1. A land classification system operable for analysis of very high resolution (VHR) remotely-sensed multispectral Earth imagery, comprising: an image store containing image data corresponding to VHR remotely-sensed multispectral Earth images; at least one feature extraction module in operative communication with the image store, the feature extraction module being operable to produce feature data regarding at least a portion of image data; a feature stack comprising the image data and the feature data; a client interface operable to receive training data regarding at least one or more pixels of the image data from a user regarding at least one class to which the one or more pixels belong; a classification compute module operable to generate a classification model at least in part based on a portion of the feature stack corresponding to the one or more pixels and the training data, wherein the classification model relates to classification of pixels of image data into one or more classes; and a plurality of feature extraction modules each selectively applicable to image data by a user to generate a feature data layer of the feature stack, wherein at least one of the feature extraction modules comprises a rotational invariant histogram of gradients (HOG) feature extraction module that is operative to generate feature data by: establishing a plurality of angular bins for an image in a gradient domain; histogramming gradients from the image with respect to the plurality of angular bins; selecting based on the histogramming the angular bin with the largest histogram value to define a primary direction; setting a plurality of pooling windows with relative offsets from a pixel of interest; defining a gradient orientation window encompassing the plurality of pooling windows; configuring the gradient orientation window to align the configuration of the gradient orientation window relative to the primary direction; shifting the positions of the pooling windows relative to the gradient orientation window based on the configuring; and rotating the internal orientation of the pooling windows based on the configuring.