's no centroid options in mergepoly.

You can use the same code that shp2dta uses to generate centroids. Just load your coordinates in memory and run this code:

bys _ID: gen float TEMPa=(_X*_Y[_n+1])-(_X[_n+1]*_Y) if _n>1 & _n<_N
bys _ID: gen float _AREA=sum(TEMPa)
bys _ID: replace _AREA=_AREA[_N]/2
bys _ID: gen float TEMPx=(_X+_X[_n+1])*(_X*_Y[_n+1]-_X[_n+1]*_Y) if _n>1 & _n<_N
bys _ID: gen float _CX=sum(TEMPx)
bys _ID: replace _CX=_CX[_N]/(6*_AREA)
bys _ID: gen float TEMPy=(_Y+_Y[_n+1])*(_X*_Y[_n+1]-_X[_n+1]*_Y) if _n>1 & _n<_N
bys _ID: gen float _CY=sum(TEMPy)
bys _ID: replace _CY=_CY[_N]/(6*_AREA)
collapse _CX _CY, by(_ID)

You can then merge these centroids to your shapefile database dataset.

[Carolin Thol]

Thanks a lot, works a charm!

[Sayoree Gooptu]

I have used the method specified in #2 to calculate the centroids since I had to use mergepoly and the shapefile needed to be modified. After that, I am trying to use the spwmatrix command in stata to calculate the weights using weights on the basis of latitudes and longitudes. However, the values of the centroids are very large. I am getting the following error with the command

spwmatrix gecon _CX _CY, wn(W)
Spherical latitudes must be in [-90,90]

Please provide me some help in this matter.