what is object-based analysis
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What is Object-Based Image Analysis?
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Kirk Benell
What is an object?
• An object is a region of interest with spatial, spectral (brightness and color), and/or texture characteristics that define the region
Object-Based Image Analysis
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region
• Pixels are grouped into objects, instead of single pixel analysis
• May provide increased accuracy and detail for classification purposes
Object-Based Image Analysis
Traditional pixel-based classification• Based on reflectance values of pixels• Works for low and medium resolution imagery• Works for mass area-based features• Multispectral or hyperspectral imagery
Limitations of pixel-based analysis
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Limitations of pixel-based analysis• Only spectral, seldom spatial and contextual• Results with inconsistent salt-and-pepper noise • Inaccurate borders for texture computation• Limited extraction of small-scale objects
Pixel-based Classification
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Reflectance
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Group materials based on their reflectance response per pixel
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Band
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Object-Based Image Analysis
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• Group contiguous pixels into objects
• Objects are classified into feature classes based on their spatial, textural and spectral attributes
ImagePixels
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Objects
• Greater accuracy from input: tone, color, texture, shape, size, orientation, pattern, shadow, situations
• Advanced visualizations: Computer vision technique using image segmentation
• Use homogeneous regions as basic analysis elements
• Additional spatial, contextual and semantic information
Object-Based Image Analysis
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• Additional spatial, contextual and semantic information
ENVI Feature Extraction• Uses an object-based approach to classify imagery
• The ENVI tool provides an easy to use method for extracting information from panchromatic, multispectral, hyperspectral, and elevation data
• Vehicles
• Buildings
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• Buildings
• Transportation
• Natural Features
Needs for Feature Extraction• Increased availability of high-
resolution images• Manual digitization, labor intensive• Semi-automated solution is highly
desired
ENVI Feature Extraction
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desired
Applications• Defense and Intelligence• Geographic Information Systems• Transportation• Urban planning and mapping
Workflow:• Spectral/spatial/texture attributes• Object-based fuzzy rule-based
classification• Object-based supervised
classification
ENVI Feature Extraction
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Preview Window for instant feedback prior to processing an entire image
Post-Classification Vector Tool• Centerline extraction• Snapping, smoothing• Vector editing
ENVI Feature Extraction
Input Data
Image Segmentation
Attribute Computation for Object Primitives
Object Generation
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Rule Base
Decision Making
Feature Selection
Supervised Classification
Extracted Features/Classes
Object-Based Classification
Image Segmentation Scale Level
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A low scale level provides moresegments in the final processed image
A high scale level provides fewersegments in the final processed image
The Preview Window provides on-the-fly feedback for the selected Scale Level
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Segmentation scale level = 50
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Under segementedscale level = 70
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Over segmentedscale level = 30
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Merge to aggregate adjacent segments
Select Classification Method• Select Classify by
Selecting Examples to select training data and perform a supervised classification
• Select Classify by Creating Rules to select attribute
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select attribute parameters to perform a classification
• Select Export Vectorsto export without performing a classification
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View attributes to characterize feature of interest
Create rules to define features of interest
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ENVI Feature ExtractionSpatial Attributes
• Region area, length, compactness, convexity, solidity, form factor, rectangular fit, roundness, elongation, main axis direction, axes length, number of holes, hole/solidity ratio
Spectral Attributes• Band minimum, maximum, average and standard deviation
Texture Attributes
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Texture Attributes• Variance, range, mean, and entropy
Color Space and Band Ratio• Hue, saturation, intensity, NDVI, NDWI, other ratios
Preview classification results and adjust training data on-the-fly
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Export features as one or individual vectors
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View Feature Extraction Report
• View parameters used and statistics of exported vectors
• Save as a text report to share with colleagues
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colleagues
• Edit vector properties
• View Attribute Information
• Square-up building sides
• Smooth vectors
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• Push data into ArcMap for further analysis and vector editing
• Add imagery and new vector layer to GIS database
Thank You
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