Reverse Relief

Does anyone else have difficulty with pictures of planets appearing in
"reverse relief"? For example, in the December issue of S&T (page 22,
the photo on left) craters and gullies appear to be mounds instead of
depressions. Sometimes I can rotate the photo and get the image to
"flip" and appear correctly. For me this seems to happen mostly on
photos of Mars. The moon photos on pages 71 and 72 appear correctly.
Can anyone explain this?

Will Parr wrote:
Does anyone else have difficulty with pictures of planets appearing in
"reverse relief"? For example, in the December issue of S&T (page 22,
the photo on left) craters and gullies appear to be mounds instead of
depressions. Sometimes I can rotate the photo and get the image to
"flip" and appear correctly. For me this seems to happen mostly on
photos of Mars. The moon photos on pages 71 and 72 appear correctly.
Can anyone explain this?

It is a common effect. It happens most often when the scene is lit
from "underneath." For scenes that are mostly flat surfaces with a bit
of surface relief, an overlit surface looks the same as an underlit
surface that is the same, except that hills and valleys are reversed.
Since most scenes in our usual environment are lit from above, a scene
that is lit from underneath can easily confuse us.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Brian Tung wrote:

Will Parr wrote:

Does anyone else have difficulty with pictures of planets appearing in
"reverse relief"? For example, in the December issue of S&T (page 22,
the photo on left) craters and gullies appear to be mounds instead of
depressions. Sometimes I can rotate the photo and get the image to
"flip" and appear correctly. For me this seems to happen mostly on
photos of Mars. The moon photos on pages 71 and 72 appear correctly.
Can anyone explain this?


It is a common effect. It happens most often when the scene is lit
from "underneath." For scenes that are mostly flat surfaces with a bit
of surface relief, an overlit surface looks the same as an underlit
surface that is the same, except that hills and valleys are reversed.
Since most scenes in our usual environment are lit from above, a scene
that is lit from underneath can easily confuse us.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Is this something that can occur on a large scale. Like while
you are flying a plane?

Steve

Steve Sherman wrote:
Is this something that can occur on a large scale. Like while
you are flying a plane?

It can, at extreme lighting angles, when the view is dim. At dusk or
dawn, if you view the terrain with the Sun illuminating the ground from
behind your vantage point, you can sometimes see the effect. However,
if the Sun is too bright, there are too many other visual cues and you
won't see the effect.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt