FAQ (Frequently Asked Questions) about TIFF Geotie Tag Initiative FAQ Prepared by: Mike Ruth SPOT Image Corp ruth@palv.spot.com Version 1.4 29 NOV 1995 This FAQ is prepared in response to email inquiries concerning the "GeoTIFF" email forum, and its activity toward definition of a geographic TIFF file format. A full version of the GeoTIFF specification, supporting documentation, and pointers to software, samples, and other GeoTIFF info is at: http://www-mipl.jpl.nasa.gov/cartlab/geotiff/geotiff.html For people who do not have WWW access, SPOT supplies "GeoTIFF Toolkits" upon request. Please send an email to SPOT at ... "products@spot.com" and put "Request GeoTIFF" in the subject ... line of your message. We generally issue new diskettes every couple of weeks with the software, specs, samples, and more on them. ***** Begin FAQ ****** Q What is TIFF format? TIFF format is the most popular 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. Q: What is the purpose of using TIFF 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. Q: 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. Q: Will IP, GIS, CAD, and Desktop Mapping systems all be able to use GeoTIFF? It is the objective of the GeoTIFF designers to create a GeoTIFF tag structure which is platform interoperable between all GeoTIFF-savvy readers. We hope that all GIS, CAD, Image Processing, Desktop Mapping and any other types of systems using geographic images will be able to read any GeoTIFF files created on any system in the GeoTIFF specification. Q: Who supports GeoTIFF today? There are many GIS software companies who have scheduled their support for GeoTIFF in their current or near-futre product updates. These include Mapinfo, Softdesk, ERDAS, Innovativ Vision, TNTMips, DataDoors, Laser-Scan, Able Software, ER Mapper, PCI, and BBN. Intergraph and ESRI support the format, though they have not yet revealed their schedule for when that support will appear. Data providers who are committed to deliver imagery in the GeoTIFF format include SPOT Image Corp, Trifid (representing LandSat data), Space Imaging, US Geological Survey, and the New York Department of Transportation. In addition, the United Kingdom Military Survey has announced it is testing the format for their products. Q: How can I get updates on what systems and providers are supporting GeoTIFF standard? Use the WWW pages for GeoTIFF, which you can access from the NASA Jet Propulsion Laboratory or the US Geological Survey. The information on industry support is in the following URL: http://www-mipl.jpl.nasa.gov/cartlab/geotiff/software.html Q: Does "GeoTIFF" imagery conform to the TIFF specification? The TIFF spec (Version 6) will be supported by the GeoTIFF format. That is, the GeoTIFF images will conform in every way to the TIFF Version 6 specification. The tags used for the "Geo" portion of the TIFF format will conform to the acceptable and customary use of 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. Q: Will systems that don't use geography be able to read a GeoTIFF image? Many such image "visualization" systems do not use geography as a basis for placement of their images. These systems should be 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 like any other TIFF image. Q: Will the Geographic data look transparent to the user? This is a software implementation question. But it is likely that most GeoTIFF-savvy systems will 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. Q: Who is working on definition of GeoTIFF? The working group includes 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 for about a year. Q: 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 softwares all implement TIFF tags to control the display and printing of TIFF images. Q: 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 support 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. Q: Are the "tags" 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: o TIFF Basic Tags (like tags for number of rows, columns, etc). These are standard and mandatory in all TIFF files. o TIFF Extended Tags (like tags for certain color systems, rather more specialized). These are optional in all TIFF files. o "Private" TIFF tags (like our proposed geographic tags at present). 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. Q: Why are you so interested in TIFF format instead of, say, GIF or BMP or LAN or IMG or other formats? 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 ite" 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 many many readers Most GIS platforms can already handle TIFF format today, using either the TIFF Basic tagset, extended tagset, or various private tagsets. Q: What are the advantages, to the user, of TIFF format for geographic data? 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 softwares. 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 softwares 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. Q: What will happen to 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, there is presently no means to let the software "georeference" the image. "Georeference" refers to the software logic to place the image within a cartographic display space based on a map projection. Thus the image usually comes up "in the wrong position", compared to other features in the computer display (such as roads, hydrology, or similar mapped features). With geotie tags, the TIFF image can be placed exactly in the correct position with respect to the earth's surface. So, 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. Q: Doesn't anyone else have TIFF tags for geographic information? ESRI has 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. We recognize the limitations on the use of this tag for general raster data exchange and have designed GeoTIFF to do everything ESRI's tag could do and a lot more. Intergraph also has a tag set which they use to support their softwares. This set is also "private" since they reserve the right to change any of its structure at any time without review or notification. 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 there are none which are known to the public domain which support geographic parameters. Q: Are Geotie tags 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 softwares. The GeoTIFF structure, by comparison, is rich in content and packs everything into a single file. The GeoTIFF tag stucture 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. Q: 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). Geotie-tag 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. Q: 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 available, as well as limited support for custom datums and ellipsoids. Q: Who is POSC? POSC is the Petrotechical Open Software Company, a non-profit corporation dedicated to open standards in the oil business. They have a geodetic model and encoding system which is international in scope, hierarchical in structure and inclusive of many cartographic options and variables. The POSC tables (maintained by EPSG) are the most comprehensive and TIFF compatible tables of crtographic inforamation available in the public domain today. Q: Where can I get a copy of the GeoTIFF spec to review? The most current version of the spec is maintained on a posted to the WWW site. This WWW site is supported by Jet Propulsion Laboratory Cartographic Applications Group and is: http://www-mipl.jpl.nasa.gov/cartlab/geotiff.html or, for those using only ftp: ftp://mtritter.jpl.nasa.gov/pub/tiff/geotiff USGS maintains a mirror site of this spec at: ftp://ftpmcmc.cr.usgs.gov/release/geotiff Q: How can I monitor the progress to the completion of the TIFF geotie tagset definition? There is an electronic forum over the Internet called "GeoTIFF" where the discussion of the TIFF geotie tagset is taking place. To subscribe to this list, send a one line email to: geotiff-request@tazboy.jpl.nasa.gov The email should say, in the body of the message: SUBSCRIBE GEOTIFF in which you substitute your name for the part in brackets, and don't type in the brackets. For example: SUBSCRIBE GEOTIFF June Smith The email list traffic is presently running 2-5 messages/week though it could flare up from time to time. To post inquiries directly to the list, send email to: geotiff@tazboy.jpl.nasa.gov Q: Is the tagset proprietary? Is the SPOT data still proprietary? The TIFF tags (remember, these are *metadata*) are not proprietary. The actual *image* (if provided by SPOT) IS PROPRIETARY and subject to all licenses applicable to the purchase of the data. This means that you cannot copy and send around SPOTView images, just because they are in TIFF format! This is against the law. The TIFF geotie tags, when they are finally specified, will be 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 geotie tags. The tags are public, the imagery is not. Q: 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. Thanks for reading to the end! ... Mike Ruth ------------- Mike Ruth Email: ruth@palv.spot.com Product Development SPOT Image Corporation 1897 Preston White Dr Reston, VA 22091 USA voice: 703) 715-3146 fax: 703) 648-1813 ------------ .