330
A.M.J. Meijerink and C.M.M. Mannaerts
15.2.1 Surveying and mapping
Surveying and mapping is basic to effective water management. Topographic maps
have long been made by photogrammetry using stereo models of aerial photographs.
Basic principles of photo gramme try can be found in Lillesand and Kieffer (1994).
Topographic base maps are usually available, but map updating is often required
nowadays. New technological developments make it possible to carry out map
updating more efficiently than used to be the case. Geometric correction programmes
for satellite imagery are a standard procedure in RS packages. Hence updating of
terrain features which are liable to change, such as land cover, river courses, reservoirs, irrigation areas and so on, can be merged with the topographic base. In addition, some RS/GIS packages offer the possibility of preparing digital orthophotos of
both satellite images and digitally scanned aerial photographs. This is of particular
importance in land and water management, because for large parts of the world, a land
use and tenure or cadastral data base does not exist, or is difficult to access. By image
processing, e.g., edge enhancement filters and multi spectral classifications, at least
parceled areas can be differentiated. The products derived have to be metrically as
accurate as possible with corrected height displacements. The ortho-image procedure
or a simpler monoplot method (lTC, 1994) allows metric registration of parcels in
hilly terrain. Packages are available for the creation of a digital elevation model
(DEM) using aerial photographs in digital format, based on automatic stereo matching
algorithms. The accuracy can be the same or even higher than that achieved by
analogue photogrammetric plotters (Ackermann & Schneider, 1992). On the same
principle, a DEM can be derived from stereo satellite imagery, provided no appreciable changes in the vegetation coridition occlL.--red during the two successive recordings. The accuracy of a DEM using spectral data is at best equal to the spatial
resolution of the system. The result must be checked and rectified for the so called
"blunders" caused by wrong stereo matching which result in fully unrealistic parallaxes. The new generation of satellites with high resolution will make the production
possible of more accurate DEM's for large catchments. A recent development is the
use of airborne laser observations. Height accuracies in the order to 5 to 10 cm have
been obtained for a spatial resolution of a few meters. Once a DEM at appropriate
resolution is available, slopes and aspect maps can be produced, which are of course
of direct interest for watershed management and small scale water development
schemes. The use of digital terrain models receives more attention in Chaps. 4, 5 and
7 of this book. The mapping of areas with important seasonal changes in inundation
and wetlands is essential for water management because of the effects of such areas
on evaporation, flood routing and water quality. Sabins (1987) discusses examples.
If a single image is used, there can, however, be confusion as to the limits of the
inundation, as discussed by Imhoff et al. (1987). On panchromatic imagery, muddy
water may not be separable from wet soil and swamp vegetation may not be spectrally
different from dense healthy vegetation on higher terrain. The use of multi temporal
and radar imagery may contribute to resolve such confusion. A simple and effective
method is to prepare a false color image by NDVI transforms of three different times
during a year. By using the additive color theory, together with and applied to the
A.M.J. Meijerink and C.M.M. Mannaerts
15.2.1 Surveying and mapping
Surveying and mapping is basic to effective water management. Topographic maps
have long been made by photogrammetry using stereo models of aerial photographs.
Basic principles of photo gramme try can be found in Lillesand and Kieffer (1994).
Topographic base maps are usually available, but map updating is often required
nowadays. New technological developments make it possible to carry out map
updating more efficiently than used to be the case. Geometric correction programmes
for satellite imagery are a standard procedure in RS packages. Hence updating of
terrain features which are liable to change, such as land cover, river courses, reservoirs, irrigation areas and so on, can be merged with the topographic base. In addition, some RS/GIS packages offer the possibility of preparing digital orthophotos of
both satellite images and digitally scanned aerial photographs. This is of particular
importance in land and water management, because for large parts of the world, a land
use and tenure or cadastral data base does not exist, or is difficult to access. By image
processing, e.g., edge enhancement filters and multi spectral classifications, at least
parceled areas can be differentiated. The products derived have to be metrically as
accurate as possible with corrected height displacements. The ortho-image procedure
or a simpler monoplot method (lTC, 1994) allows metric registration of parcels in
hilly terrain. Packages are available for the creation of a digital elevation model
(DEM) using aerial photographs in digital format, based on automatic stereo matching
algorithms. The accuracy can be the same or even higher than that achieved by
analogue photogrammetric plotters (Ackermann & Schneider, 1992). On the same
principle, a DEM can be derived from stereo satellite imagery, provided no appreciable changes in the vegetation coridition occlL.--red during the two successive recordings. The accuracy of a DEM using spectral data is at best equal to the spatial
resolution of the system. The result must be checked and rectified for the so called
"blunders" caused by wrong stereo matching which result in fully unrealistic parallaxes. The new generation of satellites with high resolution will make the production
possible of more accurate DEM's for large catchments. A recent development is the
use of airborne laser observations. Height accuracies in the order to 5 to 10 cm have
been obtained for a spatial resolution of a few meters. Once a DEM at appropriate
resolution is available, slopes and aspect maps can be produced, which are of course
of direct interest for watershed management and small scale water development
schemes. The use of digital terrain models receives more attention in Chaps. 4, 5 and
7 of this book. The mapping of areas with important seasonal changes in inundation
and wetlands is essential for water management because of the effects of such areas
on evaporation, flood routing and water quality. Sabins (1987) discusses examples.
If a single image is used, there can, however, be confusion as to the limits of the
inundation, as discussed by Imhoff et al. (1987). On panchromatic imagery, muddy
water may not be separable from wet soil and swamp vegetation may not be spectrally
different from dense healthy vegetation on higher terrain. The use of multi temporal
and radar imagery may contribute to resolve such confusion. A simple and effective
method is to prepare a false color image by NDVI transforms of three different times
during a year. By using the additive color theory, together with and applied to the