Find the coordinates (longitude and latitude) of the ground control points (GCPs) in a given image.

find_gcp_coords(gcp, sf_obj)

Arguments

gcp: A data frame containing the GCPs with columns id, x, y, longitude, and latitude.
sf_obj: the reference map, as an sf object (already cut to the extend and if needed projected)

Value

A data frame with the GCPs, including their image coordinates and corresponding geographic coordinates.

Details

NOTE: There are two conventions on how to define pixel coordinates. In this function (and more generally throughout crstools), the origin is defined as the bottom left corner of the image, with x increasing to the right and y increasing upwards. This is consistent with the convention used in many image processing libraries. However, some libraries (like OpenCV and GDAL) define the origin at the top left corner, with y increasing downwards. Be sure to check which convention your image processing library uses when working with pixel coordinates.

Examples

if (FALSE) { # rlang::is_installed("rnaturalearth") && rlang::is_interactive()
# load required packages
library(sf)
library(rnaturalearth)
# get the path to an example image included in the package and choose GCPs
img_path <- system.file("extdata/europe_map.jpeg",
  package = "crstools"
)
# choose some points
gcp_europe <- choose_gcp(img_path)
# now get some more
gcp_europe <- choose_gcp(img_path, gcp = gcp_europe)
# create a map of europe to use to get the coordinates
world <- ne_countries(scale = "medium", returnclass = "sf")
# transform it to a suitable projection
world <- st_transform(world, crs = 4326)
# crop it to the extent of the image
europe <- st_crop(world, c(xmin = -25, ymin = 25, xmax = 45, ymax = 70))
# get the coordinates for these points
new_gcp_europe <- find_gcp_coords(gcp_europe, sf_obj = europe)
# data frame with the GCPs
print(new_gcp_europe)
}