CLOSE RANGE DIGITAL PHOTOGRAMMETRY TECHNIQUES APPLIED TO LANDSLIDES MONITORING
This paper deals about the use of non metric reflex digital cameras in landslide monitoring and the analysis of landscape evolution from comparison of present digital photographs and old digitized analogue metric terrestrial photographs. The methodology has been improved in a test area (a slope along a road with instability processes in southern Spain) where terrain failures have been documented in the last ten years. Several photogrammetric techniques were checked as well as combination of both convergent (for calibration and densification) and stereoscopic (for DTM generation) networks. Considerations about the use of terrestrial LIDAR and a comparative example are also made. This paper is included in a wider research project carried out in Northern and Southern Spain for study landslides. This research project (MAPMUT: The improvement of susceptibility analysis by means of useful digital models of the terrain geometry), aims with an improvement of susceptibility maps using high quality input information.
EVALUATION OF A DIGITAL NON METRIC CAMERA ( CANON D 30 ) FOR THE PHOTOGRAMMETRIC RECORDING OF HISTORICAL BUILDINGS
This paper is about the evaluation of a digital non-metric reflex camera (Canon D30) for low cost applications in archaeology, architecture and cultural heritage. At present digital cameras of different geometric qualities are being routinely used for this purposes. The Canon D30 digital camera is a professional camera with a CMOS sensor of 3.2 Mp. Although camera resolution is relatively low, compared with more recent amateur cameras, the sensor is slightly smaller in size than conventional 35 mm film format. The focal length/angle of view conversion factor is approximately 1.6x compared to full frame 35 mm film format. This property makes very interesting the use of this camera in cultural heritage photogrammetric applications, where both accuracy and final image quality are quite important. The camera internal geometry has been solved by means of self calibration with own developed software. First self calibration was made at laboratory using digital target measurement at subpixel accuracy. Proportional accuracies with this method were between 1:20.000 and 1:30.000. But additional self calibrations where carried out with field control points. In this case, accuracy reached was poorest because images were manually measured on natural points, but it was for normal architectural/archaeological applications enough. As main drawback of this camera we can point out the low sensor resolution. So when camera/object distance increases or high oblique photographs are employed the image quality of the final product reduces considerably.
Lake Muzahi, Rwanda: limnological features and phytoplankton production
Lake Muhazi, a small lake of Rwanda (East Africa) was studied from 1986 to 1990. A dramatic decrease of the catch of Oreochromis niloticus (350 T y−1 in the fifties vs 30 T y−1 in 1982) suggested a loss of productivity or overfishing. In the same period, other ecological changes occurred: the submerged macrophytes regressed and there was a decrease in Secchi depth (0.65 m in 1987 vs 1.5 m in the fifties). Compared to other lakes of the same area, the plankton production seemed low. The results of the present study characterize lake Muhazi as a shallow lake with a rather unstable diurnal stratification and with slight differences in mixing regime between its eastern, deepest part and its western, shallowest part. Secchi disk depth does not vary seasonally to a large extent. The water has a rather high mineral content (conductivity of about 500 µS cm−1 at 25 °C) and low concentrations of dissolved N and P, except in the hypolimnion, where NHinf4sup+-N can be high.Two species, Microcystis aeruginosa and Ceratium hirundinella, account for most of the phytoplankton biomass, which is about 50–80 mg chlorophyll a m−2 in the euphotic zone, usually with little seasonal variation. Daily gross production estimates amount to about 6 to 9.5 g O2 m−2 d−1 with a significant difference between the two parts of the lake. Data on C:N and C:P ratio in the phytoplankton suggest that some N deficiency might occur in the eastern part. Moreover, the Zm:Zc ratio could also lead to rather low net production rates (0.21–0.25 d−1 for a mixed layer of 4 m)In conclusion, the primary production of lake Muhazi is medium for African lakes and the hypothesis that decreased planktonic production could account for a reduced fish production should be discarded. Whereas the present yield of the fishery is only 20 kg ha−1 y−1, the yield estimated from primary production ranges between 46 and 64 kg ha−1 y−1. This could be reached through proper management. Finally, some hypotheses are given to explain the ecological changes which occurred in the lake.