Nowadays, there is a growing interest in the application and development of new digital spatial technologies for 3D data capture, analysis and visualisation and the subsequent documentation, investigation and conservation of cultural heritage. This paper presents a topographical survey with various spatial techniques which are combined to achieve a 3D model of the Roman Walls in the city of Lugo in northwest Spain.

The walls were built in the later part of the 3^rd century and are located in the historic city centre. They are one of the best preserved in the world and the only one of it’s kind with completely intact walls. The entire length of the walls is around 2 km, enclosing an area of 34 ha. In 2000, the walls were inscribed on UNESCO's World Heritage List as "the finest example of late Roman fortifications in western Europe"; and have also held Spanish monument status since 1921.

The Roman Walls of Lugo consist of 10 gates: five ancient (from Roman times) and five recent (after 1853) and around 70 towers, spaced at irregular intervals around the walls. There are 5 stairways and a ramp to give access to the parapet walk along with a number of double staircases, within the thickness of the walls, giving access from the parapet to the towers.

We carried out a survey of the wall’s boundary including both internal and external stone facings as well as the parapet. We used mainly new and latest technologies for data capture, such as TLS (Terrestrial Laser Scanner) and UAV (Unmanned Aerial Vehicle). But we didn’t neglect other technologies such as classical surveying and GPS, always necessary for providing support for data capture and close range photogrammetry for getting a virtual model with real (photo) texture.

The data capture was carried out with a TLS (Trimble GX Advance) with different resolutions. In order to geo-reference the TLS point cloud, a series of fixed control points were placed along the entire wall, forming a local geodetic network using absolute coordinates obtained with GPS.

Once the fieldwork was completed, most of the office work and processing of results were carried out with Real Works 6.32 software by Trimble. Firstly, a process of filtering and cleansing data was needed, in order to eliminate the noise and vegetation. Then, a triangle mesh was created using the 3D mesh method, which came up with multiple meshes of the different parts of the wall, as a single mesh was unmanageable for a PC. The work continued with the georeferencing of  photographs and finally, allocation of texture. The parapet images were obtained with an UAV (Microdrone GmbH MD4-200), due to the difficulty of obtaining them with a conventional camera.

The result was the three-dimensional model of the Roman Walls, plus a series of products, like sections and orthophotos, which can be used to provide extremely precise measurements for studying the geometry of the walls and analysing its structural problems and distortions, especially in those areas that have suffered a greater degree of degradation.