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How far will the Eagle Eye spell let me see on the open ocean?

Eagle eye creates a magical sensor directly above you. The sensor can appear anywhere above you, to a maximum height equal to the spell's range.

It is a long-range spell. Obviously this is a long long way, but the spell doesn't specify how far away I can see. Obviously in many situations it'd limited by obstructions, but this spell should really shine on the open sea. How far can I see with Eagle Eye at its maximum height on the open ocean?

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The maximum height would be 1200 feet (400 + 800 for level).

I assume you could see out to the horizon on the open sea.

For an earth-like world, distance to the horizon is approximately 46.26 miles at that height.

Expanded details:

If this were a druid caster:

  • At level 3, the height would be 520 for a distance of 27.82 miles. (Minimum caster level)
  • At level 5, the height would be 600 for a distance of 29.88 miles.
  • At level 10, the height would be 800 for a distance of 34.51 miles.
  • At level 15, the height would be 1000 for a distance of 38.58 miles.
  • At level 20, the height would be 1200 for a distance of 42.26 miles.
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Expanding on Maximillian's answer:

At level 20, the height would be 1200 for a distance of 46.26 miles.

At that kind of a distance, the smallest object or detail a healthy human eye can detect under good light and atmospheric conditions must be approximately 40 feet (~12m) on its smallest dimension facing the viewer.

Under ideal conditions (clear skies, bright sun, no atmospheric haze or fog), smaller sailboats would only be visible as a single dot in the horizon, it would be impossible to identify them. It may be possible to see individual sails on a large ship and get an idea about its rigging, but it would be impossible to identify any markings. At such a distance, forget about seeing a single human being. A human would barely be detectable at around 6 miles, assuming he's dressed in blacks and walking on white salt flats under a scorching sun.

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  • \$\begingroup\$ Orange or red probably provides a better contrast against white (there's usually little dark spots wandering about, due to eyes moving and somewhat uneven light from the white background). But, yeah. \$\endgroup\$ – Vatine Sep 25 '12 at 14:56
  • \$\begingroup\$ The amount of useful detail you could see at that distance (gender, activity, weapons, etc.) would come down to a perception check with DC modifiers for distance and other conditions, certainly color of the clothing could factor in. :) \$\endgroup\$ – Tyri Sep 25 '12 at 17:34
  • \$\begingroup\$ At 6 miles, a human being would practically be a small ant (red or black). Any bits of useful information would hardly be visible even at around half a mile. \$\endgroup\$ – edgerunner Sep 25 '12 at 19:19
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    \$\begingroup\$ Would the increasing height of Eagle Vision actually make it harder to discern details as you go up in level? \$\endgroup\$ – Maximillian Sep 25 '12 at 22:03
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    \$\begingroup\$ Functionally, in game, you'd use the hypotenuse of the triangle to calculate the DC for the perception check. At DC + 1/10ft, that adds up fast. Even taking 20 on the check, it gets harder as that distance increases. \$\endgroup\$ – Tyri Sep 26 '12 at 12:54
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Horizon on Earth distance is:

d=1.22√(h)
d = distance in miles
h = height in feet.

More than you care to know about the issue is on Wikipedia's Horizon entry

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Maximillian's answer is a really good description of how the size of the field of vision increases in an area with no obstructions (i.e. on the open ocean).

I think the other part of the OP's question is, given a perfectly clear field, how far can you actually see in detail.

This would be a matter of perception, since in the description of the eagle eye spell, it specifically states:

You can see from this vantage as if you were actually there, rotating your viewpoint 360 degrees. You perceive with your normal visual senses.

So, depending on ambient light and atmospheric conditions (humidity, etc), there will be positive or negative DC modifiers... Including a +1/10ft DC modifier for distance. How far you can see is limited only by your perception.

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  • \$\begingroup\$ So, while I'll freely admit that I think DC +1/10ft is crude and unweildy, it's what is in the book. This translates to a DC +528 at one mile (5280 feet). Of course other modifiers come in to play, like size, color (camouflage -vs- contrast), movement, atmospheric conditions (clear -vs- haze) etc. So you'd see land (very low DC) LONG before a creature on the shore. The final DC is in the GM's hands. The character's perception bonus, even with a Take 20 -vs- the DC still doesn't allow you to SEE to the limits of the spell field of vision. \$\endgroup\$ – Tyri Sep 26 '12 at 13:24

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