stream segments
4.2.3. Validating the drainage analysis
Qualitative assessments of drainage accuracy were undertaken by visual comparison of the derived stream network with the 1:250,000 scale mapped streamlines and the streamlines shown on digital 1:100,000 scale topographic mapsheets at random locations across the country. More objective evidence of drainage accuracy was derived by quantitatively comparing catchment areas and the placement of streams and drainage divides against independent sources.
A comparison of derived catchment areas with independent reference values is commonly used to assess the accuracy of surface drainage pathways and catchment boundaries (Bertolo, 2000; Döll and Lehner, 2002; Vogt, Colombo, Paracchini et al., 2003). Reference values may be taken from published basin areas, but comparisons may be confounded by different definitions of the
4.2.3 Applying the new method: Validating the drainage analysis 67
Figure 4.12. The NLWRA streamflow gauging stations used as a reference set to test the accuracy of the DEM-derived catchment areas
catchment outlet or the treatment of arid areas that produce little or no effective runoff
(Vörösmarty et al., 2000). Furthermore, the reported areas for Australian basins (e.g. National Land and Water Resources Audit, 2001b) are often based on the AWRC catchment delineations (AUSLIG, 1997) that, as noted earlier (Section 3.1), are not always topographically defined. A more reliable comparison was possible with the data compiled for the NLWRA Extension of Unimpaired Monthly Streamflow Data project (Peel et al., 2000). This data set, downloaded from the NLWRA Australian Natural Resources Data Library (ANRDL)
(http://data.brs.gov.au/asdd/php/basic_search.php), provides the catchment boundary polygons and site locations of 331 streamflow gauging stations across Australia (Figure 4.12). Catchment boundaries had been provided to the NLWRA for gauging stations in Victoria, Tasmania and South Australia and were manually digitised from the 1:100,000 topographic map series for other states (Peel et al., 2000). The reference area for each catchment polygon was derived after transforming the coverage to Lambert Conformal Conic projection co-ordinates. This projection least compromises the representation of both area and shape. Where a second gauging station was located on the same river the area of the catchment polygon for the upstream station was added to that of the lower station. Catchment area for the 15 stations not supplied with a catchment boundary polygon was entered from information provided in Peel et al. (2000).
4.2.3 Applying the new method: Validating the drainage analysis 68 The reference areas were compared to the 9-second DEM-derived catchment area values for the grid cell overlaying the gauging station locations. Due to differences in the scale of base
mapping and generalisation of the stream representation in the DEM-derived flow paths, some station locations did not fall on the DEM-derived stream network. These were automatically moved to the adjacent grid cell with upstream catchment area closest to the reference values. Systematic checking to ensure the name of the mapped streamline matched the gauging station description exposed major errors in the co-ordinates provided for the gauging station. For example, stations fell on hillslopes, in the wrong catchment or even in the sea. These errors were corrected, guided by the associated catchment boundary polygons and site descriptions, and the DEM-derived catchment area extracted for all gauging stations.
Small differences in total catchment area can hide large disparities in catchment boundary locations. To test for such discrepancies the gauging station catchment polygons were compared with those automatically delineated from the DEM-derived flow paths using the gauging
stations as catchment seeds. Following Vogt, Colombo, Paracchini et al. (2003), the proportion of the DEM-derived catchment boundary length that fell within increasingly larger buffers (250, 500 and 1000m) around the reference catchment boundary was computed after transforming catchment boundary layers to Lambert Conformal Conic projection coordinates. Buffers were delineated with the Arc/Info BUFFER command.
To investigate the suitability of the DEM-derived products for finer scale applications, the 9- second DEM-derived catchment boundary for the Cotter River catchment in the ACT was compared to catchment boundaries determined from two high resolution DEMs, a 20m DEM- derived from 1:25,000 scale contour and directed streamline data and a 5m DEM produced from Airborne Laser Scanning (ALS) data (Hutchinson, Stein et al., 2004) (Figure 4.13). The ALS data have a quoted standard elevation error of about 0.1m (Hutchinson, Stein et al., 2004). As before, the proportion of the length of the 9-second DEM-derived catchment boundary falling within specified buffers of the reference boundary location was computed. Boundary co- ordinates were transformed to the Stromlo coordinate system, a local ACT projection based on the Transverse Mercator projection, consistent with the specifications of the high resolution DEMs.
Similar methods were employed to assess the accuracy of the 9-second DEM-derived stream network. Reference data for comparison included the Australia-wide 1:250,000 scale mapped streamlines and for the Cotter River catchment; digitised streamlines from the NSW 1:25,000 scale topographic map series provided by NSW Land and Property Information from their Digital Topographic Database; digitised streamlines from the ACT 1:10,000 scale topographic map series supplied by the ACT Land Information Centre; and streamlines derived from the 5m ALS DEM using a contributory area threshold of 2.5ha (Hutchinson, Stein et al., 2004)
4.2.3 Applying the new method: Validating the drainage analysis 69 Overlapping buffer polygons for the finer scale stream networks of the Cotter catchment caused the Arc/Info BUFFER command to terminate with errors, so an alternative grid-based
approach was adopted to assess the accuracy of stream placements. Each of the reference stream networks was gridded at 9-second spatial resolution, consistent with the 9-second DEM-derived network, and the proportion of the 9-second DEM-derived streamline cells that were coincident or within the immediate neighbourhood of a reference stream cell was computed. The neighbourhood of a stream cell was the default 3x3 cells used by the Arc/Info Grid FOCAL functions and thus included the adjacent grid cells in all eight directions. Coincident stream cells indicate the distance between test and reference streamlines is less than the longest axis of the grid cell, about 375m, while a neighbourhood cell is equivalent to a buffer of approximately 500 to 750m.