[Rgeos-devel] How best to work with row.names output from gUnaryUnion?

[Roger Bivand]

The list is not used. Either email  the package maintainer, post on R-sig-geo, or - much better - use the sf package for all new projects involving GEOS in R - sf does do what you appear to want. 

--
Roger Bivand
Norwegian School of Economics
Helleveien 30, 5045 Bergen, Norway
Roger.Bivand at nhh.no


________________________________________
Fra: Rgeos-devel <rgeos-devel-bounces at lists.r-forge.r-project.org> på vegne av Roger André <randre at gmail.com>
Sendt: mandag 24. februar 2020 21.02
Til: rgeos-devel at lists.r-forge.r-project.org
Emne: [Rgeos-devel] How best to work with row.names output from gUnaryUnion?

Hello,

I'm slowly learning the implementation details of rgeos after being a longtime user of GDAL and PostGIS.  I was doing some work that necessitated doing some spatial unions which are  aggregated by a feature attribute and was surprised to see that the original dissolve field (or `group by` field) wasn't returned in a data frame, but rather as row IDs.  This was inconvenient for me, so I wrote a short helper function to give me back my desired output.

dissolvePolygons <- function (
    input_SPDF, # SPDF object to dissolve
    diss_field # name of column to dissolve by, as string
    ) {
    diss_command <- sprintf("rgeos::gUnaryUnion(input_SPDF, input_SPDF$%s)", diss_field)
    data_command <- sprintf("data.frame(%s = row.names(out_polys))", diss_field)
    out_polys <- eval(parse(text = diss_command))
    data <- eval(parse(text = data_command))
    output_SPDF <- SpatialPolygonsDataFrame(out_polys,data)

    return(output_SPDF)
    }

# Usage
zones <- dissolvePolygons(sample_tracts, "zone")

I'm curious what a preferred method is to work with the original row output?  I was finding it problematic to tie it back to other data and was hoping to find some examples of how to do so.

Thanks,

Roger
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