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Help & guidance Guides to Good Practice

Image files

Armin Schmidt and Eileen Ernenwein, 2nd edition, Archaeology Data Service / Digital Antiquity, Guides to Good Practice

Although image files are only a limited representation of the original measurements (see The life of geophysical data section) they can be useful to quickly locate data in the Archive, for browsing and for comparison with re-imported data (“does it look right?”). For example, complementing Geoplot composite files with image files of the same name allows to identify quickly those composites that are most useful for a particular purpose. Image files may also be integrated into a GIS instead of a direct import of the data.

In all cases it is important to supply information about the spatial extent of the image (e.g. “120 m x 60 m”) or the resolution of the pixels, and the range of values it represents (e.g. “-2 nT white to +3 nT black”). Where possible georeferencing information should also be provided to facilitate integration into a GIS. This additional information can be supplied as a textual description (e.g. in a text document), as world- or other header files, or incorporated into a GeoTIFF file. Sometimes a scalebar is provided on the image but estimating the size of a survey area from an image is usually less accurate and more cumbersome than a direct provision of this information. The same applies to the inclusion of a value-scale as a greyscale or colour bar.

For the best results when converting data to images, loss-less image formats should be used (tiff, bmp, png) and not the lossy compression of jpg files. If ever these image formats were to become obsolete, they would also be migrated as part of the archiving process.