Buch, Englisch, 226 Seiten, Format (B × H): 250 mm x 176 mm, Gewicht: 500 g
ISBN: 978-3-937231-71-6
Verlag: Rhombos-Verlag
Following the hierarchical nature of forest resource management, the present work focuses on the natural forest cover at various abstraction levels of details, i.e. categorical land use/land cover (LU/LC) level and a continuous empirical estimation of local operational level. As no single sensor presently covers absolutely all the requirements of the entire levels of forest resource assessment, multisource imagery (i.e. RapidEye, TERRA ASTER and LANDSAT TM), in addition to other data and knowledge have been examined. To deal with this structure, an object-based image analysis (OBIA) approach has been assessed in the destabilized Blue Nile region of Sudan as a potential solution to gather the required information for future forest planning and decision making. Moreover, the spatial heterogeneity as well as the rapid changes observed in the region motivates the inspection for more efficient, flexible and accurate methods to up-date the desired information.
An OBIA approach has been proposed as an alternative analysis framework that can mitigate the deficiency associated with the pixel-based approach. In this sense, the study examines the most popular pixel-based maximum likelihood classifier, as an example of the behavior of spectral classifier toward respective data and regional specifics. In contrast, the OBIA approach analyzes remotely sensed data by incorporat-ing expert analyst knowledge and complimentary ancillary data in a way that somehow simulates human intelligence for image interpretation based on the real-world representation of the features. As the segment is the basic processing unit, various combinations of segmentation criteria were tested to separate similar spectral values into groups of relatively homogeneous pixels. At the categorical subtraction level, rules were developed and optimum features were extracted for each particular class. Two methods were allocated (i.e. Rule Based (RB) and Nearest Neighbour (NN) Classifier) to assign segmented objects to their corresponding classes.
Moreover, the study attempts to answer the questions whether OBIA is inherently more precise at fine spatial resolution than at coarser resolution, and how both pixel-based and OBIA approaches can be compared regarding relative accuracy in function of spatial resolution. As anticipated, this work emphasizes that the OBIA approach is can be proposed as an advanced solution particulary for high resolution imagery, since the accuracies were improved at the different scales applied compare with those of pixel-based approach. Meanwhile, the results achieved by the two approaches are consistently high at a finer RapidEye spatial resolution, and much significantly enhanced with OBIA.
Since the change in LU/LC is rapid and the region is heterogeneous as well as the data vary regarding the date of acquisition and data source, this motivated the implementation of post-classification change detection rather than radiometric transformation methods. Based on thematic LU/LC maps, series of optimized algorithms have been developed to depict the dynamics in LU/LC entities. Therefore, detailed change “from-to” information classes as well as changes statistics were produced. Furthermore, the produced change maps were assessed, which reveals that the accuracy of the change maps is consistently high.
Aggregated to the community-level, social survey of household data provides a comprehensive perspective addi-tionally to EO data. The predetermined hot spots of degraded and successfully recovered areas were investigated. Thus, the study utilized a well-designed questionnaire to address the factors affecting land-cover dynamics and the possible solutions based on local community's perception.
At the operational structural forest stand level, the rationale for incorporating these analyses are to offer a semi-automatic OBIA metrics estimates from which forest attrib-ute is acquired through automated segmentation algorithms at the level of delineated tree crowns or clusters of crowns. Correlation and regression analyses were applied to identify the relations between a wide range of spectral and textural metrics and the field derived forest attributes. The acquired results from the OBIA framework reveal strong relationships and precise estimates. Furthermore, the best fitted models were cross-validated with an independent set of field samples, which revealed a high degree of precision. An important question is how the spatial resolution and spectral range used af-fect the quality of the developed model this was also discussed based on the different sensors examined.
To conclude, the study reveals that the OBIA has proven capability as an efficient and accurate approach for gaining knowledge about the land features, whether at the operational forest structural attributes or categorical LU/LC level. Moreover, the methodological framework exhibits a potential solution to attain precise facts and figures about the change dynamics and its driving forces.
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Table of contents
List of tables XIII
List of figures XV
Acronyms and abbreviations XVII
ABSTRACT 1
Kurzfassung 3
1 Introduction 5
1.1 Background and motivation 5
1.1.1 Overview 5
1.1.2 Deforestation and land degradation 6
1.1.3 Remote sensing and forest resource assessment 7
1.2 Research hypotheses 9
1.3 Objectives 10
2 Study area 13
2.1 Overview 13
2.2 Introduction to the study area 13
2.2.1 General description 13
2.2.2 Vegetation 14
2.2.3 Climate 15
2.3 Test sites 17
2.3.1 LU/LC level 17
2.3.2 Forest stand level. 18
3 Theoretical background 19
3.1 Overview 19
3.2 Introduction 19
3.3 Object based image analysis 20
3.3.1 Image segmentation 21
3.3.2 Image objects hierarchy 23
3.3.3 Image object information extraction and feature measures 24
3.3.4 Nearest neighbour classification 28
3.3.5 Rule based classification 29
3.3.6 An application of object-based approach for mapping earth surface features 30
3.3.7 Quality assessment 37
4 Data and research approaches 39
4.1 Overview 39
4.2 Data 39
4.2.1 Earth observation data 39
4.2.2 Field survey data 42
4.3 Methodology 45
4.3.1 Atmospheric correction 45
4.3.2 Geometric correction 46
4.3.3 Image classification 47
4.3.4 Change detection 56
4.3.5 Classification accuracy assessment 57
4.3.6 Forest parameters estimation and model validation 57
5 Land use/ land cover analyses 63
5.1 General overview 63
5.2 Pixel-based classification of Aster imagery by maximum likelihood (ML) classifier 64
5.3 OBIA approaches 66
5.3.1 Segmentation result 66
5.3.2 Hierarchical classification 67
5.3.3 Object-based classification of Aster imagery by Nearest Neighbor
(NN) classifier 69
5.3.4 Object-based classification of Aster imagery by rule-based (RB) method 72
5.4 OBIA vs. per pixel classification 74
5.4.1 Classification of agricultural land 76
5.4.2 Classification of residential areas 78
5.4.3 Discrimination of vegetation classes 80
5.5 Scale issue and hierarchical OBIA for mapping LU/LC 82
5.5.1 Overview 82
5.5.2 Pixel-based classification of higher resolution RapidEye scene 83
5.5.3 Object-based classification of higher resolution RapidEye scene 85
5.5.4 Overall assessment of the approaches based on the two selected scales 87
5.6 Summary 88
6 Spatiotemporal analyses and driving forces 91
6.1 General overview 91
6.2 Land use/ land cover classification and accuracy assessment 92
6.2.1 Land use/ land cover classification 92
6.2.2 Accuracy assessment based on error matrix 94
6.2.3 Accuracy assessment based on best classification result 95
6.3 Distribution of LU/LC during the periods 1990, 1999 and 2009 97
6.3.1 Trend, rate and magnitude of land use/ land cover changes 98
6.4 Spatiotemporal object-based post-classification analysis 101
6.4.1 Forest cover change dynamics 1990- 1999 103
6.4.2 Forests cover change dynamics 1999- 2009 106
6.4.3 Overall evaluation of the adopted approach in mapping forest cover change dynam-ics 111
6.5 Major driving forces of forest cover loss and restoration problems 115
6.5.1 Deterioration of forest cover in the area 115
6.5.2 Driving forces of forest cover loss 115
6.5.3 Restoration of destroyed forests 117
6.6 Summary 119
7 Object-based texture measures and forest inventory 121
7.1 General overview 121
7.2 Segmentation 122
7.3 Descriptive analysis of field data 123
7.4 Image metrics and forest inventory 125
7.4.1 Volume 125
7.4.2 Height 125
7.4.3 Density 126
7.4.4 Basal area 127
7.5 Development of the regression model to estimate Acacia nilotica stand volume 128
7.5.1 Model validation 131
7.6 Mapping forest stand volume in Alambwa Acacia nilotica pure stand 133
7.7 Summary 134
8 Conclusions, recommendations and future work 137
8.1 Conclusions 137
8.1.1 Overview 137
8.1.2 Land Use/ Land cover categorical level 137
8.1.3 Integrated framework of change analysis 138
8.1.4 Continuous forest structural attributes level 139
8.2 Recommendations 140
8.3 Limitations 141
8.4 Future work 141
References 143
Appendixes 163
Editorial
Nothing is more pleasant than the country around Sennaar, in the end of August and beginning of September [ …]; instead of that barren, bare waste, which it appeared on our arrival in May (1772, editor’s note), the corn now sprung up, and covered the ground, made the whole this immense plain appear a level, green land, inters-persed with great lakes of water and ornamented at certain intervals with groups of villages, the conical tops of the houses presenting, at a distance, the appearance of small encampments. Through this immense, extensive plain, winds the Nile, a delightful river there, above a mile broad, full to the very brim, but never overflowing. Every where on these banks are seen numerous herds of the most beautiful cattle of various kinds, the tribute recently extorted from the Arabs, who, freed from all their vexa-tions, return home with the remainder of their flocks in peace, at as great a distance from the town, country, and their oppressors, as they possibly can.
Bruce of Kinnaird J (1791) Travels to discover the sources of the Nile, in the years 1768, 1769, 1770, 1771, 1772, and 1773, in six volumes, vol V, ch IX, p 231f
Sub-Saharan semiarid regions suffer from increasing local impact on vegetation canopies in terms of overgrazing by herds and flocks, overexploitation of soils by rainfed agriculture neglecting fallow cycles and fuelwood consumption as well as illegal timber extraction. Subregional to local variations in extent and severity of impact are due to varying patterns of socioeconomic and socio-political pressure on the local people and are furthermore driven by gradients of migration of people displaced from their homelands due to ethnical and economic reasons. Mapping state and changes of land use and land cover is often focusing on monitoring and analysing the effects of longterm to mediumterm climatic variations towards their impact on specific ecoclimatic zones. This approach is more and more blamed for its neglect of regional and subregional variations of drought phenomena via arguing that the whole mechanism is much more complex in terms of causes and reasons of land cover changes. Local to regional patterns of land use land cover change have to be assessed and analysed much more intensively. Besides parameters of climatic change such as variations in periodicity and decline of amount of rainfall, increasing impact of strong winds and of temperature variations, spatiotemporal information on regional and local land use and land cover change as a mirror of anthropozooic impact has to be collected in a much more holistic way. It is obvious that the latter is responsible for severe land degradation, thus creating a circulus vitiosus which more and more deteriorates the livelihood of indigenous people. Local patterns of vegetation status and change are to be assessed in high spatio-temporal resolution in order to get a better idea about the interrelated effects of human impact on land degradation, deterioration of livelihood and subsequent abandonment of the land. Remote sensing of the environment by (very) high resolution spaceborne imagery provides the perfect tool and methodological background for collecting, maintaining, analysing and visualising spatiotemporal data which are capable to describe land use and land cover change in different scales both quantitatively as well as qualitatively.
Semi-arid regions of sub-Saharan Africa differ significantly in terms of their environment, their equipment with natural resources and their socio-political condition. Availability of water throughout the year is an advantage which attracts both displaced as well as migrating people and their herds and flocks. Conflicts with regard to rights of water and land use between local and migrating people are inevitable. Thus besides eco-climatic conditions the far more significant driving force of land degradation is resulting from socio-economic and socio-political instabilities, which change both in time (from seasonal to multi-annual) as well as in space (from the water hole level to the local land management level).
It is self-evident that especially the riverine environments along the large rivers crossing the sub-Saharan drylands are and have been the focal points of settlement and land exploitation, the Nile and Niger rivers being both the most significant ancient as well as recent representatives. The kingdom and town of Sennar came into power after the invasion of the Funj people from nowadays southern Sudan (Sudd) at the beginning of the 16th century. It found its peak during the 17th century when territorial expansion reached its apogee by extending the territory as far as to Southern Kordofan in the west and Dongola in the north. The Funj Sultanate of Sennar (Sinnar) lasted till 1821 when Turkish influence which grew since the late 18th century culminated in a peaceful invasion. After the resignation of the last king the territory was assimilated into the Ottoman Egypt. The old capital of Sennar extended approximately 20km north-northwest from the new town. A milestone in rural development driven by colonial policies was the establishment of the Sennar dam in 1925 by the English colonial administration, aiming at fostering irrigation agriculture (dominantly cotton, then more and more replaced by wheat) in the Gezira scheme (founded 1911) bordering Sennar State in the northwest. Till a few years ago the town of Sinja was the capital of Sennar State, but Sennar itself was always and is still the largest town in terms of the number of inhabitants. Population has multiplied by five from 1973 to 2007, thus proving significant migration. Reasons are closely related to socio-political threats such as the Ethiopian-Eritrean and the South Sudan conflicts, associated with socio-economic pressure due to overexploitation of resources caused by accumulation of different, often controversial demands on land use. Severe impacts are driven by an ongoing trend towards mechanised farming based on rainfed agriculture which threatens sensible soil strata and leads to degradation and subsequent decline of crop yields. At the same time riverine forests along the Blue Nile are affected by often illegal fuelwood and timber extraction as well as grazing activities. The monitoring and assessment of forest resources along the Blue Nile is a matter of highest priority under the tasks of the Sudanese Forest National Cooperation (FNC). The multitude and heterogeneity of socio-ecological and socio-economic driving factors of soil and vegetation degradation calls for a sound and reliable methodology of spatio-temporal monitoring of patterns of land use and land cover changes as well as a subsequent critical analysis of parameters of efficiency and integrability of spaceborne remote sensing for management and planning of protective and restorative measures of policies of forest management and forest conservation. Mustafa El-Abbas focuses on that issue by implementing multisource and multi-resolution operational earth observation via (very) high resolution imagery of Terra Aster and RapidEye sensor systems and by analysing the potential of object-based image analysis for multiscale monitoring from the level of categorical land use land cover classes to the level of forest structural attributes such as average stand height and especially forest stand volume. Mustafa El-Abbas provides an extensive and at the same time in-depth research into the innovative assessment and evaluation of up-to-date imagery and image analysis for forest management purposes in the semiarid environment of forests along the Blue Nile. It is obvious that Mustafa El-Abbas is perfectly capable to accumulate, condense and extend as well as to apply a wealth of target-oriented knowledge of the local and regional characteristics of land cover and land use change in the Blue Nile region in general and of forest cover change and forest structure on the stand level with specific regard to pure riverine Acacia nilotica stands in particular.
Balboch is on the eastern bank of the Nile, not a quarter of a mile from the ford below. The river here runs north and south; towards the sides it is shallow, but deep in the middle of the current, and in this part it is much infested with crocodiles, Sennaar is two miles and a half S.S.W. of it. We heard the evening drum very distinctly […]
Bruce of Kinnaird J (1791) Travels …, vol V, ch VII, p 177
The banks of the Nile about Sennaar resemble the pleasantest parts of Holland in the summer season; but soon after, when the rains cease, and the sun exerts its utmost influence, the dora [millet] begins to ripen, the leaves to turn yellow and to rot, the lakes to putrify, smell, and be full of vermin, all this beauty suddenly disappears.
Bruce of Kinnaird J (1791) Travels …, vol V, ch IX, p 232
The overall scientific value and reliability of the research presented by Mustafa Elabbas is evident. It is challenging to realise that based on the well-structured, detailed and in-depth work of Mustafa El-Abbas a clear perspective for a multi-level regionalisation of multi-sensor and multi-resolution earth observation data analysis for purposes of forest management from the categorical land use land cover level to the local forest stand level is made possible. The presented performance of operational satellite imagery of very high (geometric) resolution and of object-based image analysis for monitoring and assessment of forest canopy characteristics in different scales is providing a promising baseline for a significant improvement of forest management issues in drylands of semi-arid sub-Saharan Africa, where accessibility of terrain is limited and dynamics of change both resulting from migration gradients as well as from subsequent anthropozooic impact on natural resources, especially on ligneous layers of vegetation, are highly frequent. The research of Mustafa El-Abbas contributes significantly to a better understanding of causes and reasons of degradation of forest canopies in semiarid environments at a regional to local scale. The complexity of driving factors of forest degradation along the Blue Nile valley has strong spatio-temporal characteristics. It is evident that impacts are driven by complex factors which are connected in a multifold thematic network of interrelations of both economic as well as political and ecological, climatic, geomorphological and biogeographical parameters. Mustafa El-Abbas provides an important and innovative contribution to an improvement of the monitoring of forest cover and forest use at the multi-scale and multi-temporal level which represents a cornerstone for establishing a sound forest management in the Blue Nile region of the Sudan and beyond.
Dresden, April 2017
Elmar Csaplovics
