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)\",\n BASEGEOGCRS[\"NAD83\",\n DATUM[\"North American Datum 1983\",\n ELLIPSOID[\"GRS 1980\",6378137,298.257222101,\n LENGTHUNIT[\"metre\",1]]],\n PRIMEM[\"Greenwich\",0,\n ANGLEUNIT[\"degree\",0.0174532925199433]],\n ID[\"EPSG\",4269]],\n CONVERSION[\"SPCS83 New Mexico East zone (US Survey feet)\",\n METHOD[\"Transverse Mercator\",\n ID[\"EPSG\",9807]],\n PARAMETER[\"Latitude of natural origin\",31,\n ANGLEUNIT[\"degree\",0.0174532925199433],\n ID[\"EPSG\",8801]],\n PARAMETER[\"Longitude of natural origin\",-104.333333333333,\n ANGLEUNIT[\"degree\",0.0174532925199433],\n ID[\"EPSG\",8802]],\n PARAMETER[\"Scale factor at natural origin\",0.999909091,\n SCALEUNIT[\"unity\",1],\n ID[\"EPSG\",8805]],\n PARAMETER[\"False easting\",541337.5,\n LENGTHUNIT[\"US survey foot\",0.304800609601219],\n ID[\"EPSG\",8806]],\n PARAMETER[\"False northing\",0,\n LENGTHUNIT[\"US survey foot\",0.304800609601219],\n ID[\"EPSG\",8807]],\n ID[\"EPSG\",15339]],\n CS[Cartesian,2],\n AXIS[\"(E)\",east,\n ORDER[1],\n LENGTHUNIT[\"US survey foot\",0.304800609601219,\n ID[\"EPSG\",9003]]],\n AXIS[\"(N)\",north,\n ORDER[2],\n LENGTHUNIT[\"US survey foot\",0.304800609601219,\n ID[\"EPSG\",9003]]],\n USAGE[\n SCOPE[\"unknown\"],\n AREA[\"USA - New Mexico - SPCS - E\"],\n 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")
)
Below is a more generalized way of doing it:
First you need a function to find in which direction each end is pointing:
st_ends_heading <- function(line)
{
M <- sf::st_coordinates(line)
i <- c(2, nrow(M) - 1)
j <- c(1, -1)
headings <- mapply(i, j, FUN = function(i, j) {
Ax <- M[i-j,1]
Ay <- M[i-j,2]
Bx <- M[i,1]
By <- M[i,2]
unname(atan2(Ay-By, Ax-Bx))
})
return(headings)
}
Then the main function which extends the line (one or both ends) by a given distance (in unit distance):
st_extend_line <- function(line, distance, end = "BOTH")
{
if (!(end %in% c("BOTH", "HEAD", "TAIL")) | length(end) != 1) stop("'end' must be 'BOTH', 'HEAD' or 'TAIL'")
M <- sf::st_coordinates(line)[,1:2]
keep <- !(end == c("TAIL", "HEAD"))
ends <- c(1, nrow(M))[keep]
headings <- st_ends_heading(line)[keep]
distances <- if (length(distance) == 1) rep(distance, 2) else rev(distance[1:2])
M[ends,] <- M[ends,] + distances[keep] * c(cos(headings), sin(headings))
newline <- sf::st_linestring(M)
# If input is sfc_LINESTRING and not sfg_LINESTRING
if (is.list(line)) newline <- sf::st_sfc(newline, crs = sf::st_crs(line))
return(newline)
}
So in this case, using the sf object in the OP's question as line_feature:
sf::st_geometry(line_feature) <- st_extend_line(sf::st_geometry(line_feature), 1000)