An observer at altitude H standing on the surface of a planet with radius R sees to the horizon at a distance x. The horizon is the point where the observer's line of sight meets the surface of the globe tangentially. By Pythagoras' theorem, (R+H)² = R² + X². The solution is a square root expression, X=(2RH+H²)½. For a planet of approximately Earth's size, R=6400km. For the vantage of an AT-AT walker cockpit, about twenty metres above ground, X=0.020km and the horizon distance is on the order of 16km. However variations in elevation deepen the horizon significantly. Standing on a modest rise of 100m height gives the AT-AT a view out to over 39km away. Similarly, an AT-AT at ground level can see a 100m tall hill beyond the normal horizon at a range up to 36km.
On Hoth the firing started when Imperial walkers came over the horizon seen by the most forward rebel defences. From horizon geometry we must infer that this separation is at least ~20km but probably somewhat further because the North Ridge is elevated. The initial distance from walkers to the power generator must be much greater. At this time the generators were beyond the horizon from Veers' point of view.
Essential Guide to Weapons and Technolgy, p.106, "Planetary Shields"
A small unit such as the Kuat Drive Yards DSS-02, used by the Rebel Alliance on Hoth, protects an area approximately fifty kilometers in diameter. While these units are excellent at deflecting bombardments from orbit, they do nothing to prevent troopships and bombers from landing beyond a shield's perimeter.
Implication: the distance between the generator equipment and the Imperial landing zone is at least 25km.
[MOV] Snowspeeders take a minimum of 12s to fly over the front lines and reach the walkers advancing at the start of the battle. The transit time may be longer, with footage missing during the camera cut between the trench flyover and the appearance of the speeders over open ice plains. If the snowspeeders have a speed of only 1000km/h (estimated in roleplaying games) then the 12s transit corresponds to 3.3km. However the walkers are visible on the horizon at a few tens of km away. Either the published snowspeeder speed is an underestimate or part of the flight time has been edited out of the movie. Supersonic speeds (much greater than 330m/s) are a less likely explanation than lost footage, because the bow-shock behind a supersonic speeder may disturb the ground visibly and may be audible as a sonic boom in ground-level scenes.
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