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GeoTIFF is a file format which is based on the TIFF image, but which is not actually an image and cannot be displayed in most image viewers. It differs from an image file in that:
Simile uses GDAL (Georeferencing Data Abstraction Library) to read GeoTIFFs and other similar data files and convert them into 2-D arrays of parameter values. This library is not distributed with Simile, but it is Free Software; you must go and find it on the Web, and install it on your computer before you can use this feature of Simile. A version of GDAL is included in most Linux distributions and can be installed via the package manager. When it is installed, Simile can also create GeoTIFFs using the grid helper.
Havng done this, the procedure for reading data from a GeoTIFF is much the same as for a grid of values in a .csv file. The actual values in the file are used in the data array without any conversion; the assumption being that since they can be floating-point, whoever created the dataset would have been able to include physically meaningful values.
Because getting data from this type of file is a low-level operation, it is much faster than the other options for loading data from files where very large arrays are involved. However the price for this is the loss of the "transpose" and reverse ordering options.