# What is Unsupervised change detection

##### Unsupervised Change Detection: What it is and How it Works

When we talk about change detection, we refer to the process of determining the differences between two images of the same scene taken at different times. Usually, this is done by comparing the pixel values of the two images, and these changes are highlighted by differences in intensity or color. In most cases, change detection is done in a supervised way, meaning that the changes are detected by comparing the two images using a predetermined set of criteria. However, this method has its limitations. Some changes might be missed if the criteria are too strict, while others might be falsely identified as changes if the criteria are too lax. This is where unsupervised change detection comes in – a technique for identifying changes between two images that does not require prior knowledge or manual effort.

What is Unsupervised Change Detection?

Unsupervised change detection is a technique for identifying changes between two images of a scene without any prior knowledge of the type or location of the changes. This technique is based on the idea that changes in a scene can be detected by looking for areas in which the statistical properties of the pixels have changed.

The unsupervised change detection algorithm works by comparing the statistical distribution of the pixels in two images of the same scene. The algorithm then identifies those areas in which the statistical properties of the pixels have changed. This is done by calculating the difference between the statistical distribution of the pixels in the two images. When the difference is significant, it is assumed that a change has occurred, and these areas are marked as changed areas.

How Does Unsupervised Change Detection Work?

The basic idea behind unsupervised change detection is that changes in the statistical properties of the pixels between two images of the same scene indicate a change in the scene itself. The process of unsupervised change detection involves several steps, which are as follows:

• Step 1 – Image Registration: This step involves aligning the two images to ensure that corresponding pixels in both images are at the same location. This is done to ensure that the comparison between the two images is accurate.
• Step 2 – Image Segmentation: This step involves dividing the images into smaller, non-overlapping regions, each of which contains a set of pixels with uniform statistical properties.
• Step 3 – Feature Extraction: This step involves extracting a set of features from each region of the image. The features might include the mean, standard deviation, or some other statistical property of the pixels in a region.
• Step 4 – Feature Comparison: This step involves comparing the feature vectors extracted from each region of the two images. The comparison is done using a similarity measure, such as the Euclidean distance or the Mahalanobis distance.
• Step 5 – Change Detection: This step involves identifying areas of the image where the feature vectors have changed significantly between the two images. These areas are marked as changed areas.
The Advantages of Unsupervised Change Detection:

Unsupervised change detection has several advantages over supervised methods, including:

• No Prior Knowledge Required: The technique does not require any prior knowledge of the type or location of the changes that may have occurred in the scene.
• No Manpower Required: The technique does not require any manual effort, making it ideal when working with large sets of images.
• Higher Accuracy: Unsupervised change detection is less prone to errors due to strict or lax criteria compared to supervised methods
The Limitations of Unsupervised Change Detection:

Although unsupervised change detection has several advantages, it also has some limitations, including:

• Inaccurate Results: The technique may identify false positives or negatives in areas where the statistical properties of the pixels may have changed due to other factors, such as lighting conditions or weather changes.
• High Processing Power: The technique requires high processing power, especially when comparing large sets of images.
• Difficulty in Interpretation: Unlike supervised methods, unsupervised change detection does not provide any information on the type or location of the changes, making it difficult to interpret the results correctly.
Applications of Unsupervised Change Detection:

Unsupervised change detection has several applications, including:

• Remote Sensing: The technique is widely used in remote sensing to identify changes in land cover over time.
• Surveillance: The technique is used in surveillance systems to detect changes in an environment that may indicate a security threat.
• Agriculture: The technique can be used in agriculture to track crop growth and identify areas of pest infestation or disease.
Conclusion:

Unsupervised change detection is a powerful technique for identifying changes between two images of a scene without any prior knowledge. Although it has some limitations, the technique offers several advantages, including higher accuracy and no need for manual efforts or prior training data. Unsupervised change detection has several applications, from remote sensing and agriculture to surveillance systems to detect security threat. With its capabilities, unsupervised change detection can be an important tool in remote sensing and anomaly detection systems.