Landslides - Causes, Types and Effects.pdf

Geomatic Methods for Punctual and Areal Control of Surface Changes… 155These parameters also constrain the costs and are therefore to be correctly defined beforeperforming the survey. Among these, the camera - object distance (flying height in aerialphotogrammetry) plays a relevant role, affecting the final accuracy with a quadratic influence (2).At present, with a moderately low flying height, a few centimeters accuracy can be achieved.Several approaches are available to evaluate the a priori accuracy of photogrammetricpoint restitutions, based on empirical relationships (Kraus, 1993; D’Agata and Zanutta, 2007)or on statistical methods (i.e., applying the variance propagation law to the simplifiedcollinearity model). For instance, a simple relation to evaluate the final accuracy of points isprovided by the equation (2) (Kraus, 1993):where:2⎛ Z ⎞σ =⎜ σc B⎟ ⋅⎝ ⋅ ⎠Pξ(2)σ = theoretical Root Mean Square deviation expected;Z = relative distance from the object;c = focal length of the camera;B = distance between central points on subsequent images (photobase);σ P= horizontal parallax measurement error (errors measuring image coordinates on theξimages).Furthermore, for an accurate object description, the definition of the pixel dimension onthe ground (Object Sampling Distance - OSD) is of great importance, because it constrainsthe detail of the scene (3):ZOSD = l ⋅(3)cwhere:l = pixel dimension;Z = relative distance to the object;c = focal length of the camera.Table 2. Characteristics of satellite aerial and terrestrial optical sensorsOptical sensorSatellite Aerial/balloon Close range/Fixed cameraSpatialResolution0.42 m to 80 m 0.05 m to 2 m 0.01 m to few metersTemporalResolution∼ 1-30 days On request On request/1 s to 1 daySpectralresolutionPancromatic/Multispectral Visible VisibleAccuracy 1/5 to 1 pixel 1/5 to 2-3 pixel 1/5 pixelSwath 10x10 km to 60x60 km 10x10 m to 5x5 km 10x10 m to 1x1 kmSPOT 1-4 (1986); IKONOS (1999); National and regionalHistoricalQUICKBIRD (1999) SPOT 5 (2002); photogrammetricdata archiveGEOEYE-1 (2008)coverage-