The paper is describing a 3-dimensional map algebra (<a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Map_algebra" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Map_algebra</a>), similar to what is used in array databases (<a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Array_DBMS" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Array_DBMS</a>) like SciDB and rasdaman.