期刊名称：ISPRS Annals of the Photogrammetry, Remote Sensing and Spatial Information Sciences
印刷版ISSN：2194-9042
电子版ISSN：2194-9050
出版年度：2010
卷号：XXXVIII - 4/C7
出版社：Copernicus Publications
摘要：This research presents a time-effective approach for mapping streambed and riparian zone extent from high spatial resolution LiDAR derived products, i.e. digital terrain model, terrain slope and plant projective cover. Geographic object based image analysis (GEOBIA) has proven useful for feature extraction from high spatial resolution image data because of the capacity to reduce effects of reflectance variations of pixels making up individual objects and to include contextual and shape information. This functionality increases the likelihood of generalizing classification rules, which may lead to the development of automated mapping approaches. The LiDAR data were captured in May 2005 with 1.6 m point spacing and included first and last returns and an intensity layer. The returns were classified as ground and non-ground points by the data provider. The data covered parts of the Werribee Catchment in Victoria, Australia, which is characterized by urban, agricultural, and forested land cover types. Field data of streamside vegetation structure and physical form properties were obtained in April 2008. The field data were used both for calibration of the mapping routines and to validate the mapping results. To improve the transferability of the rule set, the GEOBIA approach was developed for an area representing different riparian zone environments, i.e. urbanised areas, agricultural areas, and hilly forested areas. Results show that mapping streambed extent (R 2 = 0.93, RMSE = 3.3 m, n = 35) and riparian zone extent (R 2 = 0.74, RMSE = 3.9, n = 35) from LiDAR derived products can be automated using GEOBIA. This work lays the foundation for automatic feature extraction of biophysical properties of riparian zones to enable derivation of spatial information in an accurate and time-effective manner suited for natural resource management agencies. * Corresponding author 1. INTRODUCTION 1.1 Riparian Zones Riparian zones along rivers and creeks have long been identified as important elements of the landscape due to the flow of species, energy, and nutrients, and their provision of corridors providing an interface between terrestrial and aquatic ecosystems (Apan et al., 2002; Naiman and Decamps, 1997). Threats to riparian zones are compounded by increased anthropogenic development and disturbances in or adjacent to these environments. Riparian zones and related vegetation form corridors with distinct environmental functions and processes. To assess these functions and processes environmental indicators of riparian vegetation structure and physical form of stream banks are normally used (Werren and Arthington, 2002). Two of the most important environmental indicators to assess are the streambed extent and the riparian zone extent. Mapping streambed extent allows determination and assessment of a number of riparian environmental indicators such as streambed width, vegetation overhanging the stream, identification of stream banks for stream bank condition assessment, and water body assessment. Mapping the extent of the riparian zones defines the area within which riparian environmental indicators such as riparian zone width, plant projective cover (PPC), vegetation continuity, and other vegetation structural parameters are to be assessed. Hence, a starting point and requirement for riparian zone assessment is the accurate mapping and identification of streambed and riparian zone extents. 1.2 Remote Sensing of Riparian Zones Several papers have concluded that the use of remotely sensed image data are required for assessment of riparian zones for areas > 200 km of stream length, as field surveys become cost prohibitive at those spatial scales (Johansen et al., 2007). The availability of data from high spatial resolution sensors such as the IKONOS, QuickBird and Geoeye-1 satellite sensors and pdfMachine by Broadgun Software - a great PDF writer! - a great PDF creator! - http://www.pdfmachine.com http://www.broadgun.com var currentpos,timer; function initialize() { timer=setInterval("scrollwindow()",10);} function sc(){clearInterval(timer); }function scrollwindow() { currentpos=document.body.scrollTop; window.scroll(0,++currentpos); if (currentpos != document.body.scrollTop) sc();} document.onmousedown=scdocument.ondblclick=initialize The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol. XXXVIII-4/C7 airborne multi-spectral, hyper-spectral and light detection and ranging (LiDAR) sensors have opened up new opportunities for development of operational mapping and monitoring of small features such as narrow riparian zones (Hurtt et al., 2003). Johansen et al. (2010) found airborne LiDAR data to be suitable for mapping a number of riparian environmental indicators. They also assessed the use of LiDAR data for mapping streambed and riparian zone extents using geographic object based image analysis (GEOBIA) and obtained high mapping accuracies of streambed and riparian zone widths. However, the rule sets applied to automatically map streambed and riparian zone extents were found time-consuming, especially for large area mapping because of the use of near pixel-level segmentations and region growing algorithms. The rule sets were also found to work only in areas with streambeds clearly defined by bordering steep bank slopes. The objective of this work was to develop a new, time-effective and transferable approach for mapping streambed and riparian zone extents from high spatial resolution LiDAR derived products, i.e. digital terrain model (DTM), terrain slope and PPC. 2. DATA AND METHODS 2.1 Study Area The riparian study area was located along the Werribee and Lerderderg Rivers and Pyrites, Djerriwarrh, and Parwan Creeks in the urbanized and cultivated temperate Werribee Catchment in Victoria, 50 km northwest of Melbourne (Figure 1). The Werribee River is the major drainage stream emanating from the Werribee Catchment, and the rivers and creeks nominated for the study area confluence with it. In the northern part of the study area remnant forests of the Central Victorian Upland bioregion exist. The northern terrain is characterized by small streams cutting courses and gorges in heavily eroded hills. The water flow of the streams is generally south from the hilly areas until reaching the confluence with the Werribee River, where flows turns east and then southeast before eventually draining into Port Phillip Bay. The southern half of the study area is part of the flat Victorian Volcanic Plain bioregion characterized by disturbed terrain with agricultural (grazing and cultivation) and urban land use features. Figure 1. Area covered by the LiDAR data in the Werribee Catchment and zoomed in section showing more details of the Lerderderg River (north), Werribee River (middle), and Parwin Creek (south). Thirty-five field plots were assessed. UltracamD image data are used to illustrate the LiDAR data coverage. 2.2 Field Data Acquisition A field campaign was carried out in the Werribee Catchment between 31 March and 4 April 2008. The field data acquisition was designed to match the spatial resolution of the LiDAR data. Field measurements were obtained along transects located perpendicular to the streams of several biophysical vegetation structural and physical form parameters. However, the only field measurements used in this research included: (1) streambed width; (2) riparian zone width; (3) PPC; and (4) stream bank slope and elevation. Streambed width was measured with a laser range finder. Riparian zone width was measured with a tape measure from the toe of the stream bank to the external perimeter defined by the stream bank flattening and the vegetation species that no longer dependant on the stream for survival. Existing high spatial resolution optical image data were used to locate in-situ ground control points visible in both the field and image data to complement GPS points to precisely overlay field and image data. GPS measurements were obtained by averaging the position of the start and end of each transect until the estimated positional error was below 2.0 m. 2.3 LiDAR Data Acquisition and Preparation The LiDAR data used in this study were captured using the Optech ALTM3025 sensor between 7 and 9 May 2005 for the study area. The LiDAR data were captured with an average point spacing of 1.6 m (0.625 points per m 2 ) and consisted of two returns, first and last returns, as well as intensity. The LiDAR returns were classified as ground or non-ground by the data provider using proprietary software. The flying height when capturing the LiDAR data was approximately 1500 m above ground level. The maximum scan angle was set to 40 with a 25% overlap of different flight lines. The estimated vertical and horizontal accuracies were < 0.20 m and < 0.75 m respectively. GPS base stations were used for support to improve the geometric accuracy of the dataset. The LiDAR data were deemed suitable for integration with the field data despite the time gap between the data acquisitions. This assumption was based on existing riparian field data from 2004 provided by the Victorian Depart of Sustainability and Environment and rainfall data confirming lower than average rainfalls between 2005 and 2008 and hence no likely changes in streambed and riparian zone extents within the study area. The following three LiDAR products were produced for use in the GEOBIA: DTM; terrain slope; and fractional cover count converted to PPC (Figure 2). The DTM was produced at a pixel size of 1 m using an inverse distance weighted interpolation of returns classified as ground hits. From this DTM, the rate of change in horizontal and vertical directions was calculated to produce a terrain slope layer. Fractional cover count defined as one minus the gap fraction probability, was calculated from the proportion of counts from first returns 2 m above ground level within 5 m x 5 m pixels. The height threshold of 2 m above ground was also used in the field for measuring PPC. A detailed explanation of calculating PPC from fractional cover counts can be found in Armston et al. (2009). These LiDAR derived raster products were used for GEOBIA to map the streambed and riparian zone extents. A shapefile representing the location of the stream centres within the study area was provided by the Victorian Department of Sustainability and Environment and used in the GEOBIA.
