File:Mortar bty mark.jpg

This survey marker, likely set between 1905 and 1920, is located on top of the parapet that separates mortar pits A and B of Battery Whitman at Fort Andrews, a Coast Artillery fort on Peddocks Island in Boston Harbor. The point marked is the countersunk hole in the head of the brass or copper bolt that is embedded in a concrete-filled piece of sewage pipe buried in the ground. This marker (or a rod erected over it) would have been visible from all four mortars in each pit.

Although this marker was not strictly a directing point (since 12-inch coast defense mortars did not use these), it is of the same form as such a mark and was used to aim the mortars in azimuth.

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