High Resolution 1:10,000 scale Mapping Strategy of Multi ... - NDMA

was that ground control points (GCPs) were typically needed to produce accurate
data for any purpose beyond ad hoc visualization. By definition, the horizontal datum
is a rectangular plane coordinate system. All horizontal control shall begin and
terminate on monuments that are in the National Geodetic Reference Database
System (NGRDS). The vertical datum is normal to gravity. All vertical control shall
begin and terminate on existing benchmarks that are in the National Geodetic
Reference Database System (NGRDS).
Time consumed to collect under the best conditions, placing personnel on the ground
to collect GCP data in order to properly orthorectify images captured over
inaccessible regions such as a heavily flooded or hostile area may be impossible or
at least certainly not worth the risk. A new method was needed – direct
georeferencing.
Direct georeferencing produces an accurate relationship between airborne image
data and the terrain it represents by accurately measuring the relative position and
orientation of the sensor, removing the need for traditional ground-based GCP
measurements. This approach employs a GPS-assisted inertial navigation system to
determine the exact position and orientation of an airborne sensor at the exact
moment of data capture.
As navigation sensors (GPS, IMU, etc.) continuously record sensor travel and
rotation data before during and after image capture, ranges and bearing
measurements are produced to represent exact points on the ground via the sensor.
It is then possible to calculate the exact position each image pixel represents on the
ground through the corresponding points measured in the mapping frame; all without
ever needing to collect a single GCP. This is true remote sensing in that no work is
required on the ground.
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