From: david gudmunsen (kroctec@btopenworld.com)
Date: Fri Jul 15 2005 - 04:09:22 PDT
I hope that I may be able to partially illuminate the subject as it relates
to the "Great site" correspondence.
First of all I should point out that the imagery that is and is becoming
available to us comes from a great variety of sources and there are great
differences in terms of coverage and both spectrally and spacially. The
spectral content and spatial resolution are normally determined by the
purpose of the data, how big a pocket ($s) the primary user of the data sets
is and from there on the value of future sales which may be used to offset
investment costs. the older and more the data has and is being used the
cheaper it becomes to the point where we even get some stuff for free: High
res 3D data is crazy money.
>From a spectral point of view there is just about everything used out there
from simple black & white photographic, ultra violet, visible colour, near &
far infra red, radar and x-ray also both analogue and digital.
Spacially the resolution largely depends on how far the instrument (camera)
is from the subject, the ground speed and also the type of instrument.
Typically analogue aerial cameras have format sizes between 150mm and 300mm.
For example Leica make a range of instruments: 240mm film camera that will
ideally resolve about 5cm on the ground, a laser based instrument using
LIDAR (light detecting and ranging) horizontal and vertical resolution to
about 13cm and a CCD based scanner which can also under ideal circumstances
resolve about 5cm on the ground.
When it comes to satellite mounted instruments there is so much variety
spacially ranging from 50cm to about 10km for digital for commercial data.
Better than 8cm for some low orbit steerable military satellites. The early
Landsat instruments were about 120m per pixel and the more recent I think
are 10m per pixel.
Low orbit satellites operate at altitues from about 120 miles to typically
1500 miles and satellites below 22500 miles are also called low orbit, they
will circle the earth in times that vary from about 2 hours at 120 miles
altitude to 24 hours (geostationary or geosynchronous) at 22500 miles
altitude. To get complete earth coverage you need a satellite in a low polar
orbit, to get continuous coverage you need a geostationary satellite.
A note on coverage is that there are some areas of the world which are never
'clear sky' ie there is always cloud cover.
None of the forgoing has anything much to do with communications satellites
which are a very different subject. Also the greatest problem is the sheer
number crunching capacity required for processing such massive data sets and
the time it takes compared to the speed of data aquisitiion. Storage is also
a further problem.
Anyone who is particularly interested in the subject can email me privately
and I wil do my best to point them in appropriate directions.
Best regards from David Gudmunsen
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