A trig point is a small tower that's part of a network that forms a conceptual grid of triangles over a geographical region.
A very useful tool for building computer models of a region, but also seems like it must have involved a whole lot of work to set up; the points have to be placed such that from any one of them, you can see at least two others.
They were apparently common in the UK in the 1930s and Australia in the 1970s, but it's not clear to me how much of each country was covered by them. I'm kind of surprised not to have come across them in fiction, but maybe I just haven't noticed.
My first thought was that perhaps there was some connection here to the colonel who's a "trig / westpointer most succinctly bred" in Cummings's poem "i sing of Olaf glad and big." But it turns out that's an unrelated word meaning "stylishly or jauntily trim" (according to MW11); comes from Middle English, related to Old Norse and Old English words meaning "faithful"; distantly related to "true."