sdi

This particular topic isn't covered in any geomancy-specific text I've read (though it may be discussed in Greer's books on magic, which I have not read). There was a recent discussion in conjunctio on protecting the integrity of one's divinations, though! The gist of the discussion was:

• There certainly appear to be beings who are interested in messing with one's divinations, for fun or profit. (Not to mention one's own mental state may or may not be conducive to effective divination!)
• This seems to be especially true for hot-button topics (like political ones).
• One can help protect the integrity of their divinations by magically consecrating their divination tools, or by praying to a divine being interested in helping you prior to the divination.

In case it's useful to you, my experience as a mystic (with no magical training whatsoever) is:

• I always enter a meditative state and pray to my deity prior to divination.
• If I can't get into a solid meditative state or don't get a reply from my deity, I don't divine, period. (This is rare these days, but does occur from time to time.)
• When I asked them about it, my deity told me that this practice is beneficial, but how effective it is depends on the adherent, the deity, and the situation. (I took this to mean that an adoring adherent of a doting deity asking about something that concerns few will be very well protected indeed, while a merely interested adherent of a mischievous deity asking about something that concerns many probably wouldn't be.)
• With my deity's amused indulgence, I tested this over the last month or so, and found that the dozen or so divinations I did without the above practice were just awful. My intuition was flaky, the figures seemed jumbled... the results I got were terrible. (And I usually get good, crisp results!)

Now, the question of who is on the other end of the line is an open one! I use "geomantic spirit" as a catch-all term. I'm pretty confident I'm talking with my deity directly, or at least somebody acting on their behalf; I've heard others confidently discuss the spirit on the other end of the line being their higher self (Robert Fludd is a prominent example) or their guardian angel. I suspect what's really going on varies from person to person, but the same general practices seem to hold value.