Image rectification

Under a rectification (Latin: correction) is defined as the elimination of geometric distortions in the image data due to, for example, uneven terrain, central perspective recording or incorrect orientation of the recording system. It is a special form of georeferencing of images. So edited photos hot digital orthophotos (DOP).

With precision-engineered instrumentation rectification is also understood as precise alignment or adjustment of the measuring device. The concept is related to the adjustment. For example, each has a precise tubular spirit Rektifizierschraube, with the zero position is corrected precisely to the horizontal.

Performing image - rectification

In order to geocode an image, a transform equation has to be found, at which each pixel of the input image can be transferred to the matrix of the output image. There are basically two approaches to find this equation:

Interpolation

When interpolation is not proposed to model the imaging geometry. The transformation is only based on an interpolation between selected control points ( so-called Ground Control Points ). As control points are all distinctive, punctual, lageunveränderlichen points such as road intersections. Their coordinates are usually taken from a topographical map larger scale. Because of the relationship between at least three control points the location of all other pixels can be interpolated, but it is advisable to include as many control points.

The disadvantage of such an interpolation that differences in height of the terrain are hardly taken into account. It is only possible to include the relief indirectly by the control points are chosen so that this is to be described. If the control points placed, for example, along a slope foot on the edge and in the bed of the trench and on prominent hilltops, it is possible to describe the terrain in a limited extent. However, the precise coordinates of such terrain points must be known.

Parametric methods

The more sophisticated methods are parametric methods in which models the imaging geometry and thus provide greater accuracy is achieved. Here, two conditions must be met: ( i) the orientation (position and movement ) of the sensor is known and (ii) a digital terrain model of the area recorded is available. To make the reference to the reference system control points are also used.

Transformation

By means of certain transformation equation can now be carried out a reorganization of the data of the input image into the matrix of the output image. Such a transformation, also called resampling is done indirectly, as a rule. It is expected back from the output image in the input image, where the gray value "brought " and this is written into the output image.

The assignment of the gray values ​​can be done by different rules:

For what rule you choose depends on what claim you have and what computational effort one wants to take this into account. The Cubic convolution is the computationally most expensive, lossless transformation is not possible practically. Advantage of the nearest neighbor method is to maintain radiometric information, there is not interpolated. This can be convenient if you want to perform a classification according to the geo-referencing.

• Geoinformatics
• Cartography
• Photogrammetry