choose_gcp: Function to choose the Ground Control Point (GCP) on an image

In crstools: Tools to Work with Projections

Function to choose the Ground Control Point (GCP) on an image

Description

GCPs are used to georeference images by providing known coordinates for specific points in the image.

Usage

choose_gcp(image_obj, gcp = NULL, col = "red")

Arguments

image_obj	An array representing the image (colour images are generally imported as an array of nx x ny x 3 colour channels), or a file path to the image (currenly this can only be of type .jpg).
gcp	(optional) A dataframe of GCPs, containing columns id, x, y,longitude, latitude. This is used if we want to add additional GCP to an existing list (usually created by running this function multiple times).
col	The colour of the points to be plotted on the image. Default is "red".

Value

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

Examples

# Get the path to an example image included in the package and choose GCPs
img_path <- system.file("extdata/europe_map.jpeg", package = "crstools")
# this will open a new window where you can choose some points
gcp_europe <- choose_gcp(img_path)
# after the first set of points is chosen, we can add more points
gcp_europe <- choose_gcp(img_path, gcp = gcp_europe)

crstools documentation built on March 19, 2026, 5:08 p.m.