An orthorectified image (or orthophoto) is one where each pixel represents a true ground location and all geometric, terrain, and sensor distortions have been removed to within a specified accuracy. Orthorectification transforms the central perspective of an aerial photograph or satellite-derived image to an orthogonal view of the ground, which removes the effects of sensor tilt and terrain relief. Scale is constant throughout the orthophoto, regardless of elevation, thus providing accurate measurements of distance and direction. Geospatial professionals can easily combine orthophotos with other spatial data in a geographic information system (GIS) for city planning, resource management, and other related fields.
ENVI's existing orthorectification tools (accessed through Geometric Correction > Orthorectification in the Toolbox) allow you to orthorectify images using rational polynomial coefficients (RPCs), elevation and geoid information, and optional ground control points (GCPs). RPCs attempt to model the sensor geometry, using a polynomial approximation to model light rays from a ground location to the focal plane of the sensor. However, RPCs and elevation information do not provide enough details to build a rigorous model representing the path of light rays from a ground object to the sensor.
The ENVI Photogrammetry Module provides you with access to Rigorous Orthorectification, which builds highly accurate orthorectified images by rigorously modeling the object-to-image transformation. The details of this transformation are mostly transparent to the user, which means you can create orthorectified images without defining any detailed model parameters.
With Rigorous Orthorectification, you can easily see the spatial coverage of images, DEMs, GCPs, and tie points, along with error magnitudes for each GCP. You can adjust your GCPs and tie points to improve the root mean square error (RMSE) for the orthorectified output. ENVI uses the latest innovative research methods to perform mosaicking and color balancing in the output product.
Rigorous Orthorectification is a joint collaboration between Harris Geospatial Systems and Spacemetric AB of Stockholm, Sweden. Spacemetric designed the underlying block-adjustment model, which provides a precision orthorectification solution for various sensors. For more technical information on Spacemetric’s orthorectification models, see their website at http://www.spacemetric.com.
To use Rigorous Orthorectification, you must purchase a separate license for the ENVI Photogrammetry Module. This module also provides access to the Generate Point Clouds by Dense Image Matching tool.
See the following sections:
- Start Rigorous Orthorectification
- Collect and Edit Ground Control Points
- Collect and Edit Tie Points
- Reorder Images and Define Cutlines
- Select Output Parameters