São Jorge Castle is a historic castle in the Portuguese capital of Lisbon. Humans have occupied the castle area back to at least the 8th century BC, while the first fortifications date to the 1st century BC.

Datum Lisboa the now obsolete but first main reference system for Lisbon was established in the late 19th century. 
The angular observations that gave rise to this datum were made between 1888 and 1963 with Troughton and Repsold theodolites. The orientation of the net was given by Azimuth Lisboa -> Serves, and its scale by the base of the Chamusca measured with Repsold’s decimeter.

In 1945 the so-called Geographic and Cadastral Institute carried out work for the Lisbon City Council with the aim of creating a denser survey network in the Lisbon area for the execution of 1: 1000 scale cartography. The network thus created, called Local Triangulation of Lisbon (TLLx) , refers to the Lisbon Datum.

Since any point of this triangulation is less than 11km from São Jorge Castle, it was not considered necessary to define analytically a cartographic projection that established the correspondence between the points on the ellipsoid and those of the cartographic plane. The coordinates of the TLLx refer to a system of rectangular axes situated in the plane tangent to the ellipsoid at the geodesic vertex situated in the castle of S. Jorge.

Here are some of the photos taken by Marty of the castle from his trip to Portugal.