There are several level of products available on the remote sensing imagery market. The most basic level often provide the geometry of acquisition (sometimes called the raw geometry). In this case, pixel coordinates can not be directly used as geographical positions. For most sensors (but not for all), the different lines corresponds to different acquisition times and thus different sensor positions, and different rows correspond to different cells of the detector.
The mapping of a raw image so as to be registered to a cartographic grid is called ortho-rectification, and consist in inverting the following effects (at least):
Moreover, depending on the area of the world the image has been acquired on, different map projections should be used.
The ortho-rectification process is as follows: once an appropriate map projection has been defined, a localisation grid is computed to map pixels from the raw image to the ortho-rectified one. Pixels from the raw image are then interpolated according to this grid in order to fill the ortho-rectified pixels.
Ortho-rectification can be performed either with OTB Applications or Monteverdi. Sensor parameters and image meta-data are seamlessly read from the image files without needing any user interaction, provided that all auxiliary files are available. The sensor for which Orfeo ToolBox supports ortho-rectification of raw products are the following:
In addition, GeoTiff and other file format with geographical information are seamlessly read by Orfeo ToolBox, and the ortho-rectification tools can be used to re-sample these images in another map projection.
The OrthoRectification application allows to perform ortho-rectification and map re-projection. The simplest way to use it is the following command:
In this case, the tool will automatically estimates all the necessary parameters:
In order to use a Digital Elevation Model (see section 3.2.3) for better localisation performances, one can pass the directory containing the DEM tiles to the application:
If one wants to use a different map projection, the -map option may be used (example with lambert93 map projection):
Map projections handled by the application are the following (please note that the ellipsoid is always WGS84):
The group outputs contains parameters to set the origin, size and spacing of the output image. For instance, the ground spacing can be specified as follows:
Please note that since the y axis of the image is bottom oriented, the y spacing should be negative to avoid switching north and south direction.
A user-defined region of interest to ortho-rectify can be specified as follows:
Where the -outputs.ulx and -outputs.uly options allow to specify the coordinates of the upper-left corner of the output image. The -outputs.sizex and -outputs.sizey options allow to specify the size of the output image.
A few more interesting options are available: