Geography Reference
In-Depth Information
products derived from remote sensing images may end up presenting a lower quality than
desired, for, among other reasons, generating an expectation of intrinsic good quality. This may
occur for being considered something modern, conveying the impression that the leading-edge
technology is synonymous of - or is very near to present - guaranteed quality. In an opposite
vision of the former, it is noted that there is a group of users who reveal suspicion or misbelief
regarding the promised potentialities. Probably this occurs for different reasons, such as
insufficient information, prejudice, or still, concerns with loss of jobs. Such resistances delay the
renewal of methods and techniques, ending up to configure "pre-judgments" that end up
making the performance of an analysis difficult, even in an emergency character, regarding
their real efficacy. The results of our studies offer greater clarifications on products originated
from different sensors, leading the reader to the need to break myths - pessimistic or optimistic -
regarding the use of remote sensing, mainly at orbital level for cartographic ends.
It is important also to consider that during recent years a lot of investment was carried out
in new sensors, aiming to meet different applications. It is in place to state that we are being
overrun by growing offers of novelties associated to many promises. The universe of remote
sensors aimed at terrestrial studies is therefore very wide, involving products with several
and variable spatial, radiometric, spectral and temporal resolutions. Such diversity allows
the production of mappings with different detail and accuracy levels, of greater or minor
complexity of legend and the possibility to follow up dynamic phenomena.
Another relevant consideration is the fact that many sensors allow also the extraction of 3-D
data, also with different resolutions and accuracy levels. One fundamental mark in this area
is undoubtedly SRTM (Shuttle Radar Topography Mission), which despite not having been
the pioneer, is the most encompassing and popular Digital Elevation Model (DEM) available
worldwide. Consequently, there is a growing need to study the quality of the altimetry
obtained from such sensors, and also an assessment of how much the implemented
advances in the data acquisition process can influence the results, as for instance the along-
track stereoscopy (images acquired during satellite orbit) and interferometry.
A differentiation between DEM and Digital Terrain Models (DTM) can be useful, because
often this is one of the arguments against the use of orbital images for the generation of
terrain representations. In a simplified way, it will be assumed that the DEMs represent the
land surface added by any existent objects on it and which influence the value of the pixel
reflectance. In other words if there are trees and constructions, the surface represented refers
to the top of them. The DTMs on the other hand, represent the actual ground surface.
Despite so many investments in orbital sensors, the airborne surveys still offer greater accuracy
for their products. It is emphasized also that there are some benefits of the active sensors in
relation to passive sensors, as the case of the interferometer radar and LIDAR (LIght Detection
And Ranging), which are not subject to atmospheric interferences and allow extraction of DEMs
and DTMs - depending on the band for radar and the density of points for LIDAR.
In the case of new methodologies for generation of DEM/DTM to meet the expectations,
reaching greater detail scales, an important step was taken in the sense of generating data
with a good cost/benefit relation, mainly regarding time reduction, if compared with
