As you can see, Isomap is an Unsupervised Machine Learning technique aimed at Dimensionality Reduction.
It differs from a few other techniques in the same category by using a non-linear approach to dimensionality reduction instead of linear mappings used by algorithms such as PCA. We will see how linear vs. non-linear approaches differ in the next section.
How does Isometric Mapping (Isomap) work?
Isomap is a technique that combines several different algorithms, enabling it to use a non-linear way to reduce dimensions while preserving local structures.
Before we look at the example of Isomap and compare it to a linear method of Principal Components Analysis (PCA), let’s list the high-level steps that Isomap performs:
- Use a KNN approach to find the k nearest neighbors of every data point. Here, “k” is an arbitrary number of neighbors that you can specify within model hyperparameters.
- Once the neighbors are found, construct the neighborhood graph where points are connected to each other if they are each other’s neighbors. Data points that are not neighbors remain unconnected.
- Compute the shortest path between each pair of data points (nodes). Typically, it is either Floyd-Warshall or Dijkstra’s algorithm that is used for this task. Note, this step is also commonly described as finding a geodesic distance between points.
- Use multidimensional scaling (MDS) to compute lower-dimensional embedding. Given distances between each pair of points are known, MDS places each object into the N-dimensional space (N is specified as a hyperparameter) such that the between-point distances are preserved as well as possible.
For our example, let’s create a 3D object known as a Swiss roll. The object is made up of 2,000 individual data points. The chart is interactive so make sure to rotate it to familiarize yourself with its exact shape.
Next, we want to map this 3-dimensional swiss roll 2 dimensions using Isomap. To track what happens during this transformation, let’s select two points: A and B.
We can see that these two points are relatively close to each other within the 3D space. If we used a linear dimensionality reduction approach such as PCA, then the Euclidean distance between these two points would remain somewhat similar in lower dimensions. See PCA transformation chart below:
Continue reading: https://towardsdatascience.com/isomap-embedding-an-awesome-approach-to-non-linear-dimensionality-reduction-fc7efbca47a0?source=rss—-7f60cf5620c9—4