Apply coordinate transformations and spatially reference an image in a GIS (level 3).
Ground Control Points (GCPs) are points that can be clearly identified in the image and on the target map. The target map could be a topographic map or another image that has been transformed beforehand to the desired map projection system. The operator then needs to identify corresponding points on both images. The image and map scale determine which points are suitable. Typical examples of suitable points are road crossings, crossings of waterways and salient morphological structures. Another possibility is to identify points in the image and to measure the coordinates of these points in the field, for example by GPS, and then transform those to map coordinates. It is important to note that it can be quite difficult to identify good GCPs in an image, especially in lower-resolution space-borne images.
In the early days of geoinformation science, spatially referenced data usually originated within national boundaries, i.e. these data were derived from printed maps published by national mapping organizations. Nowadays, users of geoinformation are combining spatial data from a given country with global spatial data sets, reconciling spatial data from published maps with coordinates established by satellite positioning techniques, and integrating their spatial data with that from neighbouring countries.To perform these kinds of tasks successfully, we need to understand basic spatial referencing concepts.