increase polyline length r sf

Question

I'd like to find a way to increase a polyline 1000 feet from the beginning and 1000 feet from the end points. Any help is greatly appreciated! Below is a sample.

structure(
    list(
        TECHNICAL_ = NA_character_,
        geometry = structure(
            list(
                structure(
                    c(812697.360851467,
                      813792.18162311,
                      430678.939150205,
                      425750.102767913),
                    .Dim = c(2L, 2L),
                    class = c("XY", "LINESTRING", "sfg")
                )
            ),
            n_empty = 0L,
            crs = structure(
                list(
                    input = "+init=epsg:2257",
                    wkt = "PROJCRS["NAD83 / New Mexico East (ftUS)",
    BASEGEOGCRS["NAD83",
        DATUM["North American Datum 1983",
            ELLIPSOID["GRS 1980",6378137,298.257222101,
                LENGTHUNIT["metre",1]]],
        PRIMEM["Greenwich",0,
            ANGLEUNIT["degree",0.0174532925199433]],
        ID["EPSG",4269]],
    CONVERSION["SPCS83 New Mexico East zone (US Survey feet)",
        METHOD["Transverse Mercator",
            ID["EPSG",9807]],
        PARAMETER["Latitude of natural origin",31,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8801]],
        PARAMETER["Longitude of natural origin",-104.333333333333,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8802]],
        PARAMETER["Scale factor at natural origin",0.999909091,
            SCALEUNIT["unity",1],
            ID["EPSG",8805]],
        PARAMETER["False easting",541337.5,
            LENGTHUNIT["US survey foot",0.304800609601219],
            ID["EPSG",8806]],
        PARAMETER["False northing",0,
            LENGTHUNIT["US survey foot",0.304800609601219],
            ID["EPSG",8807]],
        ID["EPSG",15339]],
    CS[Cartesian,2],
        AXIS["(E)",east,
            ORDER[1],
            LENGTHUNIT["US survey foot",0.304800609601219,
                ID["EPSG",9003]]],
        AXIS["(N)",north,
            ORDER[2],
            LENGTHUNIT["US survey foot",0.304800609601219,
                ID["EPSG",9003]]],
    USAGE[
        SCOPE["unknown"],
        AREA["USA - New Mexico - SPCS - E"],
        BBOX[32,-105.72,37,-102.99]]]"
                ),
                class = "crs"
            ),
            class = c("sfc_LINESTRING", "sfc"),
            precision = 0,
            bbox = structure(
                c(xmin = 812697.360851467,
                  ymin = 425750.102767913,
                  xmax = 813792.18162311,
                  ymax = 430678.939150205),
                class = "bbox"
            )
        )
    ),
    row.names = 1L,
    sf_column = "geometry",
    agr = structure(c(TECHNICAL_ = NA_integer_),
                    .Label = c("constant", "aggregate", "identity"),
                    class = "factor"),
    class = c("sf", "tbl_df", "tbl", "data.frame")
)

question from:https://stackoverflow.com/questions/65928126/increase-polyline-length-r-sf

Answer 1

I found a solution. First, calculate the length of the original line using st_length() and append original coordinates similar to below data table. After your table is constructed the following formula will do the trick.

df<-structure(list(x= "abcd", surf_long = 569270.67464245, 
    surf_lat = 645729.112942885, bh_long = 569667.630944828, 
    bh_lat = 645187.374426766, length = 671.606228265491), row.names = c(NA, 
-1L), groups = structure(list(x= "abcd", .rows = structure(list(
    1L), ptype = integer(0), class = c("vctrs_list_of", "vctrs_vctr", 
"list"))), row.names = 1L, class = c("tbl_df", "tbl", "data.frame"
), .drop = TRUE), class = c("grouped_df", "tbl_df", "tbl", "data.frame"
))

Now we can increase the line length X distance proportionately.

df<- df%>%mutate(surf_lat_ext=surf_lat-(bh_lat-surf_lat)*1250/length,
                             surf_long_ext=surf_long-(bh_long-surf_long)*1250/length,
                             bh_lat_ext=bh_lat+(bh_lat-surf_lat)*1250/length,
                             bh_long_ext=bh_long+(bh_long-surf_long)*1250/length)