INDIAN INSTITUTE OF TECHNOLOGY ROORKEE

Transcription

1 SEMINAR ON A REVIEW OF CHANGE DETECTION TECHNIQUES INDIAN INSTITUTE OF TECHNOLOGY ROORKEE PRESENTED BY:- ABHISHEK BHATT RESEARCH SCHOLAR abhishekbhatt.iitr@gmail.com

2 OUTLINE This seminar is organized into eight sections as follows: 1. Background and applications of change detection techniques 2. Considerations before implementing change detection 3. A review of seven categories of change detection techniques 4. Comparative analyses among the different techniques 5. A global change analyses 6. Threshold selection 7. Accuracy assessment 8. Summary and recommendations References

3 Background In general, change detection involves the application of multitemporal datasets to quantitatively analyze the temporal effects Change detection can be defined as the process of identifying differences in the state of an object or phenomenon by observing it at different times. This process is usually applied to Earth surface changes at two or more times. understanding relationships and interactions to better manage and use resources Change detection is useful in many applications such as land use changes, habitat fragmentation, rate of deforestation, coastal change, urban sprawl, and other cumulative changes

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6 Change detection source; Norsk Regnesentral website Two main categories of land cover changes: Conversion of land cover from one category to a different category. Modification of the condition of the land cover type within the same category (thinning of trees, selective cutting, pasture to cultivation, etc.)

7 Applications of change detection techniques land-use and land-cover (LULC) change forest or vegetation change forest mortality, defoliation and damage assessment deforestation, regeneration and selective logging wetland change forest fire and fire-affected area detection landscape change urban change environmental change, drought monitoring, flood monitoring, monitoring coastal marine environments, desertification, and detection of landslide areas other applications such as crop monitoring, shifting cultivation monitoring, road segments, and change in glacier mass balance and facies.

8 Considerations before implementing change detection Before implementing change detection analysis, the following conditions must be satisfied: i. precise registration of multi-temporal images; ii. precise radiometric and atmospheric calibration or normalization between multi-temporal images; iii. selection of the same spatial and spectral resolution images if possible

9 Good change detection research should provide the following information: i. area change and change rate ii. spatial distribution of changed types iii. Change trajectories of land-cover types iv. accuracy assessment of change detection results.

10 A review of change detection techniques Because digital change detection is affected by spatial, spectral, radiometric and temporal constraints. Many change detection techniques are possible to use, the selection of a suitable method or algorithm for a given research project is important, but not easy.

11 The seven change detection technique categories 1. Algebra Based Approach image differencing image regression image ratioing vegetation index differencing change vector analysis 2. Transformation PCA Tasseled Cap (KT) Gramm-Schmidt (GS) Chi-Square 3. Classification Based Post-Classification Comparison Spectral-Temporal Combined Analysis EM Transformation Unsupervised Change Detection Hybrid Change Detection Artificial Neural Networks (ANN) 4. Advanced Models 5. GIS Li-Strahler Reflectance Model Spectral Mixture Model Biophysical Parameter Method Integrated GIS and RS Method GIS Approach 6. visual Analysis Visual Interpretation 7. other Change Detection Techniques Measures of spatial dependence Knowledge-based vision system Area production method Combination of three indicators: vegetation indices, land surface temperature, and spatial structure Change curves Generalized linear models Curve-theorem-based approach Structure-based approach Spatial statistics-based method

12 Category I Algebra Based Approach The algebra category includes image differencing, image regression Image ratioing vegetation index differencing change vector analysis (CVA)

13 Algebra based Approach These algorithms have a common characteristic, i.e. selecting thresholds to determine the changed areas. These methods (excluding CVA) are relatively simple, straightforward, easy to implement and interpret, but these cannot provide complete matrices of change information. In this category, two aspects are critical for the change detection results: selecting suitable image bands selecting suitable thresholds

14 Image Differencing Concept Date 1 - Date 2 No-change = 0 Positive and negative values interpretable Pick a threshold for change

15 Image Differencing Image Date 1 Image Date Difference Image = Image 1 - Image 2

16 Image Differencing Image differencing: Pros Simple (some say it s the most commonly used method) Easy to interpret Robust Cons: Difference value is absolute, so same value may have different meaning Requires atmospheric calibration

17 Image regression Relationship between pixel values of two dates is established by using a regression function. The dimension of the residuals is an indicator of where change occurred. Advantage Reduces impact of atmospheric, sensor and environmental differences. Drawback Requires development of accurate regression functions. Does not provide change matrix.

18 Image regression

19 Image Ratioing Concept Pros Cons Date 1 / Date 2 No-change = 1 Values less than and greater than 1 are interpretable Pick a threshold for change Simple May mitigate problems with viewing conditions, esp. sun angle Scales change according to a single date, so same change on the ground may have different score depending on direction of change; I.e. 50/100 =.5, 100/50 = 2.0

20 Change Detection source: CCRS website, CANADA Image Difference (TM99 TM88) Image Ratio (TM99 / TM88)

21 Change vector analysis In n-dimensional spectral space, determine length and direction of vector between Date 1 and Date2 No-change = 0 length Change direction may be interpretable Pick a threshold for change Band 4 Date 1 Date 2 Band 3

22 Change vector analysis Determines in n-dimensional spectral space, the length and direction of the vector between Date 1 and Date 2. Produces an intensity image and a direction image of change. The direction image can be used to classify change. source; Norsk Regnesentral website Typically used when all changes need to be investigated. Advantage Works on multispectral data. Allows designation of the type of change occurring Drawback Shares some of the drawbacks of algebra based techniques but less severe

23 Change vector analysis

24 Category I. Algebra Based Approach Techniques Characteristics Advantages Disadvantages Examples Key factors 1. Image differencing Subtracts the first date image from a seconddate image, pixel by pixel Simple and Straight forward, easy to interpret the results Cannot provide a detailed change matrix, requires selection of thresholds Forest defoliation, land-cover Change and irrigated crops monitoring Identifies suitable image bands and thresholds 2. Image regression Establishes relationships between bitemporal images, then estimates pixel values of the second-date image by use of a regression function, subtracts the regressed image from the first-date image Reduces impacts of the atmospheric, sensor and environmental differences between two-date images Requires to develop accurate regression functions for the selected bands before implementing change detection Tropical forest change and forest conversion Develops the regression function; identifies suitable bands and thresholds 3. Image ratioing Calculates the ratio of registered images of two dates, band by band Reduces impactsof Sun angle, shadow and topography Non-normal distribution of the result is often criticized Land-use mapping Identifies the image bands and thresholds

25 Techniques Characteristics Advantages Disadvantages Examples Key factors 4. Vegetation Index differencing Produces vegetation index separately, then subtracts the second-date vegetation index from the first-date vegetation index Emphasizes differences in the spectral response of different features and reduces impacts of topographic effects and illumination. random noise or coherence noise Vegetation change and forest canopy change Enhances Identifies suitable vegetation index and thresholds 5. Change vector analysis (CVA) Generates two outputs: (1) the spectral change vector describes the direction and magnitude of change from the first to the second date; and (2) the total change magnitude per pixel is computed by determining the Euclidean distance between end points through n-dimensional change space Ability to process any number of spectral bands desired and to produce detailed change detection information Difficult to identify land cover change trajectories landscape variables land-cover changes disaster assessment and conifer forest change Defines thresholds and identifies change trajectories

26 Category II. Transformation of data sets

27 Transformations Principal Component Analysis Alt1: Perform PCA on data from both dates and analyse the component images. Alt2: Perform PCA separately on each image and subtract the second-date PC image from that of the first date. Advantage Reduces data redundancy. Drawback Results are scene dependent and can be difficult to interpret. Does not provide change matrix.

28 Kauth Thomas Transformation Described the temporal spectral patterns derived from Landsat MSS imagery for crops. As crops grow from seed to maturity, there is a net increase in NIR and decrease in Red Reflectance. This effect varies based on soil Color Brightness Greenness Wetness The Brightness, Greenness, Wetness transform was first developed for use with the Landsat MSS system and called the Tasseled Cap transformation. The transform is based on a set of constants applied to the image in the form of a linear algebraic formula. Brightness primary axis calculated as the weighted sum of reflectances of all spectral bands. Greenness perpendicular to the axis of the Brightness component that passes through the point of maturity of all plants Yellow Stuff perpendicular to both Greenness and Brightness axis representing senesced vegetation.

29 Kauth Thomas Transformation Typically the first few components contain most of the information in the data so that four channels of LANDSAT MSS data or the six channels of the Thematic Mapper data may be reduced to just three principal components. The components higher than three are usually treated as being information less. Source;

30 Category II. Transformation Techniques Characteristics Advantages Disadvantages Examples Key factors 1. Principal component analysis (PCA) Assumes that multitemporal data are highly correlated and change information can be highlighted in the new components. Two ways to apply PCA for change detection are: (1)put two or more dates of images into a single file, then perform PCA and analyse the minor component images for change information; and (2) perform PCA separately, then subtract the second-date PC image from the corresponding PC image of the first date Reduces data Redundancy between bands and emphasizes different information in the derived components PCA is scene dependent, thus the change detection results between different dates are often difficult to interpret and label. It cannot provide a complete matrix of change class information and requires determining thresholds to identify the changed areas Land-cover change urban expansion,tropical forest conversion, forest mortality and forest defoliation Analyst s skill in identifying which component best represents the change and selecting thresholds 2. Tasselled cap (KT) The principle of this method is similar to PCA. The only difference from PCA is that PCA depends on the image scene, andkt transformation is independent of the scene. The change detection is implemented based on the three components: brightness, greenness and wetness Reduces data redundancy between bands and emphasizes different information in the derived components. KT is scene independent. Difficult to interpret and label change information, cannot provide a complete change matrix; requires determining thresholds to identify the changed areas. Accurate atmospheric calibration is required Monitoring forest mortality, monitoring green biomass and land-use change Analyst s skill is needed in identifying which component best represents the change and thresholds

31 Techniques Characteristics Advantages Disadvantages Examples Key factors 3. Gramm Schmidt (GS) The GS method orthogonalizes spectral vectors taken directly from bi-temporal images, as does the original KT method, produces three stable components corresponding to multitemporal analogues of KT brightness, greenness and wetness, and a change component The association of transformed components with scene characteristics allows the extraction of information that would not be accessible using other techniques It is difficult to extract more than one single component related to a given type of change. The GS process relies on selection of spectral vectors from multi-date image typical of the type of change being examined Monitoring forest mortality Initial identification of the stable subspace of the multi-date data is required 4. Chisquare Y=(X-M) T -1 *(X-M) Y:digital value of change image X:vector of the difference of the six digital values between the two dates M:vector of the mean residual of each band T:transverse of the matrix -1 = inverse covariance matrix Multiple bands Are simultaneously considered to produce a single change image. The assumption that a value of Y~0 represents a pixel of no change is not true when a large portion of the image is changed. Also the change related to specific spectral direction not identified Urban environmen tal change Y is distributed as a Chi-square random variable with p degrees of freedom ( p is the number of bands)

32 Category-III Classification based approach

33 Post-classification Post-classification (delta classification) Classify Date 1 and Date 2 separately, compare class values on pixel by pixel basis between dates Post-classification: Pros Avoids need for strict radiometric calibration Favors classification scheme of user Designates type of change occurring Cons Error is multiplicative from two parent maps Changes within classes may be interesting

34 Composite Analysis Composite Analysis Stack Date 1 and Date 2 and run unsupervised classification on the whole stack Composite Analysis: Pros May extract maximum change variation Includes reference for change, so change is anchored at starting value, unlike change vector analysis and image differencing Cons May be extremely difficult to interpret classes

35 Unsupervised techniques Objective Produce a change detection map in which changed areas are separated from unchanged ones. The changes sought are assumed to result in larger changes in radiance values than other factors. Comparison is performed directly on the spectral data. source; Norsk Regnesentral website This results in a difference image which is analysed to separate insignificant from significant changes.

36 Supervised techniques Objective Generate a change detection map where changed areas are identified and the land-cover transition type can be identified. The changes are detected and labelled using supervised classification approaches. source; Norsk Regnesentral website Main techniques: Post-classification comparison Multidate direct classification

37 Post classification comparison source; Norsk Regnesentral website Standard supervised classifiers are used to classify the two images independently. Changes are detected by comparing the two classified images. Advantage Common and intuitive. Provides change matrix. Drawback Critically depends on the accuracy of the classification maps. Accuracy close to the product of the two results. Does not exploit the dependence between the information from the two points in time.

38 Post classification comparison

39 Multidate direct classification source; Norsk Regnesentral website Two dates are combined into one multitemporal image and classified. Performs joint classification of the two images by using a stacked feature vector. Change detection is performed by considering each transition as a class, and training the classifier to recognize all classes and all transitions. Advantage Exploits the multitemporal information. Error rate not cumulative. Provides change matrix. Drawback Ground truth required also for transitions.

40 Supervised vs. Unsupervised Level of change detection Change information Change computation Supervised Change detection at decision level. Provides explicit labeling of change and class transitions Obtained directly from the classified images. Unsupervised Change detection at data level. Separates change from no change. Obtained through interpretation of the difference image. Ground truth Requires ground truth. Requires no ground truth. Spectral information. Data requirements Multispectral. Not sensitive to atmospheric conditions and sensor differences. Most methods work on one spectral band. Sensitive to atmospheric conditions and sensor differences.

41 Category III. Classification based approach Techniques Characteristics Advantages Disadvantages Examples Key factors 1. Post classification comparison 2. Spectral temporal combined analysis 3. EM detection Separately classifies multitemporal images into thematic maps, then implements comparison of the classified images, pixel by pixel Puts multi-temporal data into a single file, then classifies the combined dataset and identifies and labels the changes The EM detection is a classification-based method using an expectation maximization (EM) algorithm to estimate the a priori joint class probabilities at two times. These probabilities are estimated directly from the images under analysis Minimizes Requires a great impacts of amount of time and atmospheric, expertise to create sensor and classification environmental products. The final differences accuracy depends on between the quality of the multitemporal classified image of images; provides a each date complete matrix of change information Simple and Difficult to identify timesaving and label the change in classification classes; cannot provide a complete matrix of change information This method was reported to provide higher change detection accuracy than other change detection methods Requires estimating the a priori joint class probability. LULC change, wetland change and urban expansion Changes in coastal zone environments and forest change Land-cover change Selects sufficient training sample data for classification Labels the change classes Estimates the a priori joint class probability

42 Techniques Characteristics Advantages Disadvantages Examples Key factors 4. Unsupervised change detection 5. Hybrid change detection Selects spectrally similar groups of pixels and clusters date 1 image into primary clusters, then labels spectrally similar groups in date 2 image into primary clusters in date 2 image, and finally detects and identifies changes and outputs results Uses an overlay enhancement from a selected image to isolate changed pixels, then uses Supervised classification. A binary change mask is constructed from the classification results. This change mask sieves out the changed themes from the LULC maps produced for each date This method makes use of the unsupervised nature and automation of the change analysis process This method Excludes unchanged pixels classification to reduce classification errors from Difficulty identifying labelling trajectories in and change Requires selection of thresholds to implement classification; somewhat complicated to identify change trajectories Forest hange LULC change, vegetation change and monitoring eelgrass Identifies the spectrally similar or relatively homogeneous units Selects suitable thresholds to identify the change and nonchange areas and develops accurate classifi n output 6. Artificial neural networks (ANN) The input used to train the neural network is the spectral data of the period of change. A backpropagation algorithm is often used to train the multi-layer perceptron neural network model ANN is a nonparametric Supervised method and has the ability to estimate the properties of data based on the training samples The nature of hidden layers is poorly known; a long training time is required. ANN is often sensitive to the amount of training data used. ANN functions are not common in image processing software Mortality detection in Lake, landcover change, forest change, Urban hange The architecture used such as the number of hidden layers, and training samples

43 Category IV. Advanced models Techniques Characteristics Advantages Disadvantages Examples Key factors 1. Li Strahler The Li Strahler canopy model This method combines This method Mapping Develops the reflectance is used to estimate each conifer the techniques of digital requires a large and Stand crown model stand crown cover for two dates image processing of number of field monitoring cover images of imageries separately. remotely sensed data Measurement data. conifer and identifies Comparison of the stand crown with traditional sampling It is complex and mortality the crown covers for two dates is and field observation not available in characteristics conducted to produce the methods. It provides commercial image of vegetation change detection results statistical results and processing types maps showing the software. It is only geometric distribution of suitable for changed patterns vegetation change 2. Spectral Uses spectral mixture analysis to The fractions have This method is Land-cover Identifies mixture derive fraction images. biophysical meanings, regarded as an change, suitable model Endmembers are selected from representing the areal advanced image seasonal endmembers; training areas on the image or proportion of each processing vegetation defines suitable from spectra of materials endmember within the analysis and is patterns and thresholds for occurring in the study area or pixel. The results are somewhat Vegetation each landcover from a relevant spectral library. stable, accurate and complex change Changes are detected by repeatable using TM class based on comparing the before and data fractions after fraction images of each end member. The quantitative changes can be measured by classifying images based on the endmember fractions

44 Category V. GIS based approach Techniques Characteristics Advantages Disadvantages Examples Key factors 3. Integrated GIS and remote Sensing method Incorporates image data and GIS data, such as the overlay of GIS layers directly on image data; moves results of image processing into GIS system for further analysis Allows access of ancillary data to aid interpretation and analysis and has the ability to directly update land-use information in GIS Different data quality from various sources often degrades the results of LULC change detection LULC and urban sprawl The accuracy of different data sources and their registration accuracies between the thematic images 4. GIS approach Integrates past and current maps of land use with topographic and geological data. The image overlaying and binary masking techniques are useful in revealing quantitatively the change dynamics in each category This method allows incorporation of aerial photographic data of current and past land-use data with other map data Different GIS data with different geometric accuracy and classification system degrades the quality of results Urban change And landscape change The accuracy of different data sources and their registration accuracies between the thematic images.

45 Category VI. Visual analysis Techniques Characteristics Advantages Disadvantages Examples Key factors 1. Visual interpretation One band (or VI) from date1 image as red, the same band (or VI) from date2 image as green, and the same band (or VI) from date3 image as blue if available. Visually interprets the colour composite to identify the changed areas. An alternative is to implement on-screen digitizing of changed areas using visual interpretation based on overlaid images of diff. dates Human experience and knowledge are useful during visual interpretation. Two or three dates of images can be analysed at one time. The analyst can incorporate texture, shape, size and patterns intovisual interpretation to make a decision on the LULC change Cannot provide detailed change information. The results depend on the analyst s skill in image interpretation. Timeconsuming and difficulty in updating the results Land-use change, forest change, monitoring selectively logged areas and land cover change Analyst s skill and familiarit y with the study area

46 Category VII. Other change detection techniques 1. Measures of spatial dependence (Henebry 1993) 2. Knowledge-based vision system (Wang 1993) 3. Area production method (Hussin et al. 1994) 4. Combination of three indicators: vegetation indices, land surface temperature, and spatial structure (Lambin and Strahler 1994b) 5. Change curves (Lawrence and Ripple 1999) 6. Generalized linear models (Morisette et al. 1999) 7. Curve-theorem-based approach (Yue et al. 2002) 8. Structure-based approach (Zhang et al. 2002) 9. Spatial statistics-based method (Read and Lam 2002)

47 Factors to consider when choosing a method Objective of the change detection? Monitor/identify specific changes More efficient mapping at T2 Improved quality of mapping at T2 What type of change information to extract? Spectral changes Land cover transitions Shape changes Changes in long temporal series What type of changes to be considered? Land use and land cover change Forest and vegetation change Wetland change Urban change Environmental change

48 Factors to consider Expected amount of changes Available data at date 1 and date 2 Remote sensing data Temporal, spatial and spectral characteristics. Differences in characteristics btw. date 1 and date 2. Classified maps Ground truth Environmental considerations Atmospheric conditions Soil moisture conditions Phenological states Accuracy requirements

49 Comparing the Different Techniques Two types of change detection either detect binary change/non-change, or the detailed from-to change between different classes. Different change detection techniques are often tested and compared based on an accuracy assessment or qualitative assessment. no single method is suitable for all cases. A combination of two change detection techniques can improve the change detection results (image differencing/pca, NDVI/PCA, PCA/CVA). The most common change detection methods: image differencing, PCA, CVA, and post-classification comparison.

50 Global change analyses and image resolution For change detection at high or moderate spatial resolution: use Landsat TM, SPOT, or radar. For change detection at the continental or global scale, use coarse resolution data such as MODIS and AVHRR. AVHRR has daily availability at low cost; it is the best source of data for large area change detection. NDVI and land surface temperatures derived from MODIS or AVHRR thermal bands are especially useful in large area change detection.

51 Threshold Selection Many change detection algorithms require threshold selection to determine whether a pixel has changed. Thresholds can be adjusted manually until the resulting image is satisfactory, or they can be selected statistically using a suitable standard deviation from a class mean. Both are highly subjective methods. Other methods exist for improving the change detection results, such as using fuzzy set and fuzzy membership functions to replace the thresholds. However, threshold selection is simple and intuitive, so it is still the most extensively applied method for detecting binary change/no-change information.

52 Accuracy Assessment Accuracy assessments are important for understanding the change detection results and using these results in decision-making. However, they are difficult to do because reliable temporal field-based datasets are often problematic to collect. The error matrix is the most common method for accuracy assessment. To properly generate one, the following factors must be considered: 1. ground truth data collection, 2. classification scheme, 3. sampling scheme, 4. spatial autocorrelation, and 5. sample size and sample unit.

53 Summary and Recommendations The binary change/no-change threshold techniques all have difficulties in distinguishing true changed areas from the detected change areas. Singleband image differencing and PCA are the recommended methods. Classification-based change detection methods can avoid such problems, but requires more effort to implement. Post-classification comparison is a suitable method when sufficient training data is available. When multi-source data is available, GIS techniques can be helpful. Advanced techniques such as LSMA, ANN, or a combination of change detection methods can produce higher quality change detection results.

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