1. A method for use in identifying compound structures in overhead imagery, comprising: organizing, using a processor, a plurality of pixels of at least one input overhead image into a plurality of components of a first hierarchical data structure, wherein the input image is associated with a geographic area; deriving, using the processor, at least one image descriptor for each of the components; constructing, using the processor with at least some of the image descriptors, a second hierarchical data structure that includes a plurality of hierarchically-arranged levels of nodes, wherein the nodes of the second hierarchical data structure are arranged into a plurality of leaf paths that each extend from a root node to one of a plurality of leaf nodes, wherein each of the image descriptors depends from one of the plurality of leaf nodes, wherein at least one node at a particular one of the levels is a respective visual word, and wherein all of the visual words at the particular one of the levels collectively comprise a visual dictionary; selecting a type of compound structure to be identified in the at least one input overhead image; overlaying a grid of cells onto the at least one input overhead image, wherein the overlaying includes selecting a width of each cell in the grid to be approximately equal to a spatial extent of the selected compound structure type; determining, for at least some of the cells of the grid of cells, a frequency of each of one or more of the visual words in the cell; generating, for each of the at least some of the cells, a vector that includes a plurality of entries, wherein each entry includes a value that represents a respective one of the frequencies; and for each of the at least some of the cells, using its vector to categorize the cell as representing or not representing the selected compound structure type.