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Effectiveness of using very high resolution imagery (IKONOS) for land use mapping
2. Study area To apply such methodology, the Lebanese coastal area and 6 others sites located in Mount Lebanon have been used (Figure 1).  Figure 1. Location of the selected areas within Lebanon. The selection of the Lebanese coastal region was dependent on two main criteria, i.e. an altitude inferior to 100 m and slopes less than 10%. Its total area sums up to 335.8 km2 hence 3% of the total area of Lebanon. It extends 220 km from north to south and it spans 1.5 km from east to west. This area includes the main cities in the country and concentrates industrial, commercial and financial activities. “Mtain” and “Marjhime” zones cover approximatively the same area 35.5 km2, with the biggest part (56%) on mountainous crests rising up to 1500 m. Qornayel area (8 km2) comprises the same physiographic features as Mtain area, but with the major part on upper slopes (1000-1500 m) (90%). Dmit area, occupying 3.5 km2, is the only pilot area extending from the coastal plain (< 100 m) to the medium slopes (500-1000 m). Ramlieh area (3.56 km2) and Ras El-Matn area (12.99 km2) occupy mainly the medium slopes of Mount Lebanon (1000-1500 m). The land-cover features are diverse in these selected areas, such as agriculture area, plantation and forest. 3. Methodology 3.1. Data collection and description IKONOS images are acquired as 1-meter resolution panchromatic and 4-meter resolution multispectral images. While the panchromatic images represent the visible range of the spectrum, the four bands of multi-spectral images represent the red, green, blue and near infra-red range of the spectrum. Pan-sharpened color imageries have been generated in this study by merging 1-meter resolution panchromatic image with the 4-meter resolution multispectral bands to generate true-color or a false-color images at 1-meter resolution. These 1-meter resolution color images provide exceptional depth in color and clarity of detailed for feature extraction. In addition, the color pan-sharpened images, compared with the panchromatic ones, required lesser supervision, provided more operator ease, reduced instances of misinterpretation and provided for higher speeds in interactive interpretation-cum-delineation. Twenty two IKONOS scenes (each 121 km2) acquired in October 2000 were purchased for the study area. These images were initially registered and ortho-rectified with geographic lat-long coordinates using ground control points (GCPs) from a digital terrain model DTM (10 m), generated from toposheets at 1:20,000 scale. 3.2. Image processing A visual interpretation has been performed on pan-sharpen IKONOS imageries. This interpretation consists of marking the boundaries of areas representing single cover units on the images using on-screen digitizing to assign nomenclature headings, as well as extrapolating the established delineation and identification of different parts showing similar characteristics. The Land Cover Classification System (LCCS) software developed by FAO (2005) has been applied on these images for the formulation of the legend to comply with the CORINE (Coordination des informations sur l’environnement) multi-level classification system that enables the end-user to dynamically select the depicted types and scale. This system generates two main phases: an initial dichotomous phase, in which eight major land cover/use types are defined (artificial areas, agricultural areas, wooded lands, grasslands, wetlands, unproductive areas, water bodies and roads), followed by a subsequent modular-hierarchical phase, in which land cover/use classes are created by the combination of sets of predefined classifiers tailored to each major land cover/use type in order to use the most appropriate classifiers and to reduce the likelihood of impractical combinations of classifiers. Therefore, the structured legend meets three basic requirements namely mapping all territory leaving no heading for unclassified land, matching the headings with the needs of future users of the geographic database and avoiding vague or ambiguous heading terminology. 3.3. Ancillary data Existing land cover/use maps at 1:20,000 scale produced through visual interpretation of Landsat TM (30 m) and PAN IRS-1C (5.8 m) images acquired in October 2000 based on CORINE Land Cover methodology (level 4) (LNCRS-LMOA, 2002) have been compared to the results obtained from the interpretation of IKONOS imageries. The working scale (display of images, identification of features and digitizing) was set at the 1:5,000 in order to allow a finer presentation of the existing maps at 1:20,000. The minimum mappable unit (MMU) was then set at 500 m2 in relation with the 1:5,000 scale. Though this MMU facilitates the legibility of the printed map, allows an easy digitization from the interpretation manuscripts and is ideal in homogeneous areas like high mountains and large agricultural areas where spatial variation occurs at a minor rate, there was a need to refine it even further in urban or transitional areas, where changes in land use occur at a higher frequency. 3.4. Accuracy analysis To determine the accuracy of each land cover/use map, 200 field sites were selected for comparing ground information with interpretation results. These sites were located using a GPS with a precision of 10 m. They have been chosen by a random stratified sampling to cover all obtained land cover/use classes and sub-categories. Two methods of accuracy assessment were used in this study. First, the overall (global) accuracy was estimated from the confusion matrix by calculating the total percentage of sites correctly classified. The average accuracy was then calculated as the average of the accuracies obtained for each class. |