Calibration

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In mapping, calibration is the resizing, moving, rotating and shearing of an image to fit it into another projection. A typically case is importing a historic map or an aerial photo in an editor like JOSM. Next step is to calibrate the image to the exiting OSM elements. JOSM has the PicLayer plugin for this task.

A calibration gives six to nine vectors as result. These values can be stored internally in the image itself or externally in a wikipedia:sidecar file.

External calibration files

ESRI world file

Is a small plain text with six vectors[1]. The target projection is not stored. File extension is depending on the image format.

JOSM PicLayer

Also a small text file with nine vectors including the projection. This gives repeatable results between different systems. The file consists of the name of image including its extension and the extension .cal like grandpas-secret-mushroom-map.jpg.cal.

Internal calibration storage

  • GeoTIFF as most common file format
  • ESRI (TFW, TIFW, TFFW, TIFFW / BPW, BMPW / JGW, JPGW / EWW, ECWW)
  • GéoConcept (GXT / TXT).

See also

References