Using aerial photographs, Carl Troll (1939) has mapped the patterns and arrangements of landscape units for the first time, which means that he conducted ecological investigations on a landscape level. Subsequently he originally coined the term “landscape ecology” (in German “Landschaftsökologie” (Troll, 1950, 1963)). From the birth of landscape ecology we can see, remote sensing has been an important tool for recognizing the landscape pattern. The development of the remote sensing technology, particularly with the wide spread of high resolution remote sensing imagery, makes the Earth observation more extensive and more accurate. It is the basis of image understanding to extract and recognize geographic object information from remotely sensed images. In high spatial resolution images, the information about land surface is extremely rich and textural feature is prominent (Neubert, 2006). In addition, the availability of high resolution digital elevation data gives new possibilities to understand the landscape structure in a three dimensional perspective (Hoechstetter, 2009). As the data basis for landscape monitoring, only regularly collected data is considered in this thesis. Therefore the focus is laid on the extraction of landscape elements from remote sensing data, land-use maps and digital surface models from official land survey.
3.1.1 RapidEye images
Since 2009, new remote sensing data from the RapidEye satellite has become available. RapidEye is a German, five satellite constellation; each satellite has five spectral bands (blue, green, red, red edge and near infrared) with a 6.5 meter nominal ground resolution (resampled to 5 m pixel size). It offers different levels (e.g. 1B and 3A) of standard image products in a format that can be easily integrated into any Geographic Information System (GIS). For the RapidEye basic product (1B) delivered with radiometric and sensor correction but without any further corrections for any terrain distortions. The RapidEye ortho product (3A) is subject to radiometric, sensor and geometric corrections and the images are rectified using a SRTM (Shuttle Radar Topography Mission) DEM (level 1) or better.
A comparison between RapidEye data and other available satellite imagery of medium- resolution (1 m to 30 m) is shown in Table 3.1. Comparing to other satellites, RapidEye can quickly and reliably deliver multi-temporal data sets in relatively high resolution. It has a large collection capacity (the system records a 77 km wide swath and produces data of more than five million square kilometers of the Earth every day) and quick revisit time to any place on Earth9. This makes RapidEye unique in providing multi-temporal datasets over large areas. It is the first commercial satellite system offering a Red-Edge band (690nm-730nm), which measures variance in vegetation, allowing, e.g. species separation and monitoring vegetation health. Due to the short revisit time span of RapidEye data, it offers great potential to support a multi-temporal classification progress, which is especially useful for vegetation monitoring. Considering the task at hand, RapidEye ortho product (3A) is selected as a main image source.
Table 3.1: Satellite image products comparison (information is collected from following sources: BlackBridge10, Satellite Imaging11, esa Earth Online12)
Satellite Sensors Resolution* Spectral Bands Revisit Time Swath Width
RapidEye MS: 6.5 m (resampled to 5 m pixel size) Blue: 440-510 nm Green: 520-590 nm Red: 630-685 nm Red Edge: 690-730 nm NIR: 760-850 nm Daily (off- nadir) / 5.5 days (at nadir)
77 km SPOT-5 Pan: 5 m (2.5 m by interpolation) Pan: 480-710 nm Green: 500-590 nm 2-3 days (varies with 60 km 9
http://www.blackbridge.com/rapideye/products/images.htm (Accessed October 13, 2014)
10
http://www.blackbridge.com/rapideye/index.html (Accessed October 13, 2014)
11
http://www.satimagingcorp.com/ (Accessed October 13, 2014)
12
MS: 10 m SWIR: 20 m Red: 610-680 nm NIR: 780-890 nm SWIR: 1,580-1,750 nm latitude) SPOT-6 Pan: 1.5 m MS: 6.0 m Pan: 450-745 nm Blue: 450-520 nm Green: 530-590 nm Red: 623-695 nm NIR: 760-890 nm 1 to 5 days (varies with latitude) 60 km FORMOSAT-2 Pan: 2 m MS: 8 m Pan: 450-900 nm Blue: 450-520 nm Green: 520-600 nm Red: 630-690 nm NIR: 760-900 nm Daily 24 km ALOS Pan: 2.5 m MS: 10 m Pan: 520-770 nm Blue: 450-500 nm Green: 520-600 nm Red: 610-690 nm NIR: 760-890 nm 46 days 35km (triplet stereo observations), 70km ( at nadir) Landsat 8 Pan: 15 m MS:30 m TIR: 100 m Coastal: 435-451 nm Blue: 452-512 nm Green: 533-590 nm Red: 636-673 nm NIR: 851-879 nm SWIR-1: 1,566-1,651 nm SWIR-2: 2,107-2,294 nm Pan:503-676 nm Cirrus: 1,363-1,384 nm TIR-1: 10,600-11,190 nm TIR-2: 11,500-15,510 nm 16 days 185 km
*Pan: Panchromatic band; MS: Multispectral bands; NIR: Near Infrared; SWIR: Short Wave Infrared; TIR: Thermal Infrared.
3.1.2 High resolution elevation data
The sensitivity and accuracy of conventional sensors have significant limitations for ecological applications, and they produce only two-dimensional images, which cannot fully represent the tree-dimensional structure of, e.g. forest canopy (Lefsky et al., 2002). Alternatively, lidar (light detection and ranging) is a promising approach to both increase the accuracy of biophysical measurements and extend spatial analysis into the third dimension.
The lidar device measures the distance through laser pulses that strike and reflect from features on the surface of the earth. The distance is determined by the elapsed time between the emission of a laser pulse and the arrival of the reflection of that pulse at the sensor’s receiver. The return signals are generally collected in two ways: either as discrete signals, which measure reflected energy quantified at amplitude intervals and recorded at precisely referenced points in time and space, or as continuous wave (Lefsky et al., 2002). The “discrete return” lidar device converts scanning angle and distance from sensor information into georeferenced data points. The high point density of laser mapping systems enables achieving a detailed description of geographic objects and of the terrain.
The uses of lidar data in ecological application are numerous, especially for forested inventory. Stephens et al. (2012), for example, used airborne lidar data to estimate forest carbon stocks and proved that lidar improved the precision of stock estimates compared to ground plots alone. Tonolli et al. (2011) presented a method combining airborne lidar and multispectral data for the estimation of timber volume and found that lidar variables provided the majority of the explanative contribution for the estimation. Other applications of using lidar data for aboveground forest biomass or leaf area estimation have been studied frequently (Clark et al., 2011; Korhonen et al., 2011; Naesset et al., 2013; Zhao et al., 2009). In addition, the lidar data has also been used to characterize the forest structure (Korhonen et al., 2011; Latifi et al., 2012). The capacity to do individual tree level analysis depends on the spacing of the lidar data and the size of the trees (Richardson and Moskal, 2011; Zimble et al., 2003), because the points sampling density will have a strong influence on crown area estimation (Roberts et al., 2005). However, for a plot level analysis the tree heights can be accurately estimated and lidar-derived tree heights could be useful in the detection of differences in the continuous, nonthematic nature of vertical forest structure (Roberts et al., 2005; Zimble et al., 2003).
In this research, the lidar data was acquired by TopoSys GmbH13 (Biberach, Germany) in the spring 2005 using the Falcon II sensor system. The sensor has an effective scan rate of 83 kHz, and laser wavelength of 1,560 nm. The system measurement accuracy is 15 cm in the vertical direction and 50 cm in the horizontal direction. Data recording of the first and last echo can produce a Digital Surface Model (DSM) that includes man-made features and vegetation superimposed on the terrain with 1 meter resolution. The digital camera associated with the lidar device provides digital RGB and CIR (Color Infrared) ortho images of 50 cm resolution. See examples in Figure 3.1.
13
Figure 3.1: A sample of data acquired from lidar device in this study (Lilienstein in Saxon Switzerland, see detailed description in chapter 4.1). (a) Digital CIR ortho image. (b) Hillshade visualization of digital surface model. (c) The overlaying of both digital RGB ortho image and the terrain model.
The DSM can be processed to eliminate the man-made features and vegetation to derive the associated DEM (Csaplovics and Wagenknecht, 2000). While the DEM shows the height of the ground surface, the DSM also contains vegetation and human artefacts like buildings. To obtain object height information, the Normalized Digital Surface Model (NDSM) was calculated simply by subtracting DEM from DSM. Based on the high resolution NDSM data, detailed landscape elements (e.g. single trees, hedges or copses) can be observed (see Figure 3.2). Moreover, NDSM can also be used for the determination of transition zones created by the elevation gradient on forest boundary (Hou and Walz, 2013a). For characterizing landscape structure, the high resolution elevation model enables the analysis in a realistic way, for example the calculation of true surface area of patches or distances between patches (Hoechstetter et al., 2006; Hoechstetter et al., 2008). Therefore, incorporation of the high resolution elevation data can give an insight into the landscape structure on the vertical dimension and result in realistic descriptions of the land surface.
Figure 3.2: Visualization of different small biotopes and transition zones from a high resolution normalized digital surface model (NDSM).
3.1.3 Other data
Besides the raster data, some existing vector data can be integrated to produce the land- cover map for landscape structure analysis. The official land-use data, such as ATKIS in Germany, normally don’t have the detailed land-cover information of small biotopes, but it
can be used as ancillary data for the classification of the main land-cover classes or used as reference data to confirm the classification result. Other data source, like biotope maps or topographic maps, may also be adopted as reference data. It is dependent on the availability of the data for the investigated area.