# Data preprocessing

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| To become a usable product, the raw SPOT data as recorded by the receiving stations must be preprocessed. This operation is performed by the "Centre d'archivage et de Pretraitement (C.A.P.) at Spot Image and is designed to correct a number of parameters that affect image quality.

The data downlinked from the satellite cannot be used directly in the form in which it is received. All marketed SPOT scenes are therefore corrected. These corrections are radiometric and geometric, according to several levels:

###

     Levels of preprocessing

level | radiometric corrections | system corrections | geometric corrections in cartographic projection | + use of Ground Control Points | + use of Digital Terrain Model
--- | --- | --- | --- | --- | ---
1A |
1B |  |
2A |
2B |
Ortho |

### Availability of each level

level | SPOT Album | SPOT scene | SPOTView® full scene and geographical frame
--- | --- | --- | ---
1A

1B |  |
2A |  |
2B |  |  |
Ortho |  |  |

| Level "1A" | This is a raw image data level where only detector normalization is performed in each spectral band. There is no geometric correction.
| Level "1B" | Basic preprocessing level including standard radiometric and geometric corrections taking into account systematic distortions due to system effects (earth rotation, panoramic distortion effect)
| Level "2A" | This precision level is intended for users requiring cartographic accuracy or merging of data originating from different sources.
| Level "2B" | Similar to level 2A, but made more accurate by use of ground control points
| Level "Ortho" | In addition to level 2B, this level includes the use of a DTM to correct errors due to relief