Remote Sensing - A Primer of GIS: Fundamental Geographic and Cartographic Concepts - page 160

1. It generally only detects EMR from the surface of an object,
although some sensors allow for penetration.
2. There is no contact between the sensor and the object.
3. All remote sensing measurements use reflected energy (usually
from the sun) or emitted energy (e.g., from a radar station or
plants).
The EMR detected by remote sensing technologies varies. It depends on the
desired application as well as on the cost of different remote sensing data
collections.
Spectral Signature
The EMR emitted or ref lected by a thing or event varies. These differences
are the basis for distinguishing things and events. The ref lections and EMR
emissions of a particular thing or event can be associated with a particular
spectral signature that is used to identify where these things and events are
located in a remote sensing image.
EMR also varies by time of day, season, weather conditions, moisture
levels in the soil, wind, and a number of other factors. The physics involved
in addressing these variations in emitted or reflected EMR is critical to the
success of remote sensing and provides a commonplace solution. This solu-
tion, called “ground truthing,” involves having some people in the field
before, during, or after data collection who may take similar sensor measure-
ments or observations. These measurements and observations can be used
later to verify the remote sensing image or data and possibly to define cor-
rection parameters for adjusting the remotely sensed data to correspond to
ground observations. Needless to say, this is highly complex and requires
The electromagnet spectrum showing common examples.
From http://lmynasadata.larc.nasa.gov/images/EM_Spectrum3-new.jpg