The use of aerial photographs for identifying archaeological sites has been practised during the last several decades in Spain. Aerial surveys have been central to recording and mapping the landscape of ancient sites, and they have also been utilised on a variety of scales, ranging from the smaller internal distribution of archaeological sites to wider reconstructions of ancient landscapes.
San Juan del Viso is one of the most striking hills on the left bank of the Henares River, upon Alcalá de Henares city, 35 Km northeast of Madrid. In the Roman period, the centre of Iberian Peninsula was known as Carpetania. The Carpetania region was incorporated into the Roman polity during the first half of the 2nd century B.C. Classical sources tell of how in 192 B.C. M. Fulvius Nobilior, praetor of Hispania Ulterior, annexed Toletum, the most important pre-Roman oppidum of the area (Liv., XXXV, 7, 6 and 22, 5). Since that time, there were clashes between different populations of Carpetanians and Rome, which tried to set the northern boundary of the Hispania Ulterior in the line of the Tagus. In 151 B.C. the Carpetania region appeared, in literary sources, to be a pacified territory, which served as a base of operations for the Roman legions during the wars against the surrounding cultures of Celtiberi, Vaccaei and Lusitani (AP, Iber. 50-52). At the time of the discovery of this plan city, only two major Roman cities in the Carpetania region were known: Toletum (Toledo) and the Imperial Complutum (Alcalá de Henares).
The aerial photo survey permit a very precise mapping of the road segments and of other interesting archaeological features. In May 2011, the discovery of city map of the first Complutum was possible due to the analysis of an orthophoto (taken in May 2009), by Plan Nacional de Ortofotografia Aérea (PNOA) of the National Geographic Institute. The higher resolution of the image (0.5 m/px) significantly improves it. Furthermore, when the picture was discovered, its availability on the Internet through two simple viewing applications, i.e. Iberpix and SigPac helped to simplify the interpretation of the image. Both application are developed by Spanish institutions; Iberpix was created by the National Geographic Institute and SigPac by the Spanish Ministry of Agriculture and Environment. Using these tools, it is possible to measure crop marks, which mirror lengths and areas of the urban layout.
The area of identified urban features in the orthophoto occupies around 30 Ha, making it possible that the city was even larger, as, when the site is viewed from the air, the apparent lines not set clear limits, and the area of the plateau of the hill is around 70 Has. This orthogonal grid identifies a multitude of lines that represent the street plan as well as various and their orientation within the city. Appreciable chromatic differentiation in the orthophoto is the direct result of variations in plant water stress. Higher levels of plant water stress are often indicative of the existence of areas of higher compaction of soil in the ground, which we may interpret as the remains of foundations, or as soil of greater consistency (hydraulic floors, mosaics, etc.). The same evidence would also help reveal evidence of cobbled streets. Alternately, those areas that appear dark and inscribed in squares or rectangles are related with soil of lower density and lower compaction; these areas may perhaps be associated with rammed earth or patios.
The city is laid out in a highly regular grid pattern, with the main cross streets, cardo maximus and decumanus maximus, clearly delineated. These streets were first identified through aerial photography. The cardo has 5 m width, the same as the access road. Decumanus maximus is somewhat smaller, with an approximate width of 3.5 m. The rest of the cardines and decumani range from 3 to 4 m. width. The area at the cross of both streets has been interpreted as the Forum, because of a building whose characteristics are similar to that of a temple. The dimensions of this temple are 14 x 8.5 m and the cella, which is the part that is best appreciated in the orthophoto, measures approximately 5.5 x 8 m., with walls of 1.10 m. in thickness.
The example that best illustrates the alternation of soil compaction is located in the north-central area of the city, where a courtyard surrounded by several paved rooms was located. This structure has been interpreted as a domus. The central peristyle measures approximately 30 x 25.5 m with a courtyard of 13 x 10 m.
We also have to pay attention to a semicircular structure located at the north-eastern end of the city. It is a building of 40 m in diameter, identified as a theatre. Its location and orientation is deliberate, having been constructed so that it might be seen from the valley, offering a clear statement of political purpose. The orthophoto allows for the identification of many different features in this area. The darker feature, with two extensions at the ends, could relate to the orchestra, the approximate diameter of the structure is 19 m, and the aditus is 2 m wide. The clearer feature that encircles the orchestra, also semicircular, is interpreted as the area that would be occupied by the cavea. In addition, the rectangular space with the same contrast, of about 12 m in width, corresponds with the scaenae and the scaenae frons. Its dimensions are consistent with other Roman theatres in the 1st century A.D. such as Regina (Casas de Reina, Badajoz) or Acinipo (Ronda, Málaga), this last one also built in the top of the hill.
The first Complutum street plan (Ruiz Taboada y Azcárraga Cámara, 2014: 19, Fig. 4).