BES Aggregation using NcML

There are three main scenarios for aggregation that we have encountered so far in addition to the simple situation where a group of otherwise discreet variables are combined in a Structure to be manipulated as a single variable. Those cases are tiling, combination of parameters held in separate files and grouping M N-dimensional variables into a single N+1-dimensional variable. A fourth form of aggregation which has emerged is 'tiling in time.' That is, it is essentially tiling but it is useful to separate tiling in the abstract sense from two common cases: tiling over latitude and longitude and tiling over time. Both latitude and longitude are periodic, so tiling needs to take this into account. Time, on the other hand, is generally not periodic (although climatologies could be stored in separate files and tiled along their time dimension).

In all cases several discreet data sets (i.e., URLs) are combined into a single data set; The discreet data sets are still capable of being referenced using their DAP URLs, but the result of the aggregation is a new DAP URL which references the aggregate. Building such a data set makes it possible to query for certain images using the DAP constraint expression, effectively making it indistinguishable from a data access operation.

Increasing dimensionality

The most common example of this is the combination of a large set of satellite images, all of which are regular with respect to one another in latitude and longitude, which span some time range. The image are not generally equally spaced in time, however. The result is a N+1 dimensional grid variable with a new Grid Map that must be 'synthesized' using date/time information from some source (an attribute stored in the image file or the file name itself).