# Level 2B

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This is a high-precision preprocessing level which includes the radiometric corrections performed in level 1A and bidirectional geometric corrections involving the use of ground control points.

Visually similar to level @A, but more accurate.

### Corrections

The scene is rectified according to a given cartographic projection (conformal Lambert, UTM, oblique equatorial, polar stereographic, polyconic,etc.).

To perform level 2B preprocessing, maps of sufficient accuracy are normally required for acquiring the Ground Control Points (preferably at scale 1:25 000, otherwise 1:50 000 or 1:100 000). These maps must be provided by the customer.

If no map is available, control points can be acquired on the ground by a GPS (Global Positioning System) survey.

### Accuracy

Absolute location accuracy can be improved to 20 metres (RMS error)

The image is directly registerable with a map.

Level 2B products are aligned with true north which involves resampling the image. When stored, pixels are aligned with the map axes, thus considerably facilitating digital processing and product assembly. This level does not, however consider distortion due to terrain relief. The less pronounced the relief and the closer the viewing angle to the vertical, the more accurate the product.
In optimal conditions (vertical viewing, maximum elevation difference less than 1250m, location accuracy is specified as 20m (standard deviation). For the most oblique viewing angles, this level of accuracy can only be maintained if the elevation difference does not exceed 170 m. Elsewhere, consult level "Ortho"

### Image size

Resampling increases both the number of pixels per line and the number of lines, in proportions that vary according to viewing angle and image orientation angle. The rectangular shape of the image is restored by adding border pixels.

### File Size

In panchromatic (P) mode, the number of pixels per line is between 6000 and 10200 (depending of viewing angle and image orientation angle).
Since a pixel is coded over 1byte, the total volume of a digital product can vary between 36 Mbytes and 100 Mbytes (not including border pixels and auxiliary data files).

In multispectral (XS) mode, and for each spectral band the number of pixels per line and the number of lines can vary between 3000 and 5100.
Since each pixel is coded over 3 bytes (XS mode) the total volume of a digital product can vary between 36 Mbytes and 100 Mbytes (not including border pixels and auxiliary data files).

### Use

This is the level used when a standard cartographic projection and location accuracy are required.
Level 2B data are intermediate products, used to further generate SPOTView® products, as full scenes or geographically framed.

### Medium and format

Level 2B products are distributed as SPOTView® products under the GIS_Geospot format.

Range
CCT | P and XS
CD-Rom | P and XS
Film | P and XS
Paper print | P and XS