metria: digital photogrammetry for architecture, heritage, engineering and industry

The peculiar and surprising trajectory of the Asturian monarchy from its foundation in the VIII century, after the Muslim invasion, to the kingdom capital transfer to the city of Leon not only left very important progresses for the Reconquista but an universally well-known art like the Asturian Preromanesque art. Chronologically, the Asturian art last from the first babbling of the Asturias kingdom, after the impact of the Muslim invasion and the later Christian reorganization in the north (VIII century) to the last typical manifestations in the X century where we can already find mozarabic influences.

The work lied in the photogrammetric documentation, restitution and elaboration of the true frontal elevation orthophotographs of the church of Santa Maria del Naranco (Oviedo), San Miguel de Lillo (Oviedo), San Julian de los Prados (Oviedo), Santa Cristina de Lena (Pola de Lena) and San Salvador de Valdediós (Villaviciosa) at scales between 1:20 and 1:40, corresponding to a representation precission equivalent in elevation plans to a pixel size below 4 and 8 mm according to the scale. The documentation and three-dimensional restitutions of the five monuments were accomplished, obtaining a total of 16 orthophotographs to the following scales:

Santa Maria del Naranco – 4 elevations to 1:30
San Miguel de Lillo – 4 elevations to 1:20
San Salvador de Valdedios – 4 elevations to 1:30
Santullano – 3 elevations to 1:40
Santa Cristina de Lena – 1 elevation to 1:20

Asturian Preromanesque. Elevation true orthophotographs. Antonio González-Capitel and Fernando Nanclares, Architects.
Clic on the images to zoom

800 digital color photographs were taken, with a resolution of 4000x3000 pixels and 16 bits of color depth, using digital SLR cameras. Calibrated lenses with focal lengths between 18 and 50 mm have been chosen to take the oblique photographs from the monuments surroundings due to the available space. The photographs were taken to guarantee the cover and the point resolution suitable for each scale (between 3 and 7 mm).

Temporary adhesive targets were previously placed in easily identifiable points to establish a primary net of control points, scale, level and orientation between different facades and for different work sessions, verified by means of a laser. A redundant secondary visual net of control points was also established using circular adhesive targets.

The measurement references were correlated with the restitution. This correlation was made by means of a least squares fitting between the measures obtained by laser, and the scale and orientation control points of a primary net. The control net concordance was verified within the suitable tolerance. As a base for the orthophotographs generation it was used a secondary net of control points and lines, obtained from photogrammetric restitution by establishing several points in each separated monument surface.

Making the right projection plane choice the correspondence between the real measurements and the projected ones is optimized, increasing the utility of the true orthophoto as a metric reference presenting each facade real dimensions and real texture within the precision employed.