Floods are one of the major hazards worldwide. They are the source of huge risks in rural and urban areas, resulting in severe impacts on the civil society, industry and the economy. The Elbe River has suffered from many severe floods during recent decades. In this study, the zones flooded during 2011 were analyzed using TerraSAR-X images and a digital elevation model for the area in order to identify possible ways to mitigate flood hazards in the future, regarding sustainable land-use. Two study areas are investigated, around the Walmsburg oxbow and the Wehningen oxbow. These are located between Elbe-Kilometer (505-520) and (533-543), respectively, within the Lower Saxonian Elbe River Biosphere Reserve. Those areas are characterized by several types of land use, with agricultural land use being predominant. The study investigated the possibility of using a Decision-Tree object-based classifier for determining the major land uses and the extent of the inundation areas. The inundation areas identify for 2011 submerged some agricultural fields that must be added to existing flood risk maps, and future cultivation activities there prevented to avoid the possible economic losses. Furthermore, part of the residential area is located within the high flood zone, and must be included in risk maps to avoid the possible human and economic losses, to achieve sustainable land use for the areas studied.
The Elbe River is the fourth largest river basin in Europe, after the Danube, the Vistula, and the Rhine [
The Elbe floodplain was designated as a Biosphere Reserve (Flusslandschaft Elbe) by UNESCO in 1997. It consists of four reserves, namely the Biosphärenreservat Mittelelbe in Sachsen-Anhalt, the Biosphärenreservat Flusslandschaft Elbe-Brandenburg, the Biosphärenreservat Flusslandschaft Elbe―Mecklenburg-Vorpommern, and the Biosphärenreservat Niedersächsische Elbtalaue [
In Germany, 21 people were killed, 100 people were injured, and 200,000 people were evacuated from the 300 km2 inundation extent area along 800 km of the river, resulting in economic losses of 11.6 billion, caused by the large-scale century flooding in 2002 [
and the Elbe. However, in spite of the extent of the flooding, disastrous damage did not occur. The total damage was estimated to be more than 100 million Euro in Germany, an amount that was much smaller than the 11.6 milliard Euro in 2002, for example [
Comparing of the areas affected in 2013 with existing flood hazard and risk maps, one can see that these flood maps are usually reliable [
Flood risk is the combination of the probability of a flood event and of the possible deleterious consequences to human health, the environment and economic activity associated with the flood event. Flood risk management includes the comprehensive and continuous assessment and evaluation of flood hazard and flood risks, to mitigate the floods and/or the impact of floods. The flood risk management programs integrate five measures namely, prevention, protection, awareness, emergency response, and recovery. Prevention measures seek to prevent damage caused by floods by avoiding construction of houses and industries in present and future flood-prone areas, by adapting future developments to the risk of flooding, by promoting proper land-use, and by adapting to changing risk factors such as climate change. Protection measures seek to take both structural and non-structural measures to mitigate the impact of floods in a specific location by construction of flood dikes, and early warning systems. Awareness measures seek to inform the population about flood risks, settlement expansion in safe places using suitable forms of construction, and what to do in the event of a flood. Emergency response measures seek to develop emergency response plans in the case of a flood. Recovery measures seek to return to normal conditions as soon as possible and mitigate both the social and economic impacts on the affected population [
Flood maps include flood hazard maps, which identify the extent of flooded areas at different flood probabilities. They also include flood risk maps, which indicate the possible harmful consequences associated with floods of different probabilities, especially when correlated with land use maps. Furthermore, detection maps can show the inundation extents of former floods [
Generally, flood mapping is obtained using image classification, where each individual pixel is classified based on all existing information using the grey values and spatial information of an image [
Several studies have investigated the effect of the surface roughness on the radar backscattering. Surface roughness is the main factor affecting radar backscattering and determines the angular distribution of surface scattering, as shown in
Several studies have also investigated the effect of the wavelength of SAR sensors on mapping water and the vegetation canopy. The longer the system’s wavelength, the greater the ability of the signal to penetrate the vegetation canopy, as shown in
these wavelengths produce more surface scattering from the top layer of the forest canopy. Therefore, neither C-band nor X-band SAR sensors are effective for mapping flooding in forest environments [
The effectiveness of different SAR and optical images in water delineation and flood detection applications has been evaluated in the literature using several classification methods. Numerous studies used optical images in identifying the extent of water areas. Reference [
Numerous studies have investigated the effectiveness of SAR imagery in identifying flooded areas. Several investigations applied Thresholding―methods to SAR imagery in order to locate flooded areas. Reference [
Reference [
Although many procedures have been studied in the literature, the present research focuses on Decision-Tree methodology, which has not been comprehensively evaluated. The present research employs remote sensing tools in identifying the detection maps and land use/land cover LULC maps using TerraSAR-X (TSX) imagery for two study areas within the Elbe Biosphere reserve in Lower Saxony “Niedersächsische Elbtalaue”. Both the LULC and flood-detection maps are generated using a Decision-Tree object-based classifier. These maps can be used in updating the flood hazards and risk maps of this area, and to enable sustainable use of the land resources in the study area.
Two pilot areas are selected, around the Wehningen Oxbow, lying between Elbe-Kilometers (505-520), and the Walmsburg Oxbow, between Elbe-Kilometers (533-543), within the Lower Saxonian Elbe River Biosphere Reserve (
The water levels measured by Wasser- und Schifffahrtsverwaltung des Bundes (WSV), and provided by Bundesanstalt für Gewässerkunde (BfG), at the Neu Darchau gauge (Walmsburg Oxbow) and at the Damnatz gauge (Wehningen Oxbow) were collected for the periods from January 2010 until December 2012 and during June 2013. The readings of both gauges were converted to real water levels above mean sea level. The TerraSAR-X images (TSX) used were acquired during the period from March 2010 to January 2012 provided by the German Aerospace Centre (DLR). These images are acquired by the German Earth observation satellite. Its orbit passes over the same location every 11 days. It uses an X-band SAR, with 31 mm wavelength and 9.6 GHz, frequency
providing high-quality topographic information [
At Walmsburg Oxbow, the highest water level of 13.58 m above sea level (asl) was measured on 11 June 2013. During the high flood in January 2011, the highest water level of 13.16 m asl was measured on 23 January, while, the maximum accessible water level among the TSX images of 12.35 m asl was acquired on 19 January 2011 for that particular flood. The lowest water level of 6.98 m asl was measured on 2 September 2012 while the least accessible water level among the TSX images of 7.24 m asl was acquired on 22 June 2011. All the acquired images were taken in the same descending orbit direction. Therefore, the image acquired on 22 June 2011 was used for determining the pre-flood status, and the image acquired on 19 January 2011 was used for investigating the post-flood status.
At Wehningen Oxbow, the highest water level of 17.02 m asl was measured on 11 June 2013. During the high flood in January 2011, the highest water level of 16.57 m asl was measured on 23 January, while the maximum accessible water level among the TSX images of 16.17 m asl was acquired on 25 January 2011. The lowest water level of 10.59 m asl was measured on 22 July 2010, while the least accessible water level among the TSX
images of 10.88 m asl was acquired on 25 July 2010. Some of the acquired images were taken in descending orbit direction while others were acquired in the ascending orbit direction. The image acquired on 25 July 2010 was taken in the ascending orbit direction whereas the image acquired on 25 January 2011was taken in the descending orbit direction. It is better to process pre- and post-images with same orbit direction. Hence, another image with low water level was chosen to study the pre-flood status. On 28 June 2011, the image was acquired in the descending orbit direction, and the measured water level was 11.05 m asl. Therefore, the image acquired on 28 June 2011 was used for determining the pre-flood status, and the image acquired on 25 January 2011 was used for investigating the post-flood status.
For both oxbows, the pre-flood images were used to define land use, while the post-flood images were used in mapping the flooded areas. The land-use/land cover LULC maps and the detection maps were used in evaluating the hazards resulting from the January 2011 flood and to identify the risk zones, as shown in
For both study areas, the images with both their acquisitions were coregistered so that relative translational shift and rotational and scale differences could be corrected through performing spatial registration and, potentially, resampling. This was done using the SARSCAPE module of the ENVI program after importing the images as TerraSAR-X standard formats. The coregistered images were geocoded to provide a radiometric calibration and a cartographic reference system. The resultant geocoded images for both the HH and VV polarizations were stacked together to provide two images for pre-flood and post-flood status at both Walmsburg Oxbow and Wehningen Oxbow. Image rectification and georeference transformation were applied to the four stacked images using ERDAS Imagine 9.3 software. The images were loaded into ERDAS Imagine for data preparation and reprojection. The SAR images were geometrically transformed to the Universal Transverse Mercator (UTM) projection with spheroid WGS 84 and zone 32 North, and resampled into one meter pixel size using the projective transform model under the Geocorrect image Tool. The four images were filtered with the Lee filter in order to remove or decrease speckle noise. The Lee-filter was applied using the ERDAS Imagine software through the Speckle Suppression option under the Radar Interpreter menu. The coefficient of variation for the subset of the geocoded images was calculated for each image. The Lee filter was selected from the list of available filters and the value for the coefficient of variation was inserted. The window size was set to seven pixels.
The object-based classification method investigated for mapping the land-use/land-cover LULC maps for both study areas, as shown in
filter. It is difficult to obtain up-to-date reference maps for LULC. The available maps were old and did not accurately represent the current land cover. Therefore, reference LULC maps were digitized from the TSX imagery using the ARCGIS program to assess the classification accuracy, as shown in
As discussed in the introduction, the roughness of a surface affects the backscatter from it. The greater the roughness, the more scattering back to the radar there is, and the lighter the surface appears in radar imagery, leading to variation in the image texture [
Envi EX has a tool that utilizes object-based processing named Feature Extraction. Feature Extraction is a tool for extracting information from high-resolution panchromatic or multispectral imagery based on spatial, spectral, and texture characteristics. It uses an object-based approach to classify imagery. It is a combined process of segmenting an image into regions of pixels, computing attributes for each region to create objects, and classifying the objects (with rule-based or supervised classification) based on those attributes, to extract features. The workflow consists of two main steps: Find Objects and Extract Features. The Find Objects task is subdivided into four steps: Segment, Merge, Refine, and Compute Attributes. During the segmentation process, pixels with similar feature values (brightness, texture, color, shape) are grouped into small objects. During the region- merging process, small adjacent segments are aggregated into larger, textured areas based on a combination of
spectral and spatial information. During the Compute-Attributes process, spatial, spectral, and texture attributes are computed for each object. After completing this task, the Extract Features task can be performed; this consists of supervised or rule-based classification. In the supervised classification process, the training data (samples of known identity) are used to assign objects of unknown identity to one or more known features. The training data can be defined manually or through importing ground truth data in the form of point and polygon shape files. The supervised classifier uses either the K-Nearest-Neighbor method or the Support Vector Machine method. In the rule-based classification process, features are defined by building one or multiple rules based on object attributes. This requires human knowledge and reasoning about the extracted features. Finally, the classification results can be exported to shape files and/or raster images [
The Decision-Tree method builds classification in the form of a tree structure. The Decision Tree classifier performs multistage classifications by using a series of binary decisions to place pixels or objects into classes. Each decision divides the pixels in a set of images into two classes based on an expression. Each new class can be divided into two more classes based on another expression. As many decision nodes as needed can be utilized. The results of the decisions are classes. The Decision-Tree classification method can be applied to pixel-based classification in the same way as traditional classification algorithms would be applied. It can also be used to generate rules for knowledge-based and object-based classification with different types of attributes [
In this research, a Decision-Tree classification algorithm was applied using the rule-based classifier, as shown in
The Decision-Tree approach requires comprehensive knowledge of data about the features of the terrain and, furthermore, physical understanding of these. Since each class corresponds to a specific scattering property, decision boundaries were determined based on knowledge acquired experimentally by the analysis of the scattering characteristics of each class. The feature extraction workflow computed spectral, textural, and spatial attributes for the merged objects. In order to locate decision boundaries for separation of the various classes, the histograms of the computed attributes were analyzed to identify peaks, valleys, shoulders, and curvatures. The concepts of natural breaks and clustering were used to define the decision boundaries. Natural breaks and clustering are both methods of manual data classification through dividing data into classes based on natural groups in the data distribution. Natural breaks occur in the histogram at the low points of valley, while cluster divisions occur at the midpoints between peaks or at the shoulders of the histogram [
For mapping the flood extent areas for both study locations, the optimum classification methods with respect to the LULC classification results were applied. The flood extent maps initially produced were corrected using the digital elevation model (DEM) for this area and the resulting LULC maps. The DEM was used to remove the regions misclassified as water within the area around the river that had a land level higher than the water level measured at the gauges. The LULC maps were used to remove the forest and residential areas which were misclassified as water due to the limitations of x-band imagery in mapping the water areas beneath forest and urban coverage [
For both study areas, the raw and Lee-filtered images were processed to identify the land use classes using the object-based approach. The land uses were delineated manually to evaluate the accuracy of the resulting classification. The reference digitized LULC maps for both study areas have four main classes, as previously stated. The images at Walmsburg Oxbow has 5% water extent, 66% vegetated lands, 23% forests and 6% residential areas. The images at Wehningen Oxbow has 8% water extent, 71% vegetated lands, 17% forests and 4% residential areas. In order to generate the rule-based classifications, the workflow for feature extraction in ENVI EX was applied six times for each study area, using HH−, VV−, and HH/VV− polarization of the raw and Lee-filtered images. Three different combinations of class pairs were chosen for feature separation to represent the three branches of the Decision-Tree. These combinations are 1) water and land, 2) vegetated lands and uneven lands, and 3) residential areas and forests.
For branch (1), 64 classifications were generated to map the water and land classes. From the spectral-attributes, the band-average was used; the mean and variance were used from the texture-attributes; and solidity, convexity, compact and elongation were used from the spatial-attributes. In addition, some rule sets were used to combine certain attributes together to enhance the classification results. Rule set 11 combined the band-average and mean-texture; rule set 12 included texture-mean and solidity-attributes; rule set 13 added the roundness to texture-mean and solidity-attributes; rule set 4 incorporated elongation with texture-mean-attributes, solidity and roundness-attributes. The producer accuracy and the total accuracy are shown in
Producer Accuracy % | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Polarization | Rules | Filter Type | Wehningen Oxbow | Walmsburg Oxbow | Average | ||||||
Water | Land | Total | Water | Land | Total | Water | Land | Total | |||
HH | Average Band > 45 | Raw | 86 | 99 | 98 | 92 | 99 | 99 | 89 | 99 | 99 |
Lee | 83 | 94 | 93 | 87 | 99 | 99 | 85 | 97 | 96 | ||
Average Band > 55 | Raw | 92 | 97 | 97 | 94 | 98 | 98 | 93 | 97 | 97 | |
Lee | 88 | 92 | 92 | 91 | 99 | 98 | 89 | 95 | 95 | ||
VV | Average Band > 45 | Raw | 87 | 99 | 98 | 91 | 99 | 99 | 89 | 99 | 99 |
Lee | 84 | 94 | 93 | 87 | 99 | 99 | 85 | 97 | 96 | ||
Average Band > 55 | Raw | 92 | 95 | 95 | 94 | 98 | 98 | 93 | 97 | 96 | |
Lee | 89 | 90 | 90 | 90 | 97 | 97 | 90 | 93 | 93 | ||
HH/VV | Average Band | Raw | 92 | 97 | 97 | 94 | 98 | 97 | 93 | 97 | 97 |
Lee | 90 | 89 | 89 | 92 | 97 | 97 | 91 | 93 | 93 | ||
Rule Set 11 | Raw | 92 | 98 | 97 | 94 | 98 | 97 | 93 | 98 | 97 | |
Lee | 88 | 92 | 92 | 92 | 97 | 97 | 90 | 95 | 95 | ||
Texture Mean > 55 | Raw | 92 | 98 | 97 | 94 | 98 | 98 | 93 | 98 | 98 | |
Lee | 88 | 93 | 92 | 90 | 99 | 98 | 89 | 96 | 95 | ||
Texture Variance | Raw | 87 | 99 | 98 | 84 | 99 | 99 | 86 | 99 | 98 | |
Lee | 77 | 94 | 93 | 68 | 99 | 98 | 72 | 97 | 96 | ||
Texture Mean > 45 | Raw | 84 | 99 | 99 | 94 | 98 | 98 | 89 | 99 | 98 | |
Lee | 83 | 94 | 93 | 90 | 99 | 98 | 86 | 97 | 96 | ||
Solidity | Raw | 65 | 99 | 97 | 74 | 99 | 98 | 69 | 99 | 98 | |
Lee | 38 | 95 | 90 | 68 | 99 | 98 | 53 | 97 | 94 | ||
Convexity | Raw | 64 | 49 | 50 | 99 | 62 | 64 | 82 | 56 | 57 | |
Lee | 68 | 88 | 86 | 68 | 84 | 83 | 68 | 86 | 85 | ||
Compact | Raw | 64 | 99 | 97 | 72 | 99 | 97 | 68 | 99 | 97 | |
Lee | 74 | 87 | 86 | 68 | 99 | 98 | 71 | 93 | 92 | ||
Elongation | Raw | 76 | 26 | 29 | 73 | 99 | 99 | 74 | 63 | 64 | |
Lee | 77 | 95 | 94 | 69 | 99 | 98 | 73 | 98 | 96 | ||
Rule Set 12 | Raw | 85 | 99 | 98 | 92 | 99 | 99 | 88 | 99 | 99 | |
Lee | 83 | 94 | 93 | 87 | 99 | 99 | 85 | 97 | 96 | ||
Rule Set 13 | Raw | 87 | 98 | 97 | 92 | 99 | 99 | 90 | 98 | 98 | |
Lee | 84 | 94 | 93 | 87 | 99 | 99 | 85 | 97 | 96 | ||
Rule Set 14 | Raw | 82 | 94 | 93 | 86 | 99 | 98 | 84 | 97 | 96 | |
Lee | 82 | 94 | 93 | 82 | 99 | 99 | 82 | 97 | 96 |
land class mask was used to isolate the land areas from each image. The masked images were used in generating different classifications for the vegetated land and uneven land classes. 52 classifications were generated to map the vegetated lands and uneven lands. From the spectral-attributes, the band-average was used; the mean, variance and entropy were used from the texture-attributes; and area, solidity, convexity, compact, elongation and roundness were used from the spatial-attributes. In addition, one rule set was used to combine the band-average and mean-texture. The producer accuracy and the total accuracy are summarized in
Producer Accuracy % | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Polarization | Rules | Filter Type | Wehningen Oxbow | Walmsburg Oxbow | Average | ||||||
Vegetated Land | Uneven Land | Total | Vegetated Land | Uneven Land | Total | Vegetated Land | Uneven Land | Total | |||
HH | Average Band | Raw | 80 | 59 | 75 | 87 | 63 | 80 | 83 | 61 | 77 |
Lee | 87 | 67 | 83 | 90 | 48 | 79 | 90 | 58 | 81 | ||
VV | Average Band | Raw | 86 | 40 | 76 | 85 | 52 | 75 | 86 | 46 | 75 |
Lee | 88 | 64 | 82 | 88 | 46 | 75 | 88 | 55 | 79 | ||
HH/VV | Average Band | Raw | 80 | 62 | 76 | 80 | 64 | 75 | 80 | 63 | 75 |
Lee | 87 | 68 | 83 | 85 | 72 | 81 | 86 | 70 | 82 | ||
Rule Set 21 | Raw | 80 | 54 | 74 | 80 | 64 | 75 | 80 | 59 | 75 | |
Lee | 87 | 56 | 79 | 86 | 51 | 75 | 86 | 53 | 77 | ||
Texture Mean | Raw | 80 | 58 | 75 | 82 | 76 | 73 | 81 | 67 | 74 | |
Lee | 88 | 61 | 82 | 83 | 67 | 79 | 86 | 64 | 80 | ||
Texture Variance | Raw | 80 | 74 | 77 | 76 | 71 | 74 | 78 | 72 | 75 | |
Lee | 92 | 76 | 88 | 86 | 74 | 84 | 89 | 75 | 86 | ||
Solidity | Raw | 81 | 56 | 75 | 87 | 50 | 75 | 84 | 53 | 75 | |
Lee | 75 | 58 | 70 | 80 | 44 | 69 | 77 | 51 | 70 | ||
Convexity | Raw | 80 | 43 | 71 | 84 | 48 | 73 | 82 | 45 | 72 | |
Lee | 86 | 63 | 80 | 81 | 56 | 74 | 84 | 60 | 77 | ||
Compact | Raw | 80 | 52 | 74 | 87 | 35 | 71 | 84 | 43 | 72 | |
Lee | 86 | 61 | 80 | 74 | 55 | 68 | 80 | 58 | 74 | ||
Elongation | Raw | 87 | 68 | 82 | 85 | 66 | 80 | 86 | 67 | 81 | |
Lee | 86 | 57 | 79 | 88 | 51 | 77 | 87 | 54 | 78 | ||
Area | Raw | 85 | 47 | 76 | 80 | 35 | 66 | 82 | 41 | 71 | |
Lee | 88 | 68 | 84 | 86 | 65 | 80 | 87 | 66 | 82 | ||
Round | Raw | 79 | 53 | 73 | 77 | 64 | 73 | 78 | 58 | 73 | |
Lee | 92 | 38 | 79 | 86 | 50 | 75 | 89 | 44 | 77 | ||
Texture Entropy | Raw | 87 | 59 | 76 | 77 | 62 | 72 | 82 | 60 | 74 | |
Lee | 91 | 77 | 87 | 87 | 81 | 84 | 89 | 79 | 86 |
map the forests and residential areas. From the spectral attributes, the band average was used, the mean, variance and entropy were used from the texture-attributes; and solidity, convexity, rectangle fit and roundness were used from the spatial-attributes. In addition, one rule set was used to combine the band-average and texture-mean attributes. The producer accuracy and the total accuracy are summarized in
As shown in Tables 1-3, using dual-polarization HH/VV led to higher total producer accuracy than the total producer accuracies employing either HH− or VV− polarization only. In contrast to the vegetated/uneven land classification results, using raw images facilitated higher producer accuracies than the Lee-filtered images in the results of both land/water and forests/residential classifications. The most useful features for separating the water class from the land class were texture-mean and band-average. Other attributes are not efficient in separating water from land. Applying either rule set 11 or texture-mean attributes enabled slightly better producer accuracy for the land class than employing the average band attributes. The best producer accuracy for the water class was 94% using the raw images with either single or dual polarization based on the average-band-attribute and texture-mean-attribute rules at Walmsburg Oxbow, while the lowest producer accuracy, of only 38%, was achieved at Wahgingen Oxbow when using Lee-filter images based on the convexity-attribute rule. The best total producer accuracy was 99%, using the raw images with single or dual polarization based on many rules at Walmsburg
Producer Accuracy % | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Polarization | Rules | Filter Type | Wehningen Oxbow | Walmsburg Oxbow | Average | ||||||
Forests | Residential Areas | Total | Forests | Residential Araes | Total | Forests | Residential Araes | Total | |||
HH | Average Band | Raw | 86 | 63 | 82 | 79 | 60 | 76 | 83 | 62 | 79 |
Lee | 85 | 45 | 78 | 78 | 34 | 68 | 82 | 40 | 73 | ||
VV | Average Band | Raw | 85 | 76 | 82 | 81 | 73 | 77 | 83 | 75 | 80 |
Lee | 83 | 53 | 78 | 76 | 45 | 69 | 80 | 49 | 74 | ||
HH/VV | Average Band | Raw | 84 | 76 | 83 | 81 | 74 | 80 | 83 | 75 | 82 |
Lee | 83 | 53 | 77 | 79 | 48 | 72 | 81 | 51 | 75 | ||
Rule Set 31 | Raw | 85 | 76 | 83 | 82 | 75 | 80 | 84 | 76 | 82 | |
Lee | 83 | 52 | 77 | 79 | 50 | 75 | 81 | 51 | 76 | ||
Texture Mean | Raw | 83 | 78 | 82 | 80 | 75 | 79 | 82 | 77 | 81 | |
Lee | 75 | 60 | 72 | 76 | 59 | 70 | 76 | 60 | 71 | ||
Texture Variance | Raw | 61 | 52 | 59 | 11 | 71 | 27 | 36 | 62 | 43 | |
Lee | 64 | 59 | 62 | 66 | 43 | 69 | 65 | 51 | 66 | ||
Texture Entropy | Raw | 71 | 69 | 71 | 68 | 56 | 66 | 70 | 63 | 69 | |
Lee | 68 | 38 | 62 | 86 | 26 | 73 | 77 | 32 | 68 | ||
Solidity | Raw | 70 | 65 | 69 | 86 | 39 | 76 | 78 | 52 | 73 | |
Lee | 85 | 23 | 77 | 77 | 27 | 67 | 81 | 25 | 72 | ||
Convexity | Raw | 40 | 73 | 46 | 53 | 54 | 53 | 47 | 64 | 50 | |
Lee | 44 | 63 | 48 | 58 | 49 | 56 | 51 | 56 | 52 | ||
Rectangle-fit | Raw | 89 | 50 | 82 | 80 | 48 | 76 | 85 | 49 | 79 | |
Lee | 75 | 38 | 68 | 79 | 24 | 67 | 77 | 31 | 68 | ||
Round | Raw | 46 | 74 | 53 | 78 | 35 | 69 | 62 | 60 | 61 | |
Lee | 62 | 40 | 58 | 84 | 18 | 70 | 73 | 29 | 64 |
Oxbow, while the lowest total producer accuracy of only 50% was achieved, again at Wehningen Oxbow, using the Lee-filter images based on the convexity-attribute rule.
The objects misclassified into the water class corresponded to the shadow of the forest areas. The shadow areas have less texture-mean than the water areas of the Elbe River. Therefore, rule set 12 was applied with a lower texture-mean value to isolate the main river, as shown in
The most useful features for separating vegetated lands class from the uneven lands class were, first, texture-entropy, followed by texture-variance and band-average, respectively. Elongation and area spatial-attributes succeeded to a certain degree in differentiating the two classes. Other attributes were only negligibly successful in separating them. Applying rule set 21, led to less total producer accuracy than employing the band-average- attributes. The elongation-attribute rule enabled better classification results for raw imagery while the texture- entropy-rule enabled higher classification accuracy for Lee-filtered images. The greatest producer accuracy for vegetated lands class was 92%, using the Lee-filtered dual-polarized images at Wehningen Oxbow based on the texture-variance-rule, while the lowest producer accuracy, of only 74%, was achieved at Walmsburg Oxbow
using the Lee-filtered images based on the compact-attribute rule. The maximum producer accuracy for the uneven land class was 81%, using the Lee-filtered dual-polarized images at Walmsburg Oxbow based on the texture-entropy-rule, while the lowest producer accuracy of only 35% was achieved at same study area, again when using the raw images based on the compact-attribute-rule. The best total producer accuracy was 88%, using Lee-filtered images at Wagingen Oxbow based also on texture-variance-rule, while the lowest total producer accuracy, of only 66%, was achieved once more at Walmsburg Oxbow using raw images based on the area-attribute-rule. The objects misclassified as vegetated lands in the uneven land class mostly correspond to existing vegetated areas within the residential areas (e.g. parks or gardens), as shown in
The effective features for separating the forest class from the residential area class were texture-mean, band- average, and their combination in rule set 31. Other attributes were not able to differentiate the two classes. Applying rule set 31 led to slightly better total producer accuracy than employing the band-average-attribute or texture-mean attribute individually. This rule set produced the highest classification accuracy for both raw and Lee-filtered images. The greatest producer accuracy for the forest class was 89%, using the dual-polarized raw images at Wehningen Oxbow based on the rectangle-fit rule, while the lowest producer accuracy, of only 11%, was achieved at Walmsburg Oxbow using raw images based on the texture-variance-attribute rule. The best producer accuracy for the residential area class was 78%, using the dual-polarized raw images at Wehningen
Oxbow based on the texture-mean-attribute rule, while the lowest producer accuracy of only 23% was achieved at the same study area using the Lee-filtered images based on the solidity-attribute rule. The highest total producer accuracy was 83%, using the raw images at Wehningen Oxbow based on the average-band-attributes rule, while the lowest total producer accuracy, of only 27%, was achieved at Walmsburg Oxbow using the raw imagery based on the text-variance-attribute rule. The objects misclassified as forests in the residential area class correspond to vegetated areas within the residential areas, as shown in
Based on the LULC classifications results for identifying water areas, using the average band attribute and/or the texture-mean attribute in the rule-based classifier enabled identification of about 90% of the water cover. Therefore, post-flood images in January 2011 were processed to be used as the pre-flood ones, and classified using the rule-based classifier with the average-band attribute and the texture-mean-attribute. The initial flood extent areas were corrected using the DEM and the LULC maps. The confusion matrices for both areas before and after post-classification are shown in summary in
In
Producer Accuracy % | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Rule | Enhancement Means | Filter Type | Wehningen Oxbow | Walmsburg Oxbow | Average | ||||||
Land | Water | Total | Land | Water | Total | Land | Water | Total | |||
Average Band | NH | Raw | 97.9 | 96.4 | 97.5 | 97.3 | 98.0 | 97.4 | 97.6 | 97.2 | 97.5 |
Lee | 98.1 | 94.0 | 97.0 | 98.2 | 97.8 | 98.2 | 98.2 | 95.9 | 97.6 | ||
Residential | Raw | 98.0 | 96.4 | 97.6 | 97.6 | 97.9 | 97.7 | 97.8 | 97.2 | 97.6 | |
Lee | 98.2 | 94.0 | 97.1 | 98.6 | 97.8 | 98.4 | 98.4 | 95.9 | 97.8 | ||
Forest | Raw | 99.1 | 96.4 | 98.4 | 98.7 | 97.9 | 98.6 | 98.9 | 97.2 | 98.5 | |
Lee | 99.1 | 94.0 | 97.8 | 99.4 | 97.8 | 99.1 | 99.2 | 95.9 | 98.5 | ||
DEM | Raw | 97.9 | 96.4 | 97.5 | 97.4 | 97.8 | 97.5 | 97.7 | 97.1 | 97.5 | |
Lee | 98.2 | 94.0 | 97.1 | 98.3 | 97.7 | 98.2 | 98.2 | 95.8 | 97.6 | ||
LULC | Raw | 99.2 | 96.4 | 98.5 | 99.0 | 97.9 | 98.8 | 99.1 | 97.2 | 98.6 | |
Lee | 99.3 | 94.0 | 97.9 | 99.7 | 97.8 | 99.4 | 99.5 | 95.9 | 98.6 | ||
LULC & DEM | Raw | 99.2 | 96.4 | 98.5 | 99.0 | 97.9 | 98.8 | 99.1 | 97.2 | 98.6 | |
Lee | 99.3 | 94.0 | 97.9 | 99.7 | 97.8 | 99.4 | 99.5 | 95.9 | 98.6 | ||
Texture Mean | NH | Raw | 97.9 | 97.0 | 97.7 | 97.5 | 97.9 | 97.5 | 97.7 | 97.4 | 97.6 |
Lee | 98.0 | 94.5 | 97.1 | 97.9 | 98.5 | 98.0 | 98.0 | 96.5 | 97.6 | ||
Residential | Raw | 98.0 | 97.0 | 97.7 | 97.7 | 97.8 | 97.7 | 97.8 | 97.4 | 97.7 | |
Lee | 98.2 | 94.5 | 97.2 | 98.3 | 98.5 | 98.3 | 98.2 | 96.5 | 97.8 | ||
Forest | Raw | 99.1 | 97.0 | 98.6 | 98.8 | 97.8 | 98.6 | 99.0 | 97.4 | 98.6 | |
Lee | 99.1 | 94.5 | 97.9 | 99.2 | 98.5 | 99.1 | 99.1 | 96.5 | 98.5 | ||
DEM | Raw | 97.9 | 96.9 | 97.7 | 97.6 | 97.7 | 97.6 | 97.7 | 97.3 | 97.6 | |
Lee | 98.1 | 94.5 | 97.1 | 98.1 | 98.4 | 98.1 | 98.1 | 96.4 | 97.6 | ||
LULC | Raw | 99.2 | 97.0 | 98.6 | 99.0 | 97.8 | 98.8 | 99.1 | 97.4 | 98.7 | |
Lee | 99.2 | 94.5 | 98.0 | 99.6 | 98.5 | 99.4 | 99.4 | 96.5 | 98.7 | ||
LULC & DEM | Raw | 99.2 | 97.0 | 98.6 | 99.0 | 97.8 | 98.8 | 99.1 | 97.4 | 98.7 | |
Lee | 99.2 | 94.5 | 98.0 | 99.6 | 98.5 | 99.4 | 99.4 | 96.5 | 98.7 |
The flood detection maps produced for January 2011 were compared to the flood extent areas in June 2013 as represented by DLR (2013), and to the high flood zone maps. The flood extent areas were approximately identical for 2011 and 2013 floods, as shown in
In spite of the fact that these fields were flooded in January 2011, they were cultivated with maize and potatoes in summer 2011. During the flood of summer 2013, these cultivated areas were again inundated and caused economic losses to the owners of the land. To achieve sustainable land use in this area, these fields must be included in the flood hazard maps and regulations established to prevent cultivation in these areas, permitting the fields to be used only as grassland, in order to avoid economic losses.
In
submerged during the most recent high floods, in 2011 and 2013. Nevertheless, this part of the city should be added to the hazard and risk maps to avoid the possible human and economic losses that may occur due to higher floods. This flood mapping will support sustainable land use in this area.
In order to achieve sustainable land use on the Middle Elbe River floodplain, up-to-date land use maps during the pre-flood period are essential to determine the hazards that may arise during the post-flood period. In particular, the locations of residential areas must be verified against the maps to ensure that they are safely removed from the high flood zone. Therefore, the residential areas that lie within the extent of flood zone must be included on the risk maps to support the regulatory prevention of (further) building within these risk zones. Moreover, the arable land which has suffered partially or fully from flood events must also be added to the hazard maps to decrease potential economic losses and to achieve sustainable land use.
The merging of similar pixels into objects diminishes the problem of speckle noise in the TSX imagery and, thus, enables high producer accuracies from the raw images without filtering. The raw images lead to similar or even better results than the Lee-filtered images. Therefore, it is recommended to use the object-based classifier with the raw images to save time and effort. Especially during flood events, flood extent maps are immediately required to identify hazard areas to help reduce human and economic losses. Further, the use of dual-polarized images enhances the classification results and leads to higher producer accuracies than the mono-polarized images. Therefore, it is recommended to use dual-polarization images to attain more accurate LULC maps.
The resulting Decision-Tree procedure, using the rule-based classifier in ENVI EX, resulted in considerably better total producer accuracies, such that about 95% of the water area was accurately defined, as well as about 90% of vegetated lands being correctly determined, and around 80% of the forest and the residential area classes recognized. The 20% misclassified areas within the forest and residential areas were due to the existence of vegetated areas and trees within the residential areas around the buildings.
The use of texture and spatial attributes with the spectral attributes enhanced the classification results. Applying rules based on the band-average, as a spectral attribute, and the texture-mean facilitated correct identification of about 95% of the flood extent for post-flood imagery. Furthermore, the use of the texture-entropy attribute enabled recognition of about 90% of the vegetated lands. The texture-mean attribute enabled efficient distinguishing of residential areas and forest classes.
To conclude, the results show that similar rule sets can be used for the Decision-Tree procedure on two remote study areas in the Elbe River flood plains to achieve higher classification producer accuracies. Thus, the suggested Decision-Tree should be applicable to other remote areas. Therefore, it is recommended to continuously monitor the entire Biosphere Reserve using TSX imagery to deal with construction and/or cultivation within the flood zone. Construction and cultivation in flood plains should be carefully planned according to the flood risk maps to ensure sustainable land use within the Elbe Biosphere.
We are greatly indebted to the administration of the Biosphere Reserve “Niedersächsische Elbtalaue” (Lower Saxonian Elbe Valley Biosphere Reserve) and to the German Aerospace Centre (Deutsches Zentrum für Luft- und Raumfahrt, DLR) for providing the satellite images used in this study. We acknowledge the important support of this research provided by a PhD graduate scholarship awarded by LEUPHANA University Lüneburg to Dalia Farghaly.
Dalia Farghaly,Emad Elba,Brigitte Urban, (2016) Towards Sustainable Land Uses within the Elbe River Biosphere Reserve in Lower Saxony, Germany by Means of TerraSAR-X Images. Journal of Geoscience and Environment Protection,04,97-121. doi: 10.4236/gep.2016.43009