chapter 5: image interpretation Flashcards
Satellite imagery can be in one of the following two formats:
Analog and digital
Analog:
which data is displayed in a pictorial or photograph‐type format, independent of what type of sensor was used to collect the data and how the data were collected
interpretation and identification of targets in this imagery is performed manually or visually, ie by human interpreter
Digital
data is represented in a computer as arrays of pixels, with each pixel corresponding to a digital number, representing the brightness level of that pixel in the image
When remote sensing data are available in digital format,digital processing and analysismay be performed using a computer.
Both analogue and digital imagery can be displayed as
black and white (also called monochrome) images, or as
color images by combining different channels or bands representing different wavelengths.
Visible Imagery (VIS)
Images obtained using reflected sunlight at visible wavelengths, 0.4 to 0.7 um
Visible imagery is displayed in such a way that:
- high reflectance objects, e.g. dense cirrus from CB clusters, fresh snow, nimbostratus etc., are displayed as white, and
- low reflectance objects, e.g. much of the earth’s surface, is black
There are grey shades to indicate
different levels of albedo (or reflectivity)
Visible imagery is not available
at night
Infra Red (IR)These images are obtained by measuring
the intensity of the thermal emissions from the earth and the atmosphere, at IR wavelengths in the range 10‐12 um
This so‐called ‘window’ need to be
chosen to allow the satellite sensors to detect such radiation unhindered, and the 10‐12 um band is one such.
For example, the GOES (8‐11) sensors use
the atmospheric infrared window centered at 10.7 micrometers
For example, the GOES (8‐11) sensors use the atmospheric infrared window
centered at 10.7 micrometers
at this wavelenght
energy radiated by the earth’s surface and clouds is not significantly attenuated by atmospheric gases.
For example, the GOES (8‐11) sensors use the atmospheric infrared window centered at 10.7 micrometers.
At this wavelength, energy radiated by the earth’s surface and clouds is not significantly attenuated by atmospheric gases
in this channel
most surfaces and cloud types have an emissivity close to 1.0, with a notable exception being thin cirrus.
For example, the GOES (8‐11) sensors use the atmospheric infrared window centered at 10.7 micrometers.
In this channel, most surfaces and cloud types have an emissivity close to 1.0, with a notable exception being thin cirrus.
therefore
the brightness temperature sensed by the satellite is close to actual surface skin or cloud top temperature for other scenes.
IR imagery is so presented that warm/high intensity emissions are
dark grey or even black
IR imagery is so presented that warm/high intensity emissions are dark grey or even black, and low intensity/cold emissions are
white
IR imagery is so presented that warm/high intensity emissions are dark grey or even black, and low intensity/cold emissions are white. This convention was chosen so that the output would correspond with that from
the VIS channels
Color slicing is also frequently used whereby
different colors are assigned to various temperature ranges, thus rendering the cooling/warming of cloud tops (and thus the development/decay) easy to appreciate:
- warming/darkening of the imagery with time indicates descent and decay
- cooling/whitening images imply ascent and development
Water Vapor (WV)
This imagery is derived from
emissions in the atmosphere clustered around a wavelength of 6.7 um
The infrared water vapor channel on board GOES‐8to‐11 is located at 6.7 micrometers where the earth’s
emitted spectrum is highly attenuated by water molecules. Thus, this channel senses radiation from the mid‐and upper‐ levels of the atmosphere, from both water vapor and clouds.
IR channel, this wavelength undergoes
strong absorption by WV in the atmosphere (i.e. this isnota ‘window’), and so can also be used to infer vertical distribution and concentration of WV ‐ an important atmospheric constituent
WV imagery uses
the radiation absorbed and re‐emitted by water vapor in the troposphere.
If the upper troposphere is moist
WV emissions will be dominated by radiance from these higher levels is conventionally shown white.
If the upper troposphere is dry
then the sum of the radiation is biased towards lower altitude WV bands: and this is displayed as a shade of grey, or even black.