1 Spatial Modelling of the Terrestrial Environment - Georeferencial

Remotely Sensed Topographic Data for River Channel Research 115
Figure 6.1 Mosaiced area image of the study area in the River Waimakariri, Christchurch,
New Zealand
the increase in coverage that comes from a reduction in image scale and the improvement in
point precision that results from an increase in image scale (Lane, 2000). This is particularly
relevant with respect to large braided riverbeds, because of the low vertical relief (in the
order of metres) compared to the spatial extent (in the order of kilometres). This imposes
a minimum acceptable point precision for effective surface representation. In this study, a
photographic scale of 1:5000 was chosen for the 1999 imagery, which, given a scanning
resolution of 14 µm, results in an object space pixel size of 0.07 m (Jensen, 1996). Based
on this, the lowest DEM resolution that can be produced is 0.37 m, approximately five
times greater than the object space pixel size (Lane, 2000). Analysis of the 1999 imagery
had been completed in time for the re-design of the airborne survey to acquire imagery at a
larger scale (1:4000) in order to increase texture in the image and thereby improve stereomatching
performance. To achieve full coverage of the active bed, two flying lines were
needed. Photo-control points (PCPs) were provided by 45 specially designed targets that
were laid out on the riverbed prior to photography and positioned such that at least five (and
typically 10–12) targets were visible in each photograph. Their position was determined to
within a few centimetres using Trimble real-time kinematic (RTK) GPS survey.
DEMs of the study reach were produced using the OrthoMAX module of ERDAS
Imagine installed on a Silicon Graphics UNIX workstation. DEMs were generated using
the OrthoMAX default collection parameters with a 1 m horizontal spacing to give
around four million (x, y, z) points on the riverbed. An individual DEM was generated for
each photograph overlap: two flying lines of eight photographs each meant that in total 14
individual DEMs were produced for the 1999 surveys. The larger scale of the 2000 surveys
increased the number of DEMs to 20. In addition, image analysis was conducted in order
to obtain water depths for inundated areas of the bed. These methods are dealt with in full
in Westaway et al. (2003). This chapter only addresses exposed areas of the riverbed.
To aid in the assessment of DEM quality, and hence to identify means of improving data
collection, the generated DEMs were assessed using check point elevations measured by
NIWA and Environment Canterbury field teams using a combination of Total Station and
Trimble RTK GPS survey. An automated spatial correspondence algorithm was used to