One of the most famous instances of surmise in English literature occurs near the end of Keats's poem "On first looking into Chapman's Homer":
Then felt I like some watcher of the skies
When a new planet swims into his ken;
Or like stout Cortez, when with eagle eyes
He stared at the Pacific—and all his men
Look'd at each other with a wild surmise—
Silent, upon a peak in Darien.
Here, the implied surmise of the men who have just marched across some piece of the New World is that another, previously unsuspected ocean lies on the other side of that New World. If anything, it's a conjecture based not on disparate pieces of evidence reasoned into a meaningful fabric, but on a flash of insight about how much larger the world is than they had theretofore imagined.
I'm not sure how much of the real-world sense of any word is beholden to its use in a particular piece of literature, but to me surmise is never too far away from the shadow of "wild surmise" cast by this poem—even when the surmise in question seems relatively sober, sedate, and proportionate.
Beyond that, I agree with both AVM and deadrat that a surmise is not equivalent to a mere guess, but is indeed a conclusion drawn from one or more precedent conditions.