How can I classify forest cover into stratified classes of forest canopy density with 12-band sentinel-2 data?
I am attempting use recent sentinel-2 data to classify all forest over 5m in height. If it is possible I would also like canopy density (as a metric for percentage tree cover) however I am not sure how feasible this is without control points on the ground.
I have read that it is possible to use the shadowing in the panchromatic band in conjunction with the NIR and IR bands to determine depth but this does not appear available with Sentinel-2.
I have completed forest inventory before utilizing LiDar and on another occasion with multi-spectral imagery, ground control points and object based classification. However I have never attempted it using solely multi-spectral imagery.
A method has been developed previously which used multispectral satellite imagery from the [Landsat 7] thematic mapper plus (ETM+) sensor and a supervised learning algorithm to determine tree cover but details regarding the finer points of the method are scant.
I am unfortunately limited to Arc 10 [Advanced Licence] with just the Spatial Analyst and Geo-statistical Analyst extensions.
Any assistance with this would be greatly appreciated.
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suppose i have 10 individual observations each of size (125,59). i want to group these 10 observations based on their 2d feature matrices ((125,59)).Is this possible without flattening every observation to 125*59 1D matrix ? I cant even implement PCA or LDA for feature extraction because the data is highly variant. Please note that i am trying to implement clustering through self organizing maps or neural networks. Deep learning and neural networks are completely related to the question asked.
Of course it is.
Define an appropriate distance measure.
Then compute the 10x10 distance matrix, and run hierarchical clustering.
I've got a set of F features e.g. Lab color space, entropy. By concatenating all features together, I obtain a feature vector of dimension d (between 12 and 50, depending on which features selected.
I usually get between 1000 and 5000 new samples, denoted x. A Gaussian Mixture Model is then trained with the vectors, but I don't know which class the features are from. What I know though, is that there are only 2 classes. Based on the GMM prediction I get a probability of that feature vector belonging to class 1 or 2.
My question now is: How do I obtain the best subset of features, for instance only entropy and normalized rgb, that will give me the best classification accuracy? I guess this is achieved, if the class separability is increased, due to the feature subset selection.
Maybe I can utilize Fisher's linear discriminant analysis? Since I already have the mean and covariance matrices obtained from the GMM. But wouldn't I have to calculate the score for each combination of features then?
Would be nice to get some help if this is a unrewarding approach and I'm on the wrong track and/or any other suggestions?
One way of finding "informative" features is to use the features that will maximise the log likelihood. You could do this with cross validation.
https://www.cs.cmu.edu/~kdeng/thesis/feature.pdf
Another idea might be to use another unsupervised algorithm that automatically selects features such as an clustering forest
http://research.microsoft.com/pubs/155552/decisionForests_MSR_TR_2011_114.pdf
In that case the clustering algorithm will automatically split the data based on information gain.
Fisher LDA will not select features but project your original data into a lower dimensional subspace. If you are looking into the subspace method
another interesting approach might be spectral clustering, which also happens
in a subspace or unsupervised neural networks such as auto encoder.
I'm implementing color quantization based on k-means clustering method on some RGB images. Then, I will determine the performance the algorithm. I found some information about training and testing. As I understand, I should divide the samples of images for training and testing.
But I am confused about the terms training and testing. What does these mean ? And how to implement with a rank value ?
Training and testing are two common concepts in machine learning. Training and testing are more easily explained in the framework of supervised learning; where you have a training dataset for which you know both input data as well as additional attributes that you want to predict. Training consists in learning a relation between data and attributes from a fraction of the training dataset, and testing consists in testing predictions of this relation on another part of the dataset (since you know the prediction, you can compare the output of the relation and the real attributes). A good introductory tutorial using these concepts can be found on http://scikit-learn.org/stable/tutorial/basic/tutorial.html
However, clustering is a class of unsupervised learning, that is, you just have some input data (here, the RGB values of pixels, if I understand well), without any corresponding target values. Therefore, you can run a k-means clustering algorithm in order to find classes of pixels with similar colors, without the need to train and test the algorithm.
In image processing, training and testing is for example used for classifying pixels in order to segment different objects. A common example is to use a random forest classifier: the user selects pixels belonging to the different objects of interest (eg background and object), the classifier is trained on this set of pixels, and then the remainder of the pixels are attributed to one of the classes by the classifier. ilastik (http://ilastik.org/) is an example of software that performs interactive image classification and segmentation.
I don't know which programming language you're using, but k-means is already implemented in various libraries. For Python, both SciPy (http://docs.scipy.org/doc/scipy/reference/generated/scipy.cluster.vq.kmeans2.html#scipy.cluster.vq.kmeans2) and scikit-learn (http://scikit-learn.org/stable/modules/generated/sklearn.cluster.KMeans.html) have an implementation of K-means. Also note that, depending on your application, you may be interested in clustering pixels together using not only pixels values, but also spatial proximity of pixels. See for example the scikit-image gallery example http://scikit-image.org/docs/dev/auto_examples/plot_rag_mean_color.html
If the data to cluster are literally points (either 2D (x, y) or 3D (x, y,z)), it would be quite intuitive to choose a clustering method. Because we can draw them and visualize them, we somewhat know better which clustering method is more suitable.
e.g.1 If my 2D data set is of the formation shown in the right top corner, I would know that K-means may not be a wise choice here, whereas DBSCAN seems like a better idea.
However, just as the scikit-learn website states:
While these examples give some intuition about the algorithms, this
intuition might not apply to very high dimensional data.
AFAIK, in most of the piratical problems we don't have such simple data. Most probably, we have high-dimensional tuples, which cannot be visualized like such, as data.
e.g.2 I wish to cluster a data set where each data is represented as a 4-D tuple <characteristic1, characteristic2, characteristic3, characteristic4>. I CANNOT visualize it in a coordinate system and observes its distribution like before. So I will NOT be able to say DBSCAN is superior to K-means in this case.
So my question:
How does one choose the suitable clustering method for such an "invisualizable" high-dimensional case?
"High-dimensional" in clustering probably starts at some 10-20 dimensions in dense data, and 1000+ dimensions in sparse data (e.g. text).
4 dimensions are not much of a problem, and can still be visualized; for example by using multiple 2d projections (or even 3d, using rotation); or using parallel coordinates. Here's a visualization of the 4-dimensional "iris" data set using a scatter plot matrix.
However, the first thing you still should do is spend a lot of time on preprocessing, and finding an appropriate distance function.
If you really need methods for high-dimensional data, have a look at subspace clustering and correlation clustering, e.g.
Kriegel, Hans-Peter, Peer Kröger, and Arthur Zimek. Clustering high-dimensional data: A survey on subspace clustering, pattern-based clustering, and correlation clustering. ACM Transactions on Knowledge Discovery from Data (TKDD) 3.1 (2009): 1.
The authors of that survey also publish a software framework which has a lot of these advanced clustering methods (not just k-means, but e.h. CASH, FourC, ERiC): ELKI
There are at least two common, generic approaches:
One can use some dimensionality reduction technique in order to actually visualize the high dimensional data, there are dozens of popular solutions including (but not limited to):
PCA - principal component analysis
SOM - self-organizing maps
Sammon's mapping
Autoencoder Neural Networks
KPCA - kernel principal component analysis
Isomap
After this one goes back to the original space and use some techniques that seems resonable based on observations in the reduced space, or performs clustering in the reduced space itself.First approach uses all avaliable information, but can be invalid due to differences induced by the reduction process. While the second one ensures that your observations and choice is valid (as you reduce your problem to the nice, 2d/3d one) but it loses lots of information due to transformation used.
One tries many different algorithms and choose the one with the best metrics (there have been many clustering evaluation metrics proposed). This is computationally expensive approach, but has a lower bias (as reducting the dimensionality introduces the information change following from the used transformation)
It is true that high dimensional data cannot be easily visualized in an euclidean high dimensional data but it is not true that there are no visualization techniques for them.
In addition to this claim I will add that with just 4 features (your dimensions) you can easily try the parallel coordinates visualization method. Or simply try a multivariate data analysis taking two features at a time (so 6 times in total) to try to figure out which relations intercour between the two (correlation and dependency generally). Or you can even use a 3d space for three at a time.
Then, how to get some info from these visualizations? Well, it is not as easy as in an euclidean space but the point is to spot visually if the data clusters in some groups (eg near some values on an axis for a parallel coordinate diagram) and think if the data is somehow separable (eg if it forms regions like circles or line separable in the scatter plots).
A little digression: the diagram you posted is not indicative of the power or capabilities of each algorithm given some particular data distributions, it simply highlights the nature of some algorithms: for instance k-means is able to separate only convex and ellipsoidail areas (and keep in mind that convexity and ellipsoids exist even in N-th dimensions). What I mean is that there is not a rule that says: given the distributiuons depicted in this diagram, you have to choose the correct clustering algorithm consequently.
I suggest to use a data mining toolbox that lets you explore and visualize the data (and easily transform them since you can change their topology with transformations, projections and reductions, check the other answer by lejlot for that) like Weka (plus you do not have to implement all the algorithms by yourself.
In the end I will point you to this resource for different cluster goodness and fitness measures so you can compare the results rfom different algorithms.
I would also suggest soft subspace clustering, a pretty common approach nowadays, where feature weights are added to find the most relevant features. You can use these weights to increase performance and improve the BMU calculation with euclidean distance, for example.
I am new to hog, I am using opencv2.4.4 and visual studio 2010, i am running the sample peopledetect.cpp in the package and its compiling and running, but i want to understand the the source code in detail.In peopledetect.cpp is hog descriptors constructed/ already trained for peopledetection 3780 vectors are fed into svm classifier? when i try to debug the peopledetect.cpp i could only find HOGDescriptor creates hog descriptor and detector, i basically doesnt understand what this API does HOGDescriptor as i see peopledetect.cpp doesnt go through the steps of hog processing, it loads the already trained vectors to svm classifier to detect people/no people, am i wrong?. As there is no documentation about this.
Can anyone please brief about this.
The implementation of People Detection algorithm in opencv is based on HOG descriptors as features and SVM as classifier.
1. A training database (positives samples as person, negatives samples as non-person) is used to learn to SVM parameters (it computes and store the support vectors). Cross-validation is also perform (I assume) to optimize the soft margin parameter C and the kernel parameters (it could be linear kernel).
2. To detect people on testing video data, peopledetect.cpp loads the pre-learnt SVM, computes the HOG descriptors on different positions and scales, then merges the windows with high detection scores (outputs of binary SVM classifer).
Here is a good paper (inria) to start with.
Coming to more clearer answer, peopledetect.cpp goes through all the hog steps.
digging deeper i was more clear. Basically if you debug peopledetect.cpp goes through these steps.
Initially image is divided into several scales, scale0(1.05) is coefficient for detection window increase. For each scale of the image features are extracted from window and a classifier window is run, like above it follows scale-space pyramid method. So its pretty big computational process, very expensive, so opencv team has tried to parallelise for each scale.
I was baffled before why i was not able to debug/go through the steps, This parallel_for_(Range(0, (int)levelScale.size()),HOGInvoker()) creates several threads where each thread works on each scale, depends how much threads or created something like this.
because of this i was not able to debug, what i did was freeze all the threads and debug only the main thread. for different scales of the image hog processing steps are
Here in peopledetect.cpp hog and classifier window are kinda combined.In a single window(64x128) both feature extraction and running classifier takes place. After this is done for each scale of the image. There are a number of pedestrian windows of different scales are often associated with this region, this is grouped using grouprectangle() function
Training SVM consist to find parameters of the max margin between postive and negative samples.
if the same feature extraction is done for 1000+ negative and positive sample there is must be millions of features rite?
Yes. These coefficient are extracted from training databases. You don't have them. SVM stores only support vectors which are sufficient to characterise the margin. See dual form of linear SVM for example.
a number of pedestrian windows of different scales are often associated with the region
True. A merging function is apply. Different methods (such groupRectangles(..)) are available (see here) and take in arguments parameters given to detectMultiScale(..).
What i understood from different papers is that feature extraction using hog is done using several positive and negative images, these features which were extracted is fed to Linear SVM to train them,So peopledetect.cpp uses this trained linear SVM sample, so This feature extraction process is not done by peopledetect.cpp i.e HOGDescriptor::getDefaultPeopleDetector() consists of coefficients of the classifier trained for people detection. The features extracted from hog detection/window(64x128)gives a total of length 3780(4 cells x 9 bins x 7 x 15 blocks = 3780) features. These features are then used to train a linear SVM classifier. If the same feature extraction is done for 1000+ negative and positive sample there is must be millions of features rite? How do we get these co-efficients?
But The HOG descriptors are known to contain redundant information because of the different detection window sizes being used. So when the SVM classifier classifies a region as “pedestrian”, a number of pedestrian windows of different scales are often associated with the region. what peopledetect.cpp mainly does is (hog.detectMultiScale(img, found, 0, Size(8,8), Size(32,32), 1.05, 2);) The detection window is scanned across the image at all positions and scales, and conventional non-maximum suppression is run on the output pyramid to detect object instances.