Cl PA 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey 
As we said above the use of these filters is not limited to 
photograms used in this work and connected with this site. We 
must mention the work done by J. Del Olmo, who took oblique 
photographs of the site at Clunia. These photograms are not 
only easier and cheaper to obtain but can be done repeatedly 
during the year to highlight seasonal differences. 
2. MICRO-RELIEFS DETECTION 
From an observation of the photogrammetric model, it is clear 
that some terrain elevations are not of natural origin. Photo 
interpretation allows us to correlate these elevations with buried 
ruins, which in some case are quite visible in the photograms 
because of small colour variations. Because of the probability 
of finding hidden ruins where natural morphology has been 
changing, it is worthwhile creating a photogrammetric DTM. 
2.1 Automatic DTM by autocorrelation 
The Clunia plateau presents only small altitude variations, due 
to: 
• natural orography, which has origin from the 
geological process of sedimentation and erosion; 
• agriculture that require the formation of pits and 
furrows, boundary and field delimitations, 
accumulation of stones, terrain levelling, etc.; 
• rubble piles. 
These altitude variations are small, and it is not always possible 
to establish with certainty their origin without on-site checks. 
However, the analysis of the photogrammetric model can give 
information and measures that are difficult to acquire with the 
traditional topographic systems. The main advantages of the 
photogrammetry for archaeologists are: 
• In a stereoscopic view the observer can quickly 
estimate the altimetry; this allows the photo 
interpreter to read not only an image, but a three- 
dimensional model. 
• The measurement (automatic or not) of a great 
number of points in the photogrammetric model is 
quick, fast and cheap. The products are profiles of the 
terrain and in some case digital terrain models 
(DTM). 
Automatic DTMs can be computed using auto-correlation 
techniques (Kraus, 1993). The case of Clunia plateau is optimal 
to perform automatic DTM estimation by image correlation, 
because of the absence of trees, buildings and shadows. 
We have computed three DTMs of Clunia hill: 
• a global DTM of the hill, by using traditional 
photogrammetric techniques with Galileo Siscam 
Stereobit20 analytical stereoplotter; the photograms 
were taken at relative altitude of 2800 m with 158 
mm focal length; 
• an automatic DTM of the central area of the plateau, 
with 2 m resolution, by GeoSoft GCarto digital 
stereoplotter; the photograms were taken at relative 
altitude of 600 m with 158 mm focal length; 
• an automatic DTM of the area between Casa de 
Taracena and Termas de Los Arcos, with 1 m 
resolution. 
Better results can be achieved only using airborne laser 
scanning technique, and we hope that this will be possible in the 
future. 
Figure 1. The DTM of Clunia hill made with traditional 
photogrammetric techniques. 
Figure 2. The DTM computed by autocorrelation on the central 
area of the plateau. Some undulations (or micro 
reliefs) are clearly non-natural. The white zones 
represent lack of data; this is due to the 
photogrammetric software used for DTM estimation. 
DTM validation is in this case is performed only as internal 
reliability, observing the RMS of the estimated DTM points 
provided by the software GCarto. 
The image in fig. 3 represents these RMS values. The biggest 
errors are in the east zone, and are produced by GCarto for 
unknown motives. Thus the DTM is not useful in this zone.