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In article <1523@castle.ed.ac.uk> ct@lfcs.ed.ac.uk (Chris Thornborrow) writes:
>
>	Can satellite pictures reveal buried ruins ? Has this
>been done much ?

Not just satellites.   Much of the early twentieth century archeological
work in the UK was done using airborne photography.    Take a look at
some of the stuff published by Sir Mortimer Wheeler for details.

jon.

In article <1523@castle.ed.ac.uk>, nick@lfcs.ed.ac.uk (Nick Rothwell) writes:
> Recently I heard of satellite pictures that revealed buried ruins
> in the Sahara desert. I have a simple question. Is this true ?
> 	Can satellite pictures reveal buried ruins ? Has this
> been done much ?

The short answer is yes.  Many of the commercial satellites (such as Landsat
TM) have a thermal bands, e.g. sensors in the near or far infrared range
(2.2 um to 10um).  If you have an area with a fairly constant surface (such as
desert), then sub-surface variations show up quite clearly.  There has also
been some research in thermal profile mapping, where the rate at which
the surface heats up & cools down is used to guess at what material might
be causing the effect (for example sandstone has a different thermal
profile to granite).

sns


-- 
Stuart Nixon,	sns@erm.oz.au

Earth Resource Mapping, 130 Hay St, Subiaco, Western Australia 6008
Phone: +61 9 388 2900   Fax: +61 9 388 2901   E-mail: sns@erm.oz.au

You are getting into the area of remote sensing.

The principal instrument you want is Imaging Radar, also know as
side looking radar (SLAR), Synthetic Aperature Radar (SAR), etc.
(typical air or space borne).
There is also the use of passive microwave.  Radar has the
advantage in that is it an active sensor sending a pulse (chirp)
out which has a signal (return) which can then be analyzed.

IR has some advantages and disadvantages.  I won't digress.

The bottom line is that you are looking at secondary artfacts typically rather
than primary characteristics to learn about your subject
of interest.  Airborne imaging radar has been used to lcate runs in the
jungles of Central America and water ways in the Sahara (spaceborne
systems).  The mechanics of the subject are long, big, somewhat complex,
and not well understood (do you have 5 days?).  We create images, not photos
or maps, the geometry of creation is completely different from a photo (point
perspective) or a map (orthogonal).

Texts include: The Manual of Remote Sensing (Amer. Soc. of Photogrammetry),
(try 7 inches thick) and radar specifically:
Introduction to Radar Systems by Skolnick.  Lots of fun working
in this area, you get to go on field trips, take ground truth.

	"What do you do?"
	"I make maps."	-- Starman

Another gross generalization from

--eugene miya, NASA Ames Research Center, eugene@aurora.arc.nasa.gov
  resident cynic at the Rock of Ages Home for Retired Hackers:
  "You trust the `reply' command with all those different mailers out there?"
  "If my mail does not reach you, please accept my apology."
  {ncar,decwrl,hplabs,uunet}!ames!eugene
  Do you expect anything BUT generalizations on the net?

In article <556@erm.oz> sns@erm.oz (Stuart Nixon) writes:
>In article <1523@castle.ed.ac.uk>, nick@lfcs.ed.ac.uk (Nick Rothwell) writes:
>> Recently I heard of satellite pictures that revealed buried ruins
>> in the Sahara desert. I have a simple question. Is this true ?

Radar can see several meters below below the surface under optimal conditions
depending on wavelength, dryness, and soil grain size.
I've heard some sites have been imaged directly by radar rather than by
secondary visual effects, and I believe the Sahara was one of these
successful places.

In article <7113@lindy.Stanford.EDU> rick@hanauma.UUCP (Richard Ottolini) writes:
>In article <556@erm.oz> sns@erm.oz (Stuart Nixon) writes:
>>In article <1523@castle.ed.ac.uk>, nick@lfcs.ed.ac.uk (Nick Rothwell) writes:
>>> Recently I heard of satellite pictures that revealed buried ruins
>>> in the Sahara desert. I have a simple question. Is this true ?
>
>Radar can see several meters below below the surface under optimal conditions
>depending on wavelength, dryness, and soil grain size.
>I've heard some sites have been imaged directly by radar rather than by
>secondary visual effects, and I believe the Sahara was one of these
>successful places.

Well, er,... not quite.

We are certainly seeing substructure in some images, but.... there is
a lot we don't know or understand.

How imaging radar works is dependent on lots of things: wavelength
(which affect whether or not you can see below a surface be it ground
or a forest canopy), a set of concepts like surface roughness and surface
length, the dielectric constant of the soil (the presence of water
which affect this and the wavelength selected), the power of the system, the
structure of the chirp sent out, etc.  But it has limits: radar is like
working in the dark at night, you only see what you shine your light on.
Shadows take on a different meaning.  Images take on a quality of false
3-D.  This causes problems such as range and azimuth ambiguity.
Radar is not a miracle sensing system, but somewhat complex and needs a
lot of reasearch to understand how to use it and how it works.

Wavelength is important because one works in the region where things
like raindrops can make a difference.  Too big and you see them,
too small, you don't.  Same goes for soil particle size, etc.

Radar works by taking a small antenna and moving it linearly along a track
and synthesizing an image by integrating multiple views (looks) along
that track.  Motion is an integral part of constructing the image and no
two scan lines are taken from the same point.

One study on penetration of dense forest canopy was thought be a subsurface
example.  It turns out that was false, there were subtle variations on
vegetation not visible to eyes.  The human eye is at times an unreliable
means of information transmital.  Many people are unable to distinguish
a radar image some say a black and white negative enlargement.  It takes
a bit of training to recognize features thats why images have false depth

I worked on the Seasat-1 SAR, the preliminary studies for what is
now the Magellan spacecraft.  Along side of me were people working on the
SIR (Shuttle Imaging Radar) and various airborne radars.
Simple little description of imaging radar systems.  To learn more,
the material is complex enough you should take a class (or pay me
to explain it to you).

Another gross generalization from

--eugene miya, NASA Ames Research Center, eugene@aurora.arc.nasa.gov
  resident cynic at the Rock of Ages Home for Retired Hackers:
  "You trust the `reply' command with all those different mailers out there?"
  "If my mail does not reach you, please accept my apology."
  {ncar,decwrl,hplabs,uunet}!ames!eugene
  Do you expect anything BUT generalizations on the net?