TIF, TIFF, GeoTIFF

Adobe Tagged Image File Format (TIFF or TIF) is so universally used as a format for imagery that few people know it is a Adobe format.    TIF files can contain tags, additional items of information, that can be used to extend the standard for different purposes.  Software that does not understand how to use extra tags simply ignores them.   Manifold can read and write TIF files.   TIF is a very popular export format for interchanging images.   Manifold also reads BigTIFF, an extension of TIFF format to handle TIF images that are larger than 4 GB.

 

A GeoTIFF is a TIF file that ends in a three letter .tif extension just like other TIF files, but a GeoTIFF contains additional tags that provide projection information for that image as specified by the GeoTIFF standard.   When Manifold reads a TIF that contains GeoTIFF tags, the system will automatically capture that projection information to assign to the image.     When Manifold exports an image to TIF format, Manifold always writes GeoTIFF tags into that file to specify the projection that image used within the Manifold project.    TIFs created by Manifold therefore are always GeoTIFF compliant TIF files.    When Manifold exports a TIFF bigger than 4 GB, it is automatically exported as a BigTIFF.

 

dlg_file_import_tif.png

 

To import from TIF format:

 

  1. Choose File-Import from the main menu.

  2. In the Import dialog browse to the location of the file and double-click on the .tif or .tiff file desired.

  3. A table and an image will be created.

 

Importing the sample file shown in the dialog above results in an image and its table:

 

il_import_tif01_01.png

 

We can double-click the image to open it.  For a more interesting display, we first create a new data source using a Google street maps image server as shown in the Example: An Imageserver Tutorial topic,  we create a map and drag and drop the Google layer into the map.  We then drag and drop the image into the map.

 

il_import_tif01_02.png

 

In the illustration above we have also created a second data source using a Google image server that provides streets and labels with transparent pixels between, and we have dragged and dropped that layer into the map above the image layer.   That provides a network of streets and labels overlaid on the image to provide some context.   

 

We can see the image shows an aerial view of Washington, DC, in the United States.   The image is a false color rendition synthesized from image channels acquired in infra-red frequencies by a satellite.

Different Types of TIF Files

There are many different types of TIF files.  Manifold will automatically adapt to the different type of TIF being imported.   Some of the TIF types Manifold understands:

 

 

Manifold's TIF dataport will scan the contents of the .tif file and automatically select the correct import mechanism. Surfaces imported from .tif files with floating-point values will automatically set pixels with invalid or infinite floating-point values invisible.

 

Reading a TIFF interprets 3-channel floating-point images as RGB images and 4-channel floating-point images as RGBA images, setting the default value range to [0, 1].

TIF Format and Projections

Ordinary .tif or .tiff files are simple graphics files that have no projection (coordinate system) or georegistration information. They import as ordinary images.

 

TIF or TIFF format files can occur in a specialized form called GeoTIFF files that save projection information along with the image. GeoTIFF files embed information about the projection within tags in the .tif or .tiff format file. Images saved using GeoTIFF require only one file with a .tiff or .tif file extension. True GeoTIFF files will import automatically with correct georegistration.

 

GeoTIFF tags are different than the older  approach of using TFW "world" files together with TIF files, where a separate .tfw file provides coordinate system information to be used with the similarly-named .tif file it accompanies.  Adding to the confusion is that some packages will create both a GeoTIFF file as well as a .tfw "world" file. The .tfw file provided in such cases is not part of the GeoTIFF standard.  TFW "world" files frequently do not provide sufficient information to allow georegistration, which is one reason why GeoTIFF is used by modern packages.

 

Manifold looks first for GeoTIFF tags within the image. If these are found the image is imported using GeoTIFF standards. If GeoTIFF tags are not found Manifold checks for a .tfw file and does its best to import using whatever information may be found in that file. If neither GeoTIFF tags nor a .tfw file are found Manifold will import the .tif as an ordinary graphics file without any georegistration.

 

ico_nb_arrow_blue.png  Exports: Manifold always writes TIF / TIFF files as GeoTIFF, writing data for the coordinate system in use into the file.  Such tags are ignored by software that does not understand GeoTIFF but are highly useful for software that is GeoTIFF-aware.

 

Exporting Invisible Pixels to TIF

Exporting an image to TIFF writes values for invisible pixels as legal values set to the reserved void value specific to the pixel type, and advertises used void value for other applications. Visible pixels with values equal to the reserved void value are adjusted so they do not coincide with the reserved void value.  This may lose some data, but better the pixel is adjusted slightly than an invisible pixel is made visible or vice versa.

 

Void values for signed integer types are chosen to be the minimum allowed value.  In the -128 to127 range for pixel values, -128 is the void value.  A pixel with a valid value of -128, that is, not intended to be a void value pixel, will be adjusted to -127 on export. Void values for unsigned integer types are chosen to be the maximum allowed value. Void values for floating-point types are chosen to be the maximum allowed value.

 

Notes

Killing GeoTIFFs with Non-Spatial Image Editors -  A GeoTIFF is just an ordinary TIF image with extra tags inserted into the image that specify projection information. That works great if the image editing software we use understands such extra tags. Manifold and certain other image processing packages in the GIS world understand those extra tags.  Image editors such as some versions of PhotoShop, Gimp and many other non-GIS image editing packages will not preserve the extra tags. Opening a GeoTIFF with such non-GIS software will usually destroy the projection information it contains.

 

When a spatially unaware graphics editor loads a GeoTIFF it ignores the extra projection system tags and proceeds to display the TIF image just as if it was a plain, ordinary, dumb-format, non-GIS image. When the editor  saves that TIF or updates it in any way those projection tags have already been lost and the result is that the image is now just a plain, ordinary, dumb-format, non-GIS TIF image.  It is no longer a GeoTIFF!

 

It is therefore critically important to keep inexpert users away from GeoTIFF files to ensure they do not open them with a consumer image editing program that will ruin the GeoTIFF.  Worse yet, it is almost a law of nature that the same sort of inexpert user who does such things will deny to the end of time that they ever touched any files.

 

The result is that we will end up with a collection of .tif files that once were GeoTIFFs but which we discover, unpredictably and at random, have been converted into ordinary TIF files that no longer contain projection information and are useless for GIS.  The really annoying part of GeoTIFFs versus TIFs in that since they both end in the same .tif extension there is really nothing to distinguish the two except the presence of invisible projection tags inside the format that only GIS software like Manifold understands.

 

If we have had the bad luck of inadvertently losing a GeoTIFF by it being opened in a spatially unaware image editing package, the bad news is that those projection tags probably are gone forever. There is nothing about what remains in the .tif that will allow us to get them back.

 

If we have not yet used the image and there is no .MAPMETA or .XML accessory file for it, we will be stuck with either getting fresh copies of the GeoTIFFs from the original source, or painfully trying to find out from that original source exactly what the projections were and manually specifying them for each such image by manually launching Assign Initial Coordinate System in the Contents pane.

 

Restoring GeoTIFFs - If we have a GeoTIFF that was opened in an image editor and killed by an inexpert user,  there is a chance to save that GeoTIFF if it has ever been saved by Release 8, Radian Studio or Manifold System.    

 

When writing a TIF that is a GeoTIFF, Release 8 creates an auxiliary XML file that stores the projection information for that TIF.   When writing a TIF that is a GeoTIFF both Radian Studio and Manifold will create a .MAPMETA accessory file that saves the projection information for the TIF.    If we have either a Release 8 XML or a .MAPMETA file for the former GeoTIFF, we can restore it.

 

Place the GeoTIFF and its accompanying XML or .MAPMETA file in the same folder so that when we open the former GeoTIFF the projection information will be restored to the image.  Export the GeoTIFF again and the projection information will be re-embedded into what will again be a GeoTIFF. The inclination of inexpert users to kill GeoTIFFs was one of the main reasons Manifold introduced an auxiliary XML file to save projection information, just in case, and why the safety net of a .MAPMETA file continues into Radian technology products.

 

See Also

Style

 

Example: Spectacular Images and Data from Web Servers

 

Example: An Imageserver Tutorial