Parent Topic: Introduction
Available Types of Data
The vendors of satellite imagery sell data essentially in one of two forms:
- ``bulk'' or ``system'' corrected
- ``precision'' corrected or ``georeferenced'' data
GCPWorks only performs the geocoding corrections.
System Corrected Data
Corrections are made for distortions that are quantitatively known in
advance. These distortions include those due to the internal sensor
characteristics, the instantaneous position, height, velocity, roll,
pitch, and yaw of the sensor platform, and the easterly rotation of the
earth while the data is being acquired. Corrections for these types of
distortions are made at the satellite receiving station. This ``bulk
processed'' or ``system corrected'' data has at least one longitude and
latitude position annotated on each scene, and is therefore sometimes
referred to as ``georeferenced data''.
Precision Corrected (Geocoded) Data
Corrections are made for the residual errors remaining after the
systematic errors have been corrected. The bulk processing corrections
are based, in part, on tape header information received from the satellite.
Since this information is not very accurate, some errors will remain after
the processing is complete.
GCPs are used to correct the residual errors,
creating a geocoded image. Ground control points can be defined as
``features of known ground location that can be accurately located on
the digital imagery'' (Lillesand and Kiefer, 1987). A good example of a
suitable GCP is a highway intersection.
It is common practice to superimpose a full latitude and longitude grid
on the georeferenced image.
Parent Topic: Introduction
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