re: view/horizon determination methods

Message posted by JB737 on March 18, 2007 at 2:30:22 PST:

Excellent technical resources you mention there, thanks.

But as a first pass, I fly around in Google Earth. First I plunk myself where the viewing subject is, whether that is Groom Lake, McCarran, Half Dome, or whatever. I drop down very close to ground level usually, while tilting the view enough to see the horizon.

Then I spin around to see the potential viewpoints in all directions. In the case of Groom Lake, I look for the low points in the close, inaccessable mountains, scoot up and down to take a look to see if an accessible viewpoint is available beyond them, and then fly out there straight through the gap to it.

Then I fly past the viewpoint, spin around toward the target, drop down to just above the viewpoint elevation, and hover all around the viewpoint to see how the subject looks. A neat trick I learned yesterday is that if you pick a lower altitude than the peak and "fly through it" or "bump into it", it raises your altitude to match the highest point of ground above you. This is a better way to find peak elevations than looking straight down and fishing with the cursor.

A few iterations choosing viewpoints for a subject, will give a general idea of the best choices. Further iterations will let you know for instance, which parts of the subject can be seen from each peak of interest.

As a reality check, I snap a measurement line from viewpoint to subject, run the cursor along the line looking for the elevations of potential obstacles, then measure the distance to each limiting obstacle, and then do some simple ratios of distance vs elevation change. This will tell you pretty closely how tall something has to be at each azimuth, to be seen. Or if it is fully seen, then it tells you how far you can drop down at the viewpoint and still see it.

The same method works well regarding cityscapes, such as the McCarran terminal from hotels, if you turn on 3-D building modeling. You need a little bit of real-world knowledge to get the most out of it, such as knowing the the Luxor is not really a cube or rectangular prism, but is actually a pyramid. :-) But it will definitely tell you which hotel potentially blocks which other hotel, and to what height, from any point on the airport. And it gives you a great idea of how straight out your window the subject is, which runways are also seen best, etc.

You can go into a lot of complexity with spherical trigonometry, or worse yet, geode shapes, or worse yet, the total geometric reality....and still be only 99% right as to the view.

For instance, atmospheric refraction is not trivial when viewing out near the horizon, so even if a map-view-rendering package takes into account all sorts of geometric things, it will still be somewhat inaccurate when compared to a photo.

For instance, astronomers know very well that from absolute ground level on a perfectly spherical Earth (which it is not), your view of the sky is not exactly a 180 degree hemisphere as a mathematician would say it is after he draws the tangent plane which limits the view. In fact, you can see more like a full 181 degrees at sea level (I forget the exact number, maybe 180.8, but we're not talking about a tiny fraction of a degree here). So something like an extra "moon diameter" is seen "below the horizon" in all directions. Now, what does 0.4 degrees mean at 26 miles? A couple thousand feet. Of course, looking horizontally into space goes through more air than looking slightly below horizontal at something 26 miles away. And if your obstacle is 12 miles away, that is quite different from the obstacle being very near you (as in the case of having your eyeball at sea level). But the effect is not negligible. And the good news, is that it works in our favor, acting as a lens to bend our line of sight, like topspin on a tennis ball. Atmospheric refraction, or "seeing below the horizon" gives us slightly more than 12 hours of sunlight at the equator every day.

In Reply to: Re: traverse peak details posted by lone wolf on March 17, 2007 at 15:45:03 PST:


[ Discussion Forum Index ] [ FAQ ]