help mergepoly
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Title

    mergepoly -- Merge adjacent polygons from a shape boundary file


Syntax

        mergepoly id_var lat lon , [ generate(newvar) ]


Description

    mergepoly is used to merge adjacent polygons from a shape boundary file.
    Shape boundary files are usually obtained from outside sources and
    converted to Stata datasets using -shp2dta- (SSC). The shape database
    contains one record per feature and the coordinate dataset describes the
    shape of each feature in terms of points. The points, in order, form
    polygons.

    mergepoly combines adjacent polygons by removing all adjacent line
    segments. mergepoly creates new polygons that follow the outer border of
    the initial adjacent polygons.

    id_var is a numeric variable that identifies the geographic feature. lat
    lon are numeric variables that provide the latitude and longitude
    coordinates of each point. Note that when using -shp2dta- (available from
    SSC), the id_var is named by default _ID; the latitude variable is named
    _Y and the longitude is named _X.


Options 

    generate() specifies the name of the new numeric variable that identifies
        the new features that are created from the merged polygons.


Example

    Download the Census 2000 County Areas for the State of Texas in ArcView
    Shapefile (.shp) format

        . copy "http://www.census.gov/geo/cob/bdy/co/co00shp/co48_d00_shp.zip" 
> "co48_d00_shp.zip"
        . unzipfile "co48_d00_shp.zip"
        
    Use -shp2dta- to convert to Stata datasets

        . shp2dta using "co48_d00.shp", data("tx_counties.dta") coor("tx_coordi
> nates.dta")

    Draw the initial shapes

        . use "tx_coordinates.dta", clear
        . line _Y _X, lwidth(vvthin) cmissing(n) aspectratio(.95) xscale(off) y
> scale(off) ylabel(minmax, nogrid) xlabel(minmax)

    Merge adjacent counties

        . mergepoly _ID _Y _X, gen(newid)

    Draw the new shapes

        . line _Y _X, lwidth(vvthin) cmissing(n) aspectratio(.95) xscale(off) y
> scale(off) ylabel(minmax, nogrid) xlabel(minmax)

    Show which counties have segments on the borders of the new polygons

        . tabulate _ID newid


Author

    Robert Picard
    picard@netbox.com


Acknowledgments

    A question on Statalist from Karsten Pfaff was the stimulus for writing