The Ultimate Guide to Orthorectification of Satellite Imagery

Remote sensing satellite and orthorectification of satellite imagery play an important role in geospatial mapping, image capture, and visualization that can create and provide Super GIS Mapping Services for many applications that require accurate and precise data. Images can have a strong visual impact and put spatial concepts into perspective when displaying or analyzing data.

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The orthorectification process

• The accurate description of a sensor called a sensor model gives detailed information about the position and orientation of the sensor for each image and terrain model, for example, the World Elevation service available on ArcGIS Online.
• Many modern sensors include RPCs for geo rectification of satellite imagery with image transfer for this purpose.
• For collections of images with no associated RPC, it is often possible to create an RPC if you know a few properties about the orientation of the internal camera and the external environment. Automatic generation of tie points, DEMs, and ground control points can make the RPC orthorectification process very accurate.

How does it work?

One can generate the orthorectification of satellite imagery from elevation models (DEM) and aerial photographs. Elevations are generated from elevation models (DEMs) and aerial images. Since the elevation model can be generated from the same stereograph, the model cannot be generated from the same stereograph, and no other data source is required. You will need another data source for the purpose. 

 

Requirements for satellite imagery

·         Atmospheric and Radiometric Correction

·         Orthorectification and Alignment

·         Optimized for Cloud Computing

·         Configured for Localized Analysis

·         Fast Access and Direct Delivery

 

Orthorectification and Alignment

Like atmospheric and radiometric corrections, orthorectification of satellite imagery and alignment is necessary to understand which features in the image are relevant. Orthorectification ties pictures to their correct position on the earth and controls the alignment between roads, buildings, and other elements in stacks of photographs taken from different angles.

Orthorectification in a Nutshell

Raw satellite imagery has distortions caused by sensor orientation, variations in topography, and the earth’s curvature. After collecting satellite imagery, it has to be processed to filter out inaccuracies through a process known as orthorectification of aerial photographs. It involves the removal of image perspective (tilt) and relief (terrain) effects for creating a planimetrically correct image.

The orthorectified image contains a constant scale representing features in their accurate positions. It enables the measurement of distances, angles, and areas accurately.

The inputs required for orthorectification are:

1.     Image with accurate sensor geometry

2.     An elevation model of the ground surface in DEM, DTED, or SRIM format.

Imagery Distortions

The distortions come in the form of feature displacement, creating inconsistencies in scaling via image. The distortion levels change throughout the imagery scene. It means that a simple ground control point adjustment cannot nullify these errors. Therefore, a correction process for topographical variations and sensor orientation is necessary to rearrange the displaced pixels to their correct position. The final orthorectified image can be used for direct measurement, extracting features, and other applications that need measurement-ready data.

Accurate Elevation Models Are Crucial

Feature distortion on raw imagery is greatly affected by terrain variation. An accurate elevation model is needed        to calculate the terrain variation effects on the image pixels. This calculation is therefore used to position the pixels to the correct locations. There are free terrain data sources present, such as SRTM and ASTER.  

It is often that the importance of using accurate elevation models for orthorectification goes unnoticed. The reason is the high costs of procuring accurate elevation data and investing in hardware, software, data and skilled professionals. But these things should not be given higher value than quality elevation models. If you focus on orthorectification and geo rectification of satellite imagery in your practice, it needs to be detailed and reliable.