Chapter 5 Image Classification

Chapter 5 Image Classification
Analysis and applications of remote sensing imagery Instructor: Dr. Cheng-Chien Liu Department of Earth Sciences National Cheng Kung University Last updated: 24 May 2005

Introduction Overall objective of classification
Automatically categorize all pixels in an image into land cover classes or themes Three pattern recognitions Spectral pattern recognition  emphasize in this chapter Spatial pattern recognition Temporal pattern recognition Selection of classification No single “right” approach Depend on The nature of the data being analyzed The computational resources available The intended application of the classified data

Supervised classification
Fig 7.37 A hypothetical example Five bands: B, G, R, NIR, TIR, Six land cover types: water, sand, forest, urban, corn, hay Three basic steps (Fig 7.38) Training stage Classification stage Output stage

Supervised classification (cont.)
Classification stage Fig 7.39 Pixel observations from selected training sites plotted on scatter diagram Use two bands for demonstration, can be applied to any band number Clouds of points  multidimensional descriptions of the spectral response patterns of each category of cover type to be interpreted Minimum-Distance-to-Mean classifier Fig 7.40 Mean vector for each category Pt 1  Corn Pt 2  Sand ?!! Advantage: mathematically simple and computationally efficient Disadvantage: insensitive to different degrees of variance in the spectral response data Not widely used if the spectral classes are close to one another in the measurement space and have high variance

Classification stage (cont.) Parallelepiped classifier Fig 7.41 Range for each category Pt 1  Hay ?!! Pt 2  Urban Advantage: mathematically simple and computationally efficient Disadvantage: confuse if correlation or high covariance are poorly described by the rectangular decision regions Positive covariance: Corn, Hay, Forest Negative covariance: Water Alleviate by use of stepped decision region boundaries (Fig 7.42)

Classification stage (cont.) Gaussian maximum likelihood classifier Assumption: the distribution of the cloud of points is Gaussian distribution Probability density functions  mean vector and covariance matrix (Fig. 7.43) Fig 7.44: Ellipsoidal equiprobability contours Bayesian classifier A priori probability (anticipated likelihood of occurrence) Two weighting factors If suitable data exist for these factors, the Bayesian implementation of the classifier is preferable Disadvantage: computational efficiency Look-up table approach Reduce the dimensionality (principal or canonical components transform) Simplify classification computation by separate certain classes a prior Water is easier to separate by use of NIR/Red ratio

Training stage Classification  automatic work Assembling the training data  manual work Both an art and a science Substantial reference data Thorough knowledge of the geographic area You are what you eat!  Results of classification are what you train! Training data Both representative and complete All spectral classes constituting each information class must be adequately represented in the training set statistics used to classify an image e.g. water (turbid or clear) e.g. crop (date, type, soil moisture, …) It is common to acquire data from 100+ training areas to represent the spectral variability

Training stage (cont.) Training area Delineate boundaries (Fig 7.45) Carefully located boundaries  no edge pixels Seed pixel Choose seed pixel  statistically based criteria  contiguous pixels  cluster Training pixels Number At least n+1 pixels for n spectral bands In practice, 10n to 100n pixels is used Dispersion  representative  Training set refinement Make sure the sample size is sufficient Assess the overall quality Check if all data sets are normally distributed and spectrally pure Avoid redundancy Delete or merge

Training stage (cont.) Training set refinement process Graphical representation of the spectral response patterns Fig 7.46: Histograms for data points included in the training areas of “hay” Visual check on the normality of the spectral response distribution Two subclasses: normal and bimodal Fig 7.47: Coincident spectral plot Corn/hay overlap for all bands Band 3 and 5 for hay/corn separation (use scatter plot) Fig 7.48: SPOT HRV multi-spectral images Fig 7.49 scatter plot of band 1 versus band 2 Fig 7.50 scatter plot of band 2 versus band 3  less correlated  adequate Quantitative expressions of category separation Transform divergence: a covariance-weighted distance between category means Table 7.1: Portion of a divergence matrix (<1500  spectrally similar classes)

Training stage (cont.) Training set refinement process (cont.) Self-classification of training set data Error matrix  for training area not for the test area or the overall scene Tell us how well the classifier can classify the training areas and nothing more Overall accuracy is perform after the classification and output stage Interactive preliminary classification Plate 29: sample interactive preliminary classification procedure Representative subscene classification Complete the classification for the test area  verify and improve Summary Revise with merger, deletion and addition to form the final set of statistics used in classification Accept misclassification accuracy of a class that occurs rarely in the scene to preserve the accuracy over extensive areas Alternative methods for separating two spectrally similar classes  GIS data, visual interpretation, field check, multi-temporal or spatial pattern recognition procedures, …

Training stage (cont.) Implementation  region of interest (ROI) Three sources of ROI Manually from an image using the mouse From pixel scatter plots From vector layers

Exercise 1 Quick classification using interactive 2-D scatter plots
Rationale Sufficient information to determine appropriate training areas may not exist 2-D scatter plot  first step in determine training set Data: ca_coast.dat (TMS data) Create 2D scatter plot Tool  2-D Scatter Plots… The adjacent bands are usually highly correlated Choose band 3 for X-axis and band 8 for Y-axis Check dancing pixels hold the left-button in the image window hold the right-button in the image window Option  Density slice

Self test 1 File: ca_coast.dat Note:
Use 2D scatter plot to define 5 ROIs Note: Selection of bands for 2D scatter plot The least number of pixels required for each class Dispersion of ROIs Give each ROI an appropriate name Output the ROIs into a file

Exercise 2 Perform classification File: ca_coast.dat
Use the same ROIs that were defined earlier Classification method: Maximum likelihood method Minimum distance method Try various threshold value(s) Use Preview function Change the extent by selecting the Change View button Examine the rule image

Exercise 3 Examine class images
Load results of classification in previous exercise Link the displays and examine the differences Answer the following questions Regions of the same classification Regions of the different classification Which is better Do your ROIs seem to be appropriate? How to improve the classification by changing the ROIs Check the header and data type of the classified result Change the class color mapping

Exercise 4 Examine rule images
Display rule images in previous exercise Link the displays and examine the differences Plot the z profile for each rule image Move to an arbitrary pixel, check the value and determine which class this pixel should be

Exercise 5 Perform post classification using the rule classifier
Classification  Post Classification  Rule Classifier File: dist_rule.img Change the thresholds and press Quick Apply Examine the result Examine the rule images histogram to determine the appropriate threshold for each class Press the Hist button for open ocean class Set a threshold to encompass the first peak of the bimodel Repeat for the other classes

Exercise 6 Overlay classes Display band 7 of ca_coast.dat in gray
Overlay  Classification File: max_class.img Interactive Class Tool dialog Turn on and off class(es) Options  Class distribution Change active class Options  Associated stats data file Options  Stats for all classes Examine the min, max, mean, standard deviation for each class Display band 7 of ca_coast.dat in a new window Overlay dist_class.img Link two displays and examine the differences

Exercise 6 (cont.) Overlay classes (cont.)
Repeat setting the Interactive Class Tool dialog for the new file: dist_class.img Turn on and off class(es) Options  Class distribution Change active class Options  Associated stats data file Options  Stats for all classes Examine the min, max, mean, standard deviation for each class Compare the class distribution and stats plots Editing pixels of classification using the Interactive Class Tool

Exercise 7 Convert classes to ROIs Using Band Threshold to ROI tool
Overlay  Regions of Interest Options  Band Threshold to ROI Options  report area of ROIs

Unsupervised classification
Unsupervised  supervised Supervised  define useful information categories  examine their spectral separability Unsupervised  determine spectral classes  define their informational utility Illustration: Fig 7.51 Advantage: the spectral classes are found automatically (e.g. stressed class)

Unsupervised classification (cont.)
Clustering algorithms K-means Locate centers of seed clusters  assign all pixels to the cluster with the closest mean vector  revise mean vectors for each clusters  reclassify the image  iterative until there is no significant change Iterative self-organizing data analysis (ISODATA) Permit the number of clusters to change from on iteration to the next by Merging: distance < some predefined minimum distance Splitting: standard deviation > some predefined maximum distance Deleting: pixel number in a cluster < some specified minimum number Table 7.2 Outcome 1: ideal result Outcome 2: subclasses  classes Outcome 3: a more troublesome result The information categories is spectrally similar and cannot be differentiated in the given data set

Exercise 8 Unsupervised classification File: ca_coast.dat
Method: K-means and ISODATA Parameter: Overlay the result of classification onto the original true-color image Examine the result of classification Save both results for exercise 10

Exercise 8 (cont.)

Hybrid classification
Unsupervised training areas Image sub-areas chosen intentionally to be quite different from supervised training areas Supervised  regions of homogeneous cover type Unsupervised  contain numerous cover types at various locations throughout the scene To identify the spectral classes Guided clustering Delineate training areas for class X Cluster all class X into spectral subclasses X1, X2, … Merge or delete class X signatures Repeat for all classes Examine all class signatures and merge/delete signatures Perform maximum likelihood classification Aggregate spectral subclasses

Classification of mixed pixels
IFOV includes more than one type/feature Low resolution sensors  more serious Subpixel classification Spectral mixture analysis A deterministic method (not a statistical method) Pure reference spectral signatures Measured in the lab, in the field, or from the image itself Endmembers Basic assumption The spectral variation in an image is caused by mixtures of a limited number of surface materials Linear mixture  satisfy two basic conditions simultaneously The sum of the fractional proportions of all potential endmembers SFi = 1 The observed DNl for each pixel B band  B equations B+1 equations  solve B+1 endmember fractions Fig 7.52: example of a linear spectral mixture analysis Drawback: multiple scattering  nonlinear mixturemodel

Classification of mixed pixels (cont.)
Subpixel classification (cont.) Fuzzy classification A given pixel may have partial membership in more than one category Fuzzy clustering Conceptually similar to the K-means unsupervised classification approach Hard boundaries  fuzzy regions Membership grade Fuzzy supervised classification A classified pixel is assigned a membership grade with respect to its membership in each information class

Exercise 9 Linear spectral unmixing File: ca_coast.dat
Display the image in true color Set 5 ROIs, each has one pure pixel Spectral  mapping methods  endmember collection Import five endmembers from ROIs Algorithms  Linear spectral unmixing Set constrained Apply and examine the results

The output stage Image classification  output products  end users
Graphic products Plate 30, Fig 3 of the paper “IKONOS imagery for resource management” Tabular data Digital information files

Postclassification smoothing
Salt-and-pepper appearance Low-pass filter can not be used Must operate on the basis of logical operations, rather than simple arithmetic computations Majority filter Fig 7.53 (a) original classification  salt-and-pepper appearance (b) 3 x 3 pixel-majority filter (c) 5 x 5 pixel-majority filter Imbedded in the algorithm of classification Limited Need the technique of spatial pattern recognition Future development

Exercise 10 Postclassification smoothing File: results from exercise 8
Clump and Sieve For generalizing classification images, Sieve is usually run first to remove the isolated pixels based on a size (number of pixels) threshold. Clump is run to add spatial coherency to existing classes by combining adjacent similar classified areas Classification → Post Classification → Sieve Classes Classification → Post Classification → Clump Classes Combine Classes Classification → Post Classification → Combine Classes

Classification accuracy assessment
Significance A classification is not complete until its accuracy is assessed Classification error matrix Error matrix (confusion matrix, contingency table) Table 7.3 Omission (exclusion) 漏授(該有的沒有) Non-diagonal column elements (e.g. 16 sand pixels were omitted) Commission (inclusion) 誤授(不該有的卻有) Non-diagonal raw elements (e.g. 38 urban pixels + 79 hay pixels were included in corn) Overall accuracy Producer’s accuracy 生產者準確度 Indicate how well training set pixels of the given cover type are classified User’s accuracy 使用者準確度 Indicate the probability that a pixel classified into a given category actually represents that category on the ground Training area accuracies are sometimes used in the literature as an indication of overall accuracy. They should not be!

Classification accuracy assessment (cont.)
Sampling considerations Test area Different and more extensive than training area Withhold some training areas for postclassification accuracy assessment Being homogeneous, test areas might not provide a valid indication of classification accuracy at the individual pixel level of land cover variability Wall-to-wall comparison Expensive Defeat the whole purpose of remote sensing Random sampling Collect large sample of randomly distributed points  too expensive and difficult e.g. 3/4 of Taiwan area is covered by The Central mountain Only sample those pixels without influence of potential registration error Several pixels away from field boundaries Stratified random sampling Each land cover category  Stratum

Sampling considerations (cont.) Accomplishment of random sampling Overlay the classified output data with a grid Test cells within the grid are selected randomly and groups of pixels within the test cells are evaluated Sample unit Individual pixels, clusters of pixels or polygons Sample number General area: 50 samples per category Large area or more than 12 categories: 75 – 100 samples per category Depend on the variability of each category Wetland need more samples than open water

Evaluating classification error matrices Table 7.4: error matrix (randomly sampled test) Producer’s accuracy for Forest 84% > overall accuracy 65%  good for classify forest?! User’s accuracy for forest is only 60% Only good for classify water

Self test 2 Employ all methods and concepts of classification that you have learned so far to classify the file ca_coast.dat carefully. The ground truths in the validation region will be provided next week in the form of ROIs to assess your result.

Tutorial: multispectral classification
Read image File → Open Image File Subdirectory: envidata File: can_tmr.img RGB Color Bands 4, 3, and 2 Review Image Colors False color infrared photograph Bright red areas → high infrared reflectance → healthy vegetation → under cultivation, or along rivers Slightly darker red areas → native vegetation → coniferous trees Several distinct geologic and urbanization classes are also readily apparent as is urbanization Cursor Location/Value Examine Spectral Plots Tools → Profiles → Z Profile (Spectrum) Note the relations between image color and spectral shape Pay attention to the location of the image bands in the spectral profile, marked by the red, green, and blue bars in the plot

Tutorial: multispectral classification (cont.)
Unsupervised Classification Classification → Unsupervised → K-Means or IsoData K-Means Uses a cluster analysis approach which requires the analyst to select the number of clusters to be located in the data, arbitrarily locates this number of cluster centers, then iteratively repositions them until optimal spectral separability is achieved Choose K-Means as the method, use all of the default values and click on OK Review the results contained in can_km.img. Experiment with different numbers of classes, change thresholds, standard deviations, and maximum distance error values to determine their effect on the classification. Isodata Calculates class means evenly distributed in the data space and then iteratively clusters the remaining pixels using minimum distance techniques. Each iteration recalculates means and reclassifies pixels with respect to the new means Choose IsoData as the method, use all of the default values and click on OK, or Review the results contained in can_iso.img.

Regions of Interest (ROI) Select Training Sets Using Regions of Interest (ROI) Restore Predefined ROIs Choosing from the #1 Main Image menu bar Overlay → Region of Interest Choose File → Restore ROIs File: CLASSES.ROI Create Your Own ROIs Overlay → Region of Interest Draw a polygon Fix the polygon by clicking the right mouse button a second time New Region Edit

Supervised Classification Supervised classification requires that the user select training areas for use as the basis for classification Classification → Supervised → [method] [method] is one of the supervised classification methods in the pull-down menu (Parallelepiped, Minimum Distance, Mahalanobis Distance, Maximum Likelihood, Spectral Angle Mapper, Binary Encoding, or Neural Net). Use one of the two methods below for selecting training areas, also known as regions of interest (ROIs).

Classical Supervised Multispectral Classification Parallelepiped Uses a simple decision rule to classify multispectral data. The decision boundaries form an n-dimensional parallelepiped in the image data space. The dimensions of the parallelepiped are defined based upon a standard deviation threshold from the mean of each selected class Pre-saved results are in the file can_pcls.img Perform your own classification using the CLASSES.ROI regions of interest Maximum Likelihood Assumes that the statistics for each class in each band are normally distributed Calculates the probability that a given pixel belongs to a specific class Unless a probability threshold is selected, all pixels are classified Each pixel is assigned to the class that has the highest probability Minimum Distance Uses the mean vectors of each ROI and calculates the Euclidean distance from each unknown pixel to the mean vector for each class Mahalanobis Distance A direction sensitive distance classifier that uses statistics for each class Assumes all class covariances are equal and therefore is a faster method

Spectral Classification Methods Developed specifically for use on Hyperspectral data, but provide an alternative/improved method for classifying multispectral data The Endmember Collection Dialog Spectral → Mapping Methods → Endmember Collection (Classification → Endmember Collection) Open File File: can_tmr.img Endmember Collection: Parallel dialog Algorithm → [method] [method] represents: Parallelepiped, Minimum Distance, Manlanahobis Distance, Maximum Likelihood, Binary Encoding, and the Spectral Angle Mapper (SAM)

Spectral Classification Methods (cont.) Binary Encoding Classification Encodes the data and endmember spectra into 0s and 1s based on whether a band falls below or above the spectrum mean An exclusive OR function is used to compare each encoded reference spectrum with the encoded data spectra and a classification image is produced All pixels are classified to the endmember with the greatest number of bands that match unless the user specifies a minimum match threshold, in which case some pixels may be unclassified if they do not meet the criteria Algorithm → Binary Encoding Import → from ROI from Input File Select All Items Endmember Spectra Options → Plot Endmembers Apply Binary Encoding Parameters

Spectral Classification Methods (cont.) Spectral Angle Mapper Classification Uses the n-dimensional angle to match pixels to reference spectra Determines the spectral similarity between two spectra by calculating the angle between the spectra, treating them as vectors in a space with dimensionality equal to the number of bands Endmember Collection Algorithm → Spectral Angle Mapper

Post Classification Processing Classification Method → Rule Image Values Represent Parallelepiped Number of bands that satisfied the parallelepiped criteria Minimum Distance Sum of the distances from the class means Maximum Likelihood Probability of pixel belonging to class Mahalanobis Distance Distances from the class means Binary Encoding Binary Match in Percent Spectral Angle Mapper Spectral Angle in Radians Tools → Color Mapping → ENVI Color Tables Stretch Bottom and Stretch Top sliders Cursor Location/Value Classification → Post Classification → Rule Classifier File: can_tmr.sam Rule Image Classifier Tool

Post Classification Processing (cont.) Class Statistics Classification → Post Classification → Class Statistics Select All Items Confusion Matrix Comparison of two classified images (the classification and the “truth” image), or a classified image and ROIs The truth image can be another classified image, or an image created from actual ground truth measurements Classification → Post Classification → Confusion Matrix → [method] Using Ground Truth Image, or Using Ground Truth ROIs. Match Classes Parameters dialog

Post Classification Processing (cont.) Clump and Sieve For generalizing classification images, Sieve is usually run first to remove the isolated pixels based on a size (number of pixels) threshold. Clump is run to add spatial coherency to existing classes by combining adjacent similar classified areas Compare the pre-calculated results in the files can_sv.img (sieve) and can_clmp.img (clump of the sieve result) to the classified image can_pcls.img Classification → Post Classification → Sieve Classes Classification → Post Classification → Clump Classes Combine Classes Classification → Post Classification → Combine Classes File: can_sam.img Add Combination

Post Classification Processing (cont.) Edit Class Colors Tools → Color Mapping → Class Color Mapping To make the changes permanent, select Options → Save Changes Overlay Classes Classification → Post Classification → Overlay Classes Select can_tmr.img band 3 for each RGB band Use can_comb.img as the classification input Interactive Classification Overlays Interactively toggle classes on and off as overlays on a displayed image, to edit classes, get class statistics, merge classes, and edit class colors. Display band 4 of can_tmr.img Overlay → Classification Try the various options for assessing the classification under the Options menu Choose various options under the Edit menu to interactively change the contents of specific classes File → Save Image As → [Device]

Post Classification Processing (cont.) Classes to Vector Layers Overlay → Vectors File: can_clmp.img File → Open Vector File → ENVI Vector File Files: can_v1.evf and can_v2.evf. Select All Layers Load Selected Classification → Post Classification → Classification to Vector Raster to Vector Input Band dialog. Choose the generalized image can_clmp.img Select Region #1 and Region #2 and enter the root name canrtv Load Selected at the bottom of the dialog. Load Vector Edit→Edit Layer Properties Classification Keys Using Annotation Overlay → Annotation Object → Map Key Edit Map Key Items

Chapter 5 Image Classification

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