With both the Canon and Nikon GPS units, tree cover in a moderate canyon did not seem to have negative effects on the functionality of either brand. Deeper canyons and/or heavy tree cover and/or a non-ideal GPS constellation might alter that behavior (the available GPS constellation can be good or not so good, depending on location and time of day).
With the Nikon D800E + GP-1 stowed in my hip-pack, the GP-1 would take 10-15 seconds to reacquire the GPS signal after pulling it out of the pack. I also noted that if I took a picture right away, that the reading would be off just a little; it is best to wait that 10-15 seconds for the current reading to stabilize if the camera has just been pulled out of a pack. In short, this means that for consistent readings, one should not carry the camera in a pack, and that the camera should be set for continuous operation (GPS => Auto Meter = Disable), so that the unit always has a “fresh” GPS reading. Or some patience is required before each image. Ditto for turning the camera off between shots.
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