Chandler, Jim
associated with carrying out fieldwork are eaed, topography is traditionally measured using manual point gauges,
which again leads to difficulties in achieving appropriate data densities.
Techrologicd advances have enabled faster and hence higher resolution data aquisition methods to be used and
developed. The recat computerization and automation of point gauges (Wallingford, 1999) has enabled individual
cross ædions to be measured more efficiently in a flume. The time required for ead measurement paint is dill high
and so measuring reped parallel profiles to provide adigital elevation model (DEM) is an exceptionally slow process
Laser based instrumentation has offered pdential for flume work also, with gantry mounted laser profilers being
capable of deriving very dense DEMs. However, the time required for such measurement remains long also and
problems of defining a stable reference plane suggest that this approad is only pradicable within small areas (« 0.1m).
For natural field channels, developments in conventional surveying equipment are significant, particularly the
motorized prism tracking Total Station. This technology has enabled the number of points that can be measured per day
to be doubled (approximately 2-3000 pánts), and on seaure sites provides a 5096 reduction in personnel. The Total
Station can certainly provide data a the channel scde and in all diredions, but the density of points is only just able to
provide information at the bed scde and is certainly incapable of providing anything at the more dusive grain scde.
2 APHOTOGRAMM ETRIC SOLUTION
A photogrammetric method d deriving appropriate data for fluvial reseach would appea to dfer potential, and indeed
photogrammetric methods have been used in earlier studies. Lo and Wong (1973 used 35mm cameras to examine the
development of rills and gullies on a small sedion of wedahered granite in Hong Kong. Collins and Moon (1979
measured stream bank erosion photogrammetricdly and this was developed by Welch and Jordan (1983) where non
metric 35mm imagery was used to meæure œoss ædional profiles and 3D terrain models to represent change occurring
in a dynamic meander bed. More recently, Lane et al. (1 994) combined analyticd photogrammetry with tacheometric
methods to quantify change occurring in a rapidly evolving braided pro-gladal channel inthe Alps. A Wild P32 camera
was used to aauire highly-oblique terrestrial images and this gudy demonstrated just how improved topographic
monitoring could assst fluvial research, (Lane et al., 1996).
These studies were dependent upon traditional photogrammetric methods, required acces to expensive
analogue/analyticd plotters and relied upon manual measurement methods. Developments in digital photogrammetry
have provided fresh impetus to the use of photogrammetry in fluvial reseach. There ae many major advantages with a
purely digital solution, particularly if a high-resolution digital camera is included. The equipment necessary for image
measurement is becoming progressively less expensive, particularly with increasing use of PC platforms. The costs of
high-resolution digital cameras remain high, but are deceæing (Ahmad and Chandler, 1999) and the spatial acaracy
adievable is impressive, (Fraser, 1997, Shortis & al., 1998). Competition in the photogrammetric software market is
intense dso, with prices reducing and capabiliti es increasing. Of significanceto fluvial researchers is the trend towards
software which demystifies photogrammetry, encouraging ron-photogrammetrists to apply the techniques, (Chandler,
1999). Most sigrificantly, the software is capable of extrading very dense digital elevation models automaticdly and at
very high rates. This means that DEMs can be derived at spatial and temporal frequencies that are far more appropriate
to understanding the fluvial processes eff ecting change.
Water worked surfaces are ided for automated photogrammetric measurement. The lack of vegetation and natural
texture ensures that the surfacemeæured coincides with the desired surface urlike automatic terrain measurement for
conventional urban mapping where vegetation and buildings crede undesirable atifacts. The DEM acquisition
procedure itself is also enhanced by obtaining imagery using a digital camera, which has a higher dynamic range and
improved contrast when compared to an analogue image and scanning solution, (Graham, 1998. Image contrast is also
maximized through the short camera to oljed distances used, which minimizes image degradation due to atmospheric
haze Finally, the digital camera provides the obvious advantage of instant appraisal of exposures and cf course removes
thetime consuming and expensive film processing and scanning phases.
These aombined advantages, reinforced by work in related fields (e.g. Hel ming et al., 1992 Gruen, 1994 Brunsden and
Chandler, 1996 convinced the authors that a stream bed measurement system based upon digital photogrammetry and a
high-resolution digital camera was an appropriate technology to invest in and develop. Reseach grant funding enabled
the purchase of a Kodak DCS460 and DCS420 and purchase and maintenance of the Erdas I magi ne/OrthoM AX and
OrthoBASE software packages. This combination has subsequently bee used upon a diverse range of fluvial projeds,
bath in terms of scde, location and desired cutcomes.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000. 251