What is the exact process of georeferencing

Georeferencing

description

Through the process of Georeferencing position or location information on earth ("geo") is associated with an object of the earth image (map, raster file, information).

This is mostly done with the help of coordinates in a reference or coordinate system. The problem here is to project a 3D object (sphere or ellipsoid) onto a 2D object (map sheet). In principle, this cannot be done without loss.

The process is also known as localization in connection with land information systems (LIS) or geographic information systems (GIS).

See also:Geodesy, geocoding, cartography, latitude and longitude, sphere, earth ellipsoid, projection, navigation, global positioning system

In the case of image files, the results of the georeferencing can either be saved internally in the header of the image file (img, sid) or externally as a separate world file (gif -> gfw, bmp -> bpw, jpg -> jgw). The Tiff format supports both the storage of GeoTiff files with information in the header and the storage of tfw world files.


A TFW file is an ASCII file and consists of the following 6 lines (example):

2.50000000000000 width of a pixel (in x-direction) 0.00000000000000 rotation term for the x-axis (rows) 0.00000000000000 rotation term for the y-axis (columns) 2.50000000000000 height of a pixel (in y-direction), often negative because the origin of raster images at the top left is 4450933.75000000000000 x-coordinate of the upper left corner of the image 5407046.25000000000000 y-coordinate of the upper left corner of the image

aims

There are essentially three possible reasons for georeferencing:

  1. Elimination of geometric distortions in image data, caused by, for example, uneven terrain, central perspective recording or incorrect orientation of the recording system (also Rectification called)
  2. Fitting the image coordinates into a geodetic reference system, i.e. into real "world coordinates" (also Geocoding called)
  3. relative adjustment of two differently oriented or scaled images to one another.

Tools

if you're comfortable with commandline tools GDAL Utilities, part of the FWTools suite will help you. For GUI's check out Thuban and Quantum GIS. And of course there's Grass, which has everything including the geospatial kitchen sink but will take some time to learn. (all of these tools use gdal under the hood).

Commercial tools

  • Blue Marble Geographics - Geographic Transformer
    'Master Raster' with our image georeferencing, reprojection, tiling and mosiacing tool. As raster images are becoming more prevalent in GIS projects, the need to reproject images into the appropriate coordinate system becomes increasingly important.
  • In ArcGIS 8.x georeferencing is carried out with its own toolbar. By defining identical points in the raster image and in a reference map, a link table is generated and an error calculation (residual error) is carried out.

execution

Finding a transformation equation

In order to be able to georeference an image, a Transformation equation can be found with which each pixel of the input image is transferred into the matrix of the output image. There are basically two approaches to finding this equation:

Interpolation method:

In the case of interpolation methods, it is not necessary to model the imaging geometry. The transformation is based only on an interpolation between selected ones Control points (so-called Ground Control Points). All distinctive, punctual, position-invariable places, such as road crossings, are suitable as control points. Their coordinates are usually taken from a larger-scale topographic map. Due to the relationship between at least three control points, the position of all other image points can be interpolated, but it is advisable to include as many control points as possible.

The disadvantage of such an interpolation method is that differences in height of the terrain are hardly taken into account. It is only possible to include the relief indirectly by choosing the control points so that it is also described. If the control points are placed e.g. along the foot of a slope, on the edge as well as in the bed of a ditch and on protruding hilltops, it is possible to describe the shape of the terrain to a limited extent. However, the exact coordinates of such terrain points must be known.

Parametric methods:

The more complex methods are the parametric methods, in which the recording geometry is modeled and thus a far greater accuracy is achieved. Two prerequisites must be met: (i) the orientation (position and movement) of the sensor is known and (ii) a digital terrain model of the recorded area is available. Control points are also used to establish the relationship to the reference system.

transformation

By means of the specific transformation equation, the data of the input image can now be rearranged into the matrix of the output image. Such a transformation, too Resampling called, can be done directly or indirectly. In the direct transformation the new coordinates are first calculated for each input image element and then gray values ​​are assigned to these output image elements. In the case of the indirect transformation the other way around, it is calculated back from the output image to the input image, the gray value is "fetched" there and this is written into the output image.

The gray values ​​can be assigned according to various rules:

  1. Nearest Neighbor: the gray value of the pixel is assigned which

References