# GeoTIFF FAQ Version 2.4

## Last Updated: February, 2011

The FAQ is intended to provide an introduction for persons new to GeoTIFF, as well as pointers to significant resources on the WWW for further information. Comments concerning this FAQ, and suggestions for improvement and addition to the FAQ may be emailed to warmerdam@pobox.com.

For general WWW information and resources concerning GeoTIFF, please point your browser to GeoTIFF WWW Homepage.

## Begin GeoTIFF FAQ

1. What is TIFF format?
2. What is the purpose of GeoTIFF format for satellite data?
3. What is GeoTIFF and how is this different from TIFF?
4. Who owns GeoTIFF Format?
5. Can IP, GIS, CAD, and Desktop Mapping systems all use GeoTIFF?
6. Does GeoTIFF imagery conform to the TIFF specification?
7. Can systems that don't use geography read a GeoTIFF image?
8. Does the Geographic data look transparent to the user?
9. Who is working on definition of GeoTIFF?
10. What's a TIFF tag?
11. What is the relationship between a TIFF tag and the actual image?
12. Are the tags in separate files from the tiff files ...
13. Why is GeoTIFF based only on TIFF format?
14. Is the tagset proprietary?
15. What are the advantages of TIFF format for geographic data?
16. Is GeoTIFF really platform interoperable?
17. What will happen in my GIS if I load a GeoTIFF image?
18. Doesn't anyone else have TIFF tags for geographic information?
19. Is GeoTIFF based on the world reference files that ESRI uses?
20. How do I convert between ESRI world files, and GeoTIFF?
21. How do I preserve GeoTIFF info when editing my image?
22. What are the requirements to make a TIFF image ...
23. Can I make customized geographic projections using GeoTIFF?
24. What is the effect of PixelIsArea vs. PixelIsPoint?
25. What is the Axis Order in GeoTIFF?
26. How should Vertical Datums and Vertical Coordinate Systems be encoded in GeoTIFF?
27. Who are POSC and EPSG and why is GeoTIFF involved with them?
28. What EPSG Version does GeoTIFF Use?
29. Where can I get a copy of the GeoTIFF spec to review?
30. What can I do to support GeoTIFF ...
31. How can I monitor the progress and changes to future GeoTIFF specs?
32. Is the GeoTIFF community still active?

### What is TIFF format?

TIFF format is the most popular and versatile raster data format in the world today. TIFF is a format for storage, transfer, display, and printing of raster images, such as clipart, logotypes, scanned documents. Today, TIFF is being used for storage of map information, too.

More information on TIFF can be found at Joris Van Damme's TIFF Page.

### What is the purpose of GeoTIFF format for satellite data?

The TIFF imagery file format can be used to store and transfer digital satellite imagery, scanned aerial photos, elevation models, scanned maps or the results of many types of geographic analysis. Over the past several years many users of such images have urged geographic data suppliers to provide imagery in TIFF format. TIFF is the only full-featured format in the public domain, capable of supporting compression, tiling, and extension to include geographic metadata. GeoTIFF implements the geographic metadata formally, using compliant TIFF tags and structures.

### What is GeoTIFF and how is this different from TIFF?

"GeoTIFF" refers to TIFF files which have geographic (or cartographic) data embedded as tags within the TIFF file. The geographic data can then be used to position the image in the correct location and geometry on the screen of a geographic information display.

GeoTIFF is a metadata format, which provides geographic information to associate with the image data. But the TIFF file structure allows both the metadata and the image data to be encoded into the same file.

GeoTIFF makes use of a public tag structure which is platform interoperable between any and all GeoTIFF-savvy readers. Any GIS, CAD, Image Processing, Desktop Mapping and any other types of systems using geographic images can read any GeoTIFF files created on any system to the GeoTIFF specification.

### Who owns GeoTIFF Format?

The GeoTIFF format is completely open, public domain, non-proprietary. It was produced by Dr. Niles Ritter, while at NASA-JPL (Jet Propulsion Laboratory), and changes, or additions to the format are proposed through a public review using email and WWW resources. Two meetings of developers from all areas of the geographic information processing industry have lead to proposals for revision, since early 1994.

There is no restriction on licensing, implementation, promulgation, or any uses of the format. The format is entirely open, and available to all. The specifications are public, there are abundant free software source libraries, toolkits, data samples, and technical support through the email forum.

### Can IP, GIS, CAD, and Desktop Mapping systems all use GeoTIFF?

The format is available to any software company, for free with no restrictions on license. It is the decision of the product managers for each software to include GeoTIFF capability in the geographic data management products which they sell.

Many systems today can read GeoTIFF files into correct geographic position in a variety of Geographic Information Systems (GIS), Image Processors (IP) and Computer Aided Design (CAD) software. You need to check with your system provider to find out if they have written the software module to read/write GeoTIFF imagery in the software version which you use.

### Does GeoTIFF imagery conform to the TIFF specification?

The TIFF spec (Version 6) is be supported by the GeoTIFF format. That is, the GeoTIFF images conform in every way to the TIFF formal specifications. The tags used for the "Geo" portion of the TIFF format conform to the acceptable and customary use of "private" or "registered" TIFF tagsets. The GeoTIFF tags and definitions are considered completely "orthogonal" to the baseline and extended TIFF tag definitions currently supported in TIFF V 6 by Adobe/Aldus.

### Can systems that don't use geography read a GeoTIFF image?

Many such image "visualization" systems do not use geography as a basis for placement of their images. These systems are all able to view a GeoTIFF image just as though there were no geographic information in the TIFF file. To non-GeoTIFF-savvy readers, the GeoTIFF image should look and behave like any other TIFF image.

Note, that some systems that don't support the double precision tag type of TIFF 6.0, or that fail with TIFF files with unexpected private tags, cannot read GeoTIFF files. It seems that ArcView 2 suffered from this problem.

### Does the Geographic data look transparent to the user?

Or does the user have to do something with the GeoTIFF to be able to load and place it ?

This is a software implementation question. But most GeoTIFF-savvy systems look at the geographic information and use it without any requirement that the user know the content of the geographic tags. The GeoTIFF format provides enough information that the software can automatically place an image without requirement of any user intervention, such as typing in coordinates, digitizing points, or other labor intensive and technical actions. One aim of GeoTIFF is to reduce the need of users to be geographic experts in order to load a map-projected image or scanned map.

### Who is working on definition of GeoTIFF?

The original working group included representatives of SPOT Image Corp, and SPOT Image S.A. (France), Jet Propulsion Laboratory (NASA), US Geological Survey, Intergraph, ESRI, ERDAS, Trifid, MapInfo, and Softdesk. In addition, a consortium of over 140 international subscribers have been monitoring and contributing to the spec's evolution through an email discussion forum. In April, 1996, over 40 industry representatives, including several international participants, members of US DMA (Defence Mapping Agency), and additional software companies (Bentley, E Systems, and others) and additional satellite and aerial imaging providers (Space Imaging, Orbital, Vargis Aerial and others).

### What's a TIFF tag?

A TIFF tag conveys information about the imagery. It is TIFF's version of metadata (data about data). TIFF Tags are the only way that a TIFF reading software can obtain fundamental information about the TIFF image. TIFF reading software all implement TIFF tags to control the display and printing of TIFF images.

### What is the relationship between a TIFF tag and the actual image?

The image is the "product" or the "purpose" of the TIFF format. The "tags" merely describe the image with information that the TIFF reader needs to know in order to control the appearance of the image on the user's display screen or printer.

### Are the tags in separate files from the tiff files ...

... or are they built into the tiff file format?

The TIFF tags are built into the TIFF format. They include:

- TIFF Basic Tags (like tags for number of rows, columns, etc). These are standard and mandatory in all TIFF files.
- TIFF Extended Tags (like tags for certain color systems, rather more specialized). These are optional in all TIFF files.
- "Private" TIFF tags (like the GeoTIFF tags). These include tags which are registered to specific users, in blocks of five tags. The users who register their tags, may put any information they like on those tags to support their software community. Then they can write software to use those tags in whatever way they please.

### Why is GeoTIFF based only on TIFF format?

... instead of, say, GIF or BMP or LAN or IMG or other formats? TIFF is already used for geographic imagery, using certain proprietary tag structures. GeoTIFF merely expands this to involve public domain tags and structures that are interoperable between any GIS.

TIFF is the only format in the public domain which can support the wide variety of image types already used for geographic imagery. TIFF can support 8-bit black and white images, up to 16 and 32 bit elevation models, 24 bit color imagery, and any other type of image. TIFF also supports the most varieties of compression and tiling options, to increase the efficiency of image transfer and utilization. The other data formats possible are limited in these areas, or are still merely proposals, while TIFF has been implemented for a decade or so.

It is almost the only format which allows the flexibility to support tag structures, like the geographic metadata of GEOTIFF, without causing a conflict with non-geographic uses of the raster imagery. TIFF is already the most popular image format in the world, and most GIS platforms can already handle TIFF format today, using either the TIFF Basic tagset, extended tagset, or various private tagsets.

### Is the tagset proprietary?

The TIFF tags (remember, these are *metadata*) are not proprietary. The GeoTIFF tags are completely public and published for anyone to use for any imagery. So, anyone may create any images which use the same tagset as SPOT or other users of the GeoTIFF structure.

### What are the advantages of TIFF format for geographic data?

The user of a GeoTIFF file, and a GeoTIFF-savvy software reader will notice that the software will let the user load and place the image on a simplified "point and click" basis. The users should not have to enter projection strings, control points, cartographic definitions, etc in order to support the loading of the image.

### Is GeoTIFF really platform interoperable?

The TIFF structure is the only image format which is extensible, allows private implementations and has the flexibility to support geographic information. For users, GeoTIFF files provide the possibility of a single image format compatible with all GIS vendor software. At present, many GIS software do implement the ability to read the exact same GeoTIFF specification for any compliant image file. This means that users of different software can now exchange and share raster files without needing to run translators, assuming they are exchanging between software that read GeoTIFF format.

The TIFF tag structure is designed to be simple and platform independent, so any GIS software developer can make use of the tags in just a few hours of program development. TIFF files which use the proposed GeoTIFF tagset can be transported from one display environment to another, provided both system software support the GeoTIFF spec. For the user, this means greater ease of use of images, less hassle importing and exporting raster formats, and less duplication of images in everyone's proprietary format in a single network of heterogeneous GIS displays.

### What will happen in my GIS if I load a GeoTIFF image?

When users "click on" a TIFF image in a GeoTIFF-savvy reader and it comes up on their screen the image should be automatically "georeferenced" to correct position and scale, with respect to the cartographic parameters of the users "map projection." Thus the GeoTIFF image should display "in the right position", compared to other features in the computer display (such as roads, hydrology, or similar mapped features). In this way, a satellite image of, say, San Francisco, will position itself in the GIS in San Francisco, and the roads, streams, and coastlines in the GIS coverages will overlay on top of the image. This is useful for any GIS applications where the user wants to see both the digital map, and a picture of the same area in the same geometric reference. From this a user can "heads-up" digitize, interpret features visible on the image, or conduct other raster/vector processes supported by the GIS.

### Doesn't anyone else have TIFF tags for geographic information?

Both ESRI and Intergraph have been involved in the GeoTIFF definition since the inception of GeoTIFF and have released software which support the open GeoTIFF tag structure.

ESRI has a history of using a private tag which Arc/Info uses to support image placement. This tag is basically a simple tag implementation of their WORLD file on a TIFF tag, internal to the TIFF format file. ESRI prefers to keep this tag private. GeoTIFF tags do everything ESRI's tag could do and a lot more.

Intergraph also has a tag set which they have used for several years to support their raster geographic software. This set is completely private since they reserve the right to change any of its structure at any time without review or notification outside of their user base. There are many many other TIFF private tagsets, as well as private "IFD's" (image file directories) which can exist transparently within the TIFF structure. But only GeoTIFF is known to the public domain and supports geographic parameters.

### Is GeoTIFF based on the world reference files that ESRI uses?

ESRI's world file is a separate file with limited metadata content designed only for use in Arc/Info and other ESRI software, though widely supported by other software. The world reference file only contains the coefficients for an affine transformation between raster coordinates, and world coordinates, but no definition of the coordinate system.

The GeoTIFF structure, by comparison, is rich in content and packs everything into a single file. The GeoTIFF tag structure covers more diverse options for projection spaces, datums, ellipsoids, coordinate types, and related geographic features than the ESRI tag. And the GeoTIFF structure is intended to become entirely public domain, unlike tags withheld for private implementations.

The contents of the worldfile are six numbers (each on their own line):

    A
    B
    C
    D
    E
    F

The georeferenced location of a pixel center can be computed as:

    Xgeo = E + Xpixel * A + Ypixel * C
    Ygeo = F + Ypixel * D + Xpixel * B

So, a typical (north up) arrangement would be:

    pixel width in geounits
    0
    0
    pixel height in geounits (normally negative)
    top left X geo location
    top left Y geo location

### How do I convert between ESRI world files, and GeoTIFF?

While it is presumably possible to accomplish this with ESRI software, it is also possible to use the listgeo and geotifcp executables distributed with libgeotiff to accomplish this. Recent binaries for selected platforms, and source, can be found at:

ftp://ftp.remotesensing.org/geotiff/libgeotiff/

To convert a TIFF file (abc.tif) and a world file (abc.tfw) into a simple GeoTIFF file (geo_abc.tif) without a projection definition use the following command:

      % geotifcp -e abc.tfw abc.tif geo_abc.tif

If a geotifcp compatible metadata file defining the coordinates system is also available (such as produced by listgeo run on a comparible GeoTIFF file), it can be combined with the georeferencing information of the world file to produce a fully defined GeoTIFF file using a command like this:

      % geotifcp -e abc.tfw -g projdef.txt abc.tif geo_abc.tif

The listgeo executable can be used to produce a geo_abc..tfw file from a GeoTIFF file (geo_abc.tif) using a command like this:

      % listgeo -tfw geo_abc.tif > projdef.txt

### How do I preserve GeoTIFF info when editing my image?

When I edit my GeoTIFF image in Photoshop or other applications, and then save the result the geotiff information is lost. How can I avoid this?

If a given applications does not support the GeoTIFF tags it will generally discard them when reading a GeoTIFF image and they won't be present after a save. However, it is possible to save the GeoTIFF metadata and reapply it to the image later using commandline utilities distributed with libgeotiff.

Given a GeoTIFF file named original.tif, and a modified file (modified.tif) without the GeoTIFF tags, but still the same size and region:

      listgeo -no_norm original.tif > original.geo
      geotifcp -g original.geo modified.tif modified_geotiff.tif

The listgeo utility is saving a text version of the geotiff tags in the file original.geo, and then geotifcp is copying the modified image, and in the process reapplying the GeoTIFF tags to produce a modified GeoTIFF file. This process only works properly if the modified.tif is the same number of pixels and lines as the original file, and represents the same region on the earth.

### What are the requirements to make a TIFF image ...

... which is geographically correct for the proposed tagset?

The images should be pre-corrected to a standard map projection, such as UTM, StatePlane, an international grid coordinate system (such as "British NationalGrid", or "Nigeria East Belt" or similar). Geotiff-savvy TIFF readers can then use the geotie tags to place the image (say a satellite image) into the correct position in the GIS display. For SPOT's products (called our SPOTView product line) the images will be accurate within 12 meters horizontal anywhere in the USA (a looser spec prevails in most international SPOTViews). Images which are not georeferenced to a map projection can still be entered into a GeoTIFF format but with some compromises on precision/accuracy of placement.

### Can I make customized geographic projections using GeoTIFF?

The GeoTIFF spec contains fields for definition of projection variables in 20 different coordinate transformation systems (such as Lambert Conformal Conic, Albers Equal Area, Transverse Mercator, etc). The datums and ellipsoids supported by POSC are all available, as well as limited support for most custom datums and ellipsoids. The GeoTIFF format provides rules, fields, and "cookbooks" for defining customized cartographic spaces if you need them. Since they will be coded as "User Defined" in some fields, not all GeoTIFF readers will be able to understand all possibilities.

### What is the effect of PixelIsArea vs. PixelIsPoint?

Setting the GTRasterTypeGeoKey value to RasterPixelIsPoint or RasterPixelIsArea alters how the raster coordinate space is to be interpreted. This is defined in section 2.5.2.2 of the GeoTIFF specification. In the case of PixelIsArea (default) a pixel is treated as an area and the raster coordinate (0,0) is the top left corner of the top left pixel. PixelIsPoint treats pixels as point samples with empty space between the "pixel" samples. In this case raster (0,0) is the location of the top left raster pixel.

The net effect is a half pixel offset when interpreting PixelIsPoint vs. PixelIsArea images. There has been some confusion on the interpretation of this value in the GeoTIFF community for some time and some held the opinion that this parameter should have no effect on the georeferencing of the image. While this matter has been settled, the result is that some software packages, particularly those built on GDAL 1.7.x or older incorrectly interpret the georeferenced location of PixelIsPoint images - placing them offset by half a pixel. One way of avoiding compatibility problems is to produce GeoTIFF files with the default PixelIsArea interpretation.

### What is the Axis Order in GeoTIFF?

The axis order of a coordinate system is used to interpret coordinates. For instance should (-5,45) in EPSG:4326 be interpreted as -5 degrees longitude, and 45 degrees latitude or -5 degrees latitude and 45 degrees longitude?

The GeoTIFF specification does not appear to specifically address axis order. The EPSG dataset does define axis order for each coordinate system. Unfortunately for most (all?) geographic coordinate systems it defines a lat/long order that conflicts with common GIS practice and the examples and existing practice in GeoTIFF files.

For that reason, it is suggested that geographic coordinate systems in GeoTIFF be assumed to be with longitude, latitude coordinate ordering in the GEOTIEPOINTS and related fields.

For projected coordinate system the situation is less clear. Examples are all easting, northing but then most projected coordinate systems are defined this way in EPSG. However, a few coordinate system (such as Krovak - EPSG:2065) actually use other axis orientations. In the case of Krovak the first coordinate is actually southing, and the second is westing. Note that axis orientation can also affect the direction of positive (east vs. west, north vs. south) as well as the order of the coordinates.

While there are few examples of files in these coordinate systems, it is suggested practice to honour EPSG axis orientation definitions for projected coordinate systems.

### How should Vertical Datums and Vertical Coordinate Systems be encoded in GeoTIFF?

The GeoTIFF specification only lightly addresses vertical coordinate systems. In support of .LAS format (which uses GeoTIFF tags for coordinate systems) and for general GeoTIFF use there are some vertical coordinate system best practices being written up in the wiki.

### Who are POSC and EPSG and why is GeoTIFF involved with them?

POSC is the Petrotechical Open Software Company, a non-profit corporation dedicated to open data standards and interoperability in the oil and gas business. They distribute a geodetic model and encoding system which is international in scope, hierarchical in structure and inclusive of many cartographic options and variables. The geodesy system and tables are created and supported by the OGP, the International Association of Oil & Gas Producers who have absorbed the work of the European Petrotechnical Survey Group (EPSG). The POSC tables (maintained by EPSG) are the most comprehensive and TIFF compatible tables of cartographic inforamation available in the public domain today.

### What EPSG Version does GeoTIFF Use?

The GeoTIFF 1.0 Specification references the EPSG 2.1 database. Without a specification revision this remains the case; however, it is common practice among vendors producing and consuming GeoTIFF files to support recent versions of the EPSG database, currently 6.4 or higher. The libgeotiff reference implementation is distributed with .csv files from a recent release of the EPSG database (used by normalization code); however, the names enumerated in the various .inc files have not been kept up to date.

### Where can I get a copy of the GeoTIFF spec to review?

The most current version of the spec is maintained on the GeoTIFF WWW Homepage. The spec it self is at:

      http://www.remotesensing.org/geotiff/spec/geotiffhome.html

The GeoTIFF WWW site is supported by Frank Warmerdam. It contains pointers to other GeoTIFF and TIFF resources, including software, sample data sets, email archives, compiled utilities, and more.

### What can I do to support GeoTIFF ...

... and platform interoperable raster imagery?

TELL your GIS software provider that you need to read raster imagery which is in GeoTIFF format. Log formal change requests with your software vendor, especially if they are not on the current list of GeoTIFF supporters. Most software companies develop on the basis of logged formal client change requests. If nobody puts in a change request, then the software doesn't get developed. You will be doing yourself, your GIS provider, and your fellow users a service by making this issue visible to the GIS providers.

You can also participate in a variety of standardization activities throughout the geographic information processing industry and government. GeoTIFF has been proposed to the Open GIS Consortium, for example, and other entities are evaluating whether GeoTIFF should be proposed to international or other standards groups. Let your opinion be known. "Wreak yourself upon the world" (Robert Bork).

### How can I monitor the progress and changes to future GeoTIFF specs?

There is a GeoTIFF email list where the GeoTIFF file format and specification is discussed. Subscription and mailing list archive information is available at:

http://lists.maptools.org/mailman/listinfo/geotiff

### Is the GeoTIFF community still active?

GeoTIFF standard improvement efforts have been quiet since about early 1997. In early 1998 the original GeoTIFF mailing list stopped working due to a machine failure at JPL, and a new list wasn't started till the fall of 1998. Dr. Niles Ritter has left JPL, and has less time to put towards GeoTIFF efforts. Mike Ruth has left Spot, and is less active in GeoTIFF efforts.

However, in the meantime most geographically aware software produced that could utilize TIFF imagery has added some level of support for GeoTIFF ingest, and/or production. Discussions on mailing lists and between software vendors has resulted in some refinements to interpretations of some GeoTIFF tags.

In a sense GeoTIFF has become a large success despite a light hand at the helm.

However, the quality of GeoTIFF implementations varies greatly, there are no means to do conformance testing on produced GeoTIFF files.

The GeoTIFF mailing list is still active, and is a good place to ask questions about GeoTIFF topics. As well, clarifications are developed there for details about the list of available projections, their parameters, units issues and so forth. The libgeotiff reference library is also periodically released with updates to the EPSG .csv tables, and bug fixes.

There have been some efforts to put the GeoTIFF specification into the care of either NASA or OGC, but those have not been successful. In 2008 libgeotiff joined the OSGeo MetaCRS Project and to some sense stewardship of the GeoTIFF specification is also in the care of that project.<p. In short, the GeoTIFF community is alive, but relatively quiet.