What is Object-Based Analysis

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

Visual Information Solutions

region

• Pixels are grouped into objects, instead of single pixel analysis

• May provide increased accuracy and detail for classification purposes


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


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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Group materials based on their reflectance response per pixel

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• Group contiguous pixels into objects

• Objects are classified into feature classes based on their spatial, textural and spectral attributes

ImagePixels

SegmentedObjects

FeatureThe Letter ‘E’

MergedSegmented

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



• 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


• 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


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



Preview Window for instant feedback prior to processing an entire image

Post-Classification Vector Tool• Centerline extraction• Snapping, smoothing• Vector editing


Input Data

Image Segmentation

Attribute Computation for Object Primitives

Object Generation


Rule Base

Decision Making

Feature Selection

Supervised Classification

Extracted Features/Classes

Object-Based Classification

Image Segmentation Scale Level


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


Segmentation scale level = 50


Under segementedscale level = 70


Over segmentedscale level = 30


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


select attribute parameters to perform a classification

• Select Export Vectorsto export without performing a classification


View attributes to characterize feature of interest

Create rules to define features of interest


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


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


Export features as one or individual vectors


View Feature Extraction Report

• View parameters used and statistics of exported vectors

• Save as a text report to share with colleagues


colleagues

• Edit vector properties

• View Attribute Information

• Square-up building sides

• Smooth vectors



• Push data into ArcMap for further analysis and vector editing

• Add imagery and new vector layer to GIS database

Thank You


What is Object-Based Analysis

Technology

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