R/extract_spatial.R

In jhollist/miscPackage: Package with JWHs Miscellaneous Functions

#' define an area on a spatial plot and extract selected polygon or 
#' point features
#' 
#' This function utilizes `sp::over` to select features that overlay 
#' on an input polygon.  As no method is defined for SpatialLines, 
#' only Polygons and Points may be selected.
#' 
#' @param in_spatial The sp object from which to extract features
#' @param over_poly A polygon that specifies the features to extract.  
#'                  used in non-interactive mode.  NOT YET IMPLEMENTED
#' @param plot_result Boolean to plot the resulting features and selection 
#'                    poly.
#'                    
#' @import sp 
#'                  
#' @noRD
#' @examples
#' \dontrun{
#'  x <- c(0.5, 0.5, 1.2, 1.5)
#'  y <- c(1.5, 0.5, 0.5, 0.5)
#'  xy <- cbind(x,y)
#'  dimnames(xy)[[1]] <- c('a', 'b', 'c', 'd')
#'  pts <- SpatialPoints(xy)
#'  xpol <- c(0,1,1,0,0)
#'  ypol <- c(0,0,1,1,0)
#'  pol <- SpatialPolygons(list(
#'  Polygons(list(Polygon(cbind(xpol-1.05,ypol))), ID='x1'),
#'  Polygons(list(Polygon(cbind(xpol,ypol))), ID='x2'),
#'  Polygons(list(Polygon(cbind(xpol,ypol-1.05))), ID='x3'),
#'  Polygons(list(Polygon(cbind(xpol+1.05,ypol))), ID='x4'),
#'  Polygons(list(Polygon(cbind(xpol+.4, ypol+.1))), ID='x5')
#'  ))
#'  extract_spatial(pts,T)
#'  extract_spatial(pol,T)
#'  }

extract_spatial <- function(in_spatial, over_poly = NULL, plot_result = FALSE) {
    plot(in_spatial)
    message("Select at least 3 points to define the extent")
    coords <- locator() %>% data.frame()
    poly <- rbind(coords, coords[1, ]) %>% Polygon() %>% list() %>% Polygons(ID = 1) %>% list() %>% SpatialPolygons(proj4string = CRS(proj4string(in_spatial)))
    idx <- over(poly, in_spatial, returnList = TRUE) %>% unlist()
    if (plot_result) {
        plot(in_spatial[idx, ])
        plot(poly, add = T)
    }
    return(in_spatial[idx, ])
}