A network of longitudes (in red) was created based on Mercator’s prime meridian passing through Fuerteventura in the Canary Islands. This represents a 14.1° westward shift in relation to today’s prime meridian passing through Greenwich (in blue).

The process used to photograph the globes produced 42 images centred on selected longitude and latitude intersections.

The colour, lighting and sharpness of the images had to be as uniform as possible. A usable circular area was therefore determined for each one.

Each rectangular photo was georeferenced so that it could then be laid precisely over the virtual globe.

To correct the distortions due to perspective as much as possible, the globe was displayed using the World Vertical Perspective projection from an altitude of 35,800,000 metres. On scale, this altitude corresponds to the distance from the camera to the globe.

The graticule intersections on each image were then matched with those of the virtual grid. Each of these anchor points made it possible to improve the georeferencing.

Since the globes are not perfectly spherical, a non-parametric alignment using the Spline algorithm yielded better results.

On average about 30 of these anchor points were needed to ensure an optimal alignment.

To highlight this process, in the above animation the point focal is shifted in relation to the centre of the globe. Actually, the globe was centred so that its geometry would resemble the conditions of the image capture as closely as possible.

The centre of each image was cropped so as to keep only the most uniform portion in terms of colour, lighting and sharpness.

The cropping process was repeated for all 42 photos.

Even so, the round tiles were not perfectly matched and this was particularly visible at their edges. To reduce these discrepancies, the tiles were assembled in a mosaic dataset.

The “blend” function made it possible to weight the tiles’ display in relation to the distance from their centre and thus to fit them together more seamlessly.

Now the Mercator globe is georeferenced, users can process the information at will, for example by changing the system of coordinates: