Extract coordinates from a georeferenced image

This function allows to extract coordinates from a georeferenced image by clicking on points of interest. Every time a point is clicked, its latitude and longitude are captured, and a unique ID is assigned to it. If a dataframe of coordinates is supplied, the new points will be added to it, and the IDs will be numbered sequentially.

extract_coords(georef_image, coords_df = NULL, col = "red")

Arguments

georef_image

A spatraster object representing the georeferenced image.

coords_df

An optional dataframe containing the coordinates of points previously extracted from the image. It has to consist of 3 columns, named: "id", "longitude" and "latitude". Default is NULL.

col

The colour of the points to be plotted on the image. Default is "red".

Value

A dataframe with ID and coordinates of the points extracted from the image.

Examples

if (FALSE) { # rlang::is_interactive()
# Load required packages
library(terra)
# Georeference the image using the created GCPs
# get the gcp coordinates
gcp_europe_coords <-
  readRDS(system.file("vignettes/img/europe_gcp_georef_v2.RDS",
    package = "crstools"
  ))
# get the path to the image
img_path <- system.file("extdata/europe_map.jpeg", package = "crstools")
georef_path <-
  georeference_img(
    image_obj = img_path,
    gcp = gcp_europe_coords,
    output_path = file.path(tempdir(), "europe_map_georef")
  )
# georeference the image using the GCPs
map_warp <- rast(georef_path)
# get the coordinates of the points
coords_df <- extract_coords(map_warp)
# if needed, extract additional points by supplying the coords_df argument
# and re run the function
coords_df <- extract_coords(map_warp, coords_df)
# dataframe with ID and coordinates of the points extracted from the image.
print(coords_df)
}