The Battle of Grand Gulf was fought on April 29, 1863, during th: These standards acknowledge that standard reference systems also exist for measuring elevation using vertical datums and  time (e.g. ISO 8601), which may be combined with a spatial reference system to form a compound coordinate system for representing three-dimensional and/or spatio-temporal locations. There are also internal systems for measuring location within the context of an object, such as the rows and columns of pixels in a  raster image, Linear referencing measurements along linear features (e.g., highway mileposts), and systems for specifying location within moving objects such as ships. The latter two are often classified as subcategories of engineering coordinate systems.