By default, ELKI will produce a directory with one file per cluster. Unless the output file already exists, in which case you will get all the clusters written into the same file, separated with comments as seen above.
With a hierarchical result, such as OPTICSXi
, your should however also treat all members of the child clusters to be also part of the parent. These are clusters nested into the parent. They are not repeated in the parent, to reduce redundancy in the output.
Compare the output of OPTICSXi
to OPTICS
output. What the Xi approach does, is split the data for you, based on sudden drops in reachability-distance. All clusters of Xi should be subsequences of the original OPTICS cluster order.
In your case, you may have chosen minPts
too small, if your cluster has just 4 elements. (Although, you may have truncated the file, or you may have a lot of elements in child clusters; so the output may be fine).
Also note that you will usually want to validate whether you want the first element(s) of your cluster to belong to the cluster or not; similarly the last elements. OPTICSXi
tends to err on the first elements, but not in a systematic way that would be trivial to fix. The first and last elements are those that bridge the gap from one cluster to another. You really should verify these manually (which is a good reason to not choose minPts
too small).
I strongly recommend to build/use a visualization for your specific use case. Then you could just load such a cluster into your visualization and visually inspect if the result makes sense to you. I have used OPTICSXi
on geographic data, and that worked very well for me.