Microsoft TerraServer Microsoft TerraServer Collapse the table of content Expand the table of content This documentation is archived and is not being maintained. This documentation is archived and is not being maintained. Microsoft TerraServer Tom Barclay, Robert Eberl, Jim Gray, John Nordlinger, Guru Raghavendran, Don Slutz, Greg Smith, Phil Smoot Microsoft Research and Development John Hoffman, Natt Robb III Aerial Images Hedy Rossmeissl, Beth Duff, George Lee, Theresa Mathesmier, Randall Sunne Lee Ann Stivers, Ken Goodman Digital Equipment Corporation June 1998 Summary: The Microsoft® TerraServer stores aerial and satellite images of the earth in a Microsoft SQL Server™ database served to the public through the Internet. It is the world's largest atlas, combining five terabytes of image data from the United States Geodetic Survey, SOVINFORMSPUTNIK, and Encarta® Virtual Globe. Internet browsers provide intuitive spatial and gazetteer interfaces to the data. The TerraServer demonstrates the scalability of Microsoft Windows NT® Server, Enterprise Edition version 4.0 and SQL Server, Enterprise Edition running on Digital hardware including the AlphaServer 8400 and StorageWorks™ storage system.

The TerraServer is also an E-Commerce application. Users can buy the right to use the imagery using Microsoft Site Servers managed by the USGS and Aerial Images. This paper describes the TerraServer's design and implementation. Contents The Microsoft TerraServer Figure 1. The TerraServer hardware The TerraServer has five terabytes of uncompressed satellite and aerial images of urban areas, compressed to one terabyte of database data. It serves these images onto the Internet with a graphical and intuitive user interface. The application demonstrates several things: Information at your fingertips. This is the most comprehensive world atlas anywhere—and it is available to anyone with access to the Internet. Windows NT Server, Enterprise Edition and SQL Server, Enterprise Edition version 7.0 scale. The TerraServer fills eight large cabinets: one for the Digital Alpha 8400 processors, and seven cabinets for the 324 disks—almost three terabytes (TB) of raw disk storage and 2.3 TB of RAID5 storage. Windows NT and SQL Server, Enterprise Edition are excellent for serving multimedia and spatial data onto the Internet. Microsoft Site Server Commerce Edition can help sell images over the Internet.

TerraServer is a multimedia database that stores both classical text and numeric data, as well as multimedia image data. In the future, most huge databases will be comprised primarily of document and image data. The relational metadata is a relatively small part of the total database size. TerraServer is a good example of this new breed of multimedia databases. The Application An Interesting Internet Server. TerraServer is designed to be a compelling Internet application. It tries to be interesting to almost everyone, everywhere, to be offensive to no one, and to be relatively inexpensive to build and operate. It is hard to find data like that—especially a terabyte of such data. A terabyte is nearly a billion pages of text—four million books. A terabyte holds 250 full-length movies. It is a lot of data. Figure 2. World population density Satellite Images of the Urban World. Pictures have a universal appeal, so it was natural to pick a graphical application. Aerial images of the urban world seemed to be a good application. The earth's surface is about 500 square tera-meters. 75 percent is water, 20 percent of the rest is above 70° latitude.

This leaves about 100 square tera-meters. Most of that is desert, mountains, or farmland. Less than 4 percent of the land is urban. The TerraServer primarily stores images of urban areas. Right now, it has nearly five square tera-meters—and it grows as more data becomes available. Cooperating with the United States Geological Survey (USGS): The USGS has published aerial imagery of many parts of the United States. These images are approximately one-meter resolution (each pixel covers one square meter). We have a Cooperative Research Agreement (CRADA) with the USGS to make this data available to the public. We have loaded all the published USGS data (3 TB raw, 0.6 TB compressed). This is 30 percent of the United States. As additional data becomes available, it will be loaded into the TerraServer. This data is unencumbered and can be freely distributed to anyone. It is a wonderful resource for researchers, urban planners, and students. The picture at left shows a baseball game in progress near San Francisco. You can see the cars, but one-meter resolution is too coarse to show people. Figure 3.

A USGS 1-meter resolution image of Candlestick Park near San Francisco Working with SOVINFORMSPUTNIK (the Russian Space Agency) and Aerial Images. To be interesting to everyone everywhere, TerraServer must have worldwide coverage. The USGS data covers much of the continental United States. There is considerable imagery of the planet, but much of it either has poor quality (10 meter to 1-km resolution), has not been digitized, or is encumbered. SOVINFORMSPUTNIK and their representative, Aerial Images, have some of the best data and were eager to cooperate. The Russians and Aerial Images contributed two square tera-meters of imagery (1.56-meter resolution). This data is trademarked SPIN-2, meaning satellite-2-meter imagery. They intend to deliver an additional 2.4 square terra-meters over the next year. Figure 4. A SPIN-2 1.6-meter image of Atlanta's Olympic stadium TerraServer is the largest world atlas. The SOVINFORMSPUTNIK SPIN-2 imagery covers Rome, Athens, Hong Kong, New York, Chicago, Seattle, and many other cities.

TerraServer has more data in it than all the HTML pages on the Internet. If printed in a paper atlas, with 500 pages per volume, the information would fill a collection of 2,000 volumes. It grows by 10,000 pages per month. Clearly, this atlas must be stored online. The USGS data (the three square tera-meters) is seven times larger. This data is a world asset that will likely change the way geography is taught in schools, the way maps are published, and the way we think about our planet. TerraServer as a business. Slicing, dicing, and loading the SPIN-2 and USGS data is a continuing process. Today, the TerraServer stores a terabyte. Aerial Images, Digital, and Microsoft are operating the TerraServer on the Internet ( ). Microsoft views TerraServer as a demonstration of the scalability of Windows NT Server and Microsoft SQL Server, Enterprise Edition. Digital views it as a demonstration of their Alpha and StorageWorks servers. The USGS is participating as an experiment to present USGS data to a wider audience through the Internet. They operate an online store that allows anyone to download copies of the USGS images.

SOVINFORMSPUTNIK and Aerial Images view TerraServer as a try-and-buy distributor for their intellectual property. They make coarse-resolution (8-meter, 16-meter, and 32-meter) imagery freely available. The fine-resolution data is viewable in small quantities, but customers must buy the right to use the good imagery. All the SPIN-2 images are watermarked, and the high-resolution images are lightly encrypted. Site Server Commerce Edition, a new business model for the Internet. Aerial Images' business model is likely to become a textbook case of Internet commerce. Because they use the Internet to sample and distribute their images, Aerial Images has very low distribution costs. This allows them to sell imagery in small quantities and large volumes. Microsoft helped USGS and Aerial Images set up Microsoft Site Servers that accept credit-card payments for the imagery. A Download button on the image page takes the user to these Site Servers (Microsoft has no financial interest in these transactions). You can buy a detailed image of your neighborhood for a few dollars. Figure 5. Navigation through TerraServer User Interface to the Microsoft TerraServer Navigation through database searches. The TerraServer can be accessed from any Web browser (for example, Internet Explorer, Netscape Navigator).

Full resolution SPIN-2 imagery requires the Web browser to support Java applets. Any Web browser that supports HTML tables and display of Jpeg data can host the TerraServer user interface. Navigation can be spatial through a point-and-click map control based on Microsoft's Encarta World Atlas. Clients only knowing the place name can navigate textually by presenting a name to the Encarta Virtual Globe Gazetteer. The gazetteer knows the names and locations of 1.1 million places in the world. For example, "Moscow" finds 28 cities, while "North Pole" finds 5 cities, a mining district, a lake, and a point-of-interest. There are 378 listings for San Francisco in the gazetteer. The user can select the appropriate member from the list. The map control displays the 40-km map of that area. The user can then pan and zoom with this map application and can select the USGS and SPIN-2 images for the displayed area. Navigation using coverage map. The USGS gave us a shaded relief map (Mercator projection), which includes political (state and province) boundaries. We shaded this map green where we have some imagery.

Then we built an image pyramid: one image for the whole planet, and two levels of zoom that cover a continent and a region on the continent. The bottom panels of the previous figure show an example of zooming in on New York City. We added this interface last, but it is the most popular way to navigate the database. Spatial navigation using the map control. A dynamic HTML page allows the client browser to talk with the Microsoft Expedia™ travel service map server ( ) that provides the basic features of Microsoft Encarta World Atlas and Microsoft Automap® Streets as GIF images. The applet lets users pan and zoom over graphic images of the earth and of US street maps. The applet decides what the client wants to see and sends a request for that map to the Expedia map server. That server, given the corners of a rectangle and an altitude, generates the view of the earth inside that rectangle. It generates a GIF image that is downloaded to the applet in the client browser. The map server is provided by MSN™ to any Internet customer. We have wrapped it in our Java applet. This application works on Windows, Macintosh, and UNIX clients. Coverage map and spatial access is especially convenient for those who do not understand English.

Zooming in and out. The map controls allow the browser to zoom out and see a larger area, or zoom in and see finer detail. The coarsest view shows the whole planet. The user can spin the globe to see the other side and place the point of interest in the center of the screen. Then the user can zoom in to see fine detail. Where we have street maps (Microsoft Automap Streets), the zoom can go all the way down to a neighborhood. Figure 6. Moving Around: Once you find the spot you are looking for, you can see nearby places by pushing navigation buttons to pan and zoom. By doing this, you can "drive" cross-country. Encarta, USGS, and SPIN-2 Themes. TerraServer has several different views of the earth: the coverage map view, the Encarta World Atlas view, the USGS image view, and the SOVINFORMSPUTNIK/SPIN-2 view. We call each of these views a theme. The user may switch from one theme to another, perhaps starting with the Encarta theme, then the SPIN-2 theme, and then the USGS theme of the same spot.

With time, we expect to have multiple images of the same spot. Then the user will be able to see each image in turn. Your grandchildren will be able to see how your neighborhood evolved since 1990. Download. If you like the SPIN-2 imagery or USGS image, you can push the DownLoad Image button. That takes you to the Aerial Images or USGS Site Server. Both Aerial Images and USGS e-commerce sites run Microsoft's Commerce Server, which is a component of Microsoft Site Server. That is where the similarity between Aerial Images and the USGS e-commerce site ends. The Aerial Images Web site allows Internet users to select one of three sizes for a digital image. The user can also select a choice of format (TIFF or JPEG) and can have a photograph printed and delivered overnight by Kodak. The USGS site allows users to download the image viewed on Microsoft TerraServer as a single digital image in JPEG format at no charge. The USGS site offers an easy method to purchase one or more of the original data sets used to form these images. The original USGS imagery is intended for use by professional geographic information system (GIS) users.

The Site Server allows you to shop for imagery and quotes you a price. If you want to purchase a high-quality digital copy of these images, Site Server asks for your credit card account number, debits it, and downloads the images you purchased or ships them to you on the media of your choice. The USGS and SPIN-2 sites are good examples of selling soft goods over the Internet. Server Design The TerraServer has several components that combine to make a seamless Internet application: Internet Information Server and Active Server Pages. Clients send requests to the TerraServer's Internet Information Server (IIS), built into Windows NT Server 4.0. These requests are passed to Active Server Pages (ASPs) programmed in Visual Basic®, Scripting Edition (VBScript). These ASPs send queries to stored procedures in the SQL Server, Enterprise Edition database to query the Gazetteer and to fetch image tiles. The ASPs dynamically construct the HTML Web pages needed to mosaic the tiles together to make a complete image. The server first returns the HTML for the outer frame, and the HTML table referencing the two-dimensional array of tiles. The number of tiles displayed in the HTML table is controlled by the user and the current page's image resolution.

TerraServer stores imagery in 32 m/p (square meters/pixel), 16 m/p, 8 m/p, and full resolution (1 m/p or 1.56 m/p). The user decides if they want to see a small, medium, or large view. A full resolution small view page displays 4 image tiles whereas a large view page displays 16 image tiles. A 32 m/p Web page ranges from 64 tiles in a small view to 256 image tiles in a large view. When the client pans an image, 50 percent of the images on the current frame are moved to another location within the frame and the other 50 percent are downloaded from the database. Figure 7. TerraServer hardware and software The VBScript program dynamically creates the necessary HTML to render that image. It sends this HTML back to the client's browser. The client browser then requests the images needed to fill in the picture. Depending on the image size the user selects, this can be between 4 and 256 tiles. These URL requests generate between 30 and 500 database accesses. Tiled Image Database.

The database stores both the USGS data and the SPIN-2 data as small (10 kilobytes or less) tiles compressed with JPEG. Larger images are created as a mosaic of these tiles. This allows quick response to users over slow voice-grade phone lines. It also allows the application to pan and zoom across the images. Gazetteer: The Encarta World Atlas Gazetteer has over a million entries describing most places on earth. All these records are stored and indexed by Microsoft SQL Server, Enterprise Edition. Stored procedures look up these names and produce an HTML page describing the top 10 hits, with hot links to the images if they are in the TerraServer. Site Server Commerce Edition. If a client wants to buy some imagery from Aerial Images, the client pushes the DownLoad Image button. Site Server uses secure HTTP to authenticate the user and quote the user a price for the requested data. The data providers, Aerial Images and the USGS, have built electronic "stores" tailored to their existing products and their markets. The sites differ in pricing philosophies and market focus. The Aerial Images site is designed for unsophisticated users.

Imagery size and formats are designed to be attractive to nonprofessional users interested in a digital photo of a small area. The USGS site on the other hand, is targeted towards the USGS' traditional market of GIS professionals and data resellers. Both systems use credit cards as authorization for payment. Aerial Images' site downloads Digital images immediately over the user's Internet connection. Paper photographs are delivered by ground mail or overnight express services. USGS images are distributed by ground mail carriers only. Terra Server uses Microsoft SQL Server, Enterprise Edition. TerraServer uses the 1998 version of Microsoft SQL Server, Enterprise Edition. This version supports larger page sizes, parallel load, backup and restore utilities, has better support for multimedia, supports parallelism within queries, and supports much larger databases. TerraServer has been a good stress test for SQL Server, Enterprise Edition. Loading the Database. SOVINFORMSPUTNIK and the USGS delivered the data to us on several hundred tapes. We had to sort, reformat, slice, and dice this data before it could be inserted into the database. We wrote several programs to do this image processing.

We also wrote a load manager that consumes these files and feeds the data into the TerraServer using the SQL Server, Enterprise Edition loader APIs (ODBC BCP). Using several parallel streams, it loads at approximately 2 MBps. At this rate, the load takes 6 days. The load is constrained more by the scan, slice, and dice process than by the SQL Server load rate. Indeed, the database load rate is 15 MBps, eight times faster than the load program can produce the data. Summary. TerraServer is a new world atlas—far larger than any seen before. It is a relatively simple database application, but it demonstrates how to build a real Internet application using Windows NT Server Enterprise Edition version 4.0 and SQL Server, Enterprise Edition version 7.0, running on Digital AlphaServer 8400 and StorageWorks servers. Database Design The TerraServer presents an interesting geo-spatial database design problem. It contains data from three different sources represented in different coordinate systems. It has to integrate all this data into a single intuitive user interface. This section describes how the data is represented in the database and how it is indexed. Coordinate Systems The earth is not flat.

It is not round, either—it is a bumpy oblate spheroid. When measuring the earth at one-meter resolution, this becomes a very important issue. USGS DOQs. The USGS has aerial photographs of most of the United States. It has corrected these aerial photographs for elevation and camera optics. The USGS then maps the true image into the Universal Transverse Mercator (UTM) coordinate system. The resulting digital images are mosaiced into Digital Orthorectified Quarter Quadrangles (DOQQs for short). A quadrangle is a one-eighth of a degree square (about 3.5 kilometers on a side). The USGS has published paper quadrangle maps for many decades. A DOQQ is one-quarter of a USGS Digital Orthorectified Quadrangle (DOQ). The UTM system. UTM divides the earth into 60 zones. Each zone is two 6º spherical triangles going from the equator to the poles. The continental United States occupies nine UTM zones (Alaska and Hawaii add 7 more zones). A UTM projection flattens each of these spherical triangles (projects them onto a plane).

The meridian of the triangle is represented perfectly, but all the other pixel-points are slightly distorted to be trapezoids rather than squares. In particular, the pixels at the edges of a zone have north at 3º from the vertical. The UTM system maps latitude lines into curves. This is barely noticeable to the eye, but is very noticeable when images that lie on zone boundaries are concatenated. Figure 8. UTM USGS data uses UTM. We decided to use the USGS coordinate system for the USGS data. To be exact, the USGS uses UTM with the NAD83 datum. It would be too much work for us to remap the USGS data into a coordinate system that gives a seamless mosaic of the earth. In UTM, each point has a zone number, then a Northing (meters from the equator), and an Easting (meters from the west meridian of the triangle). TerraServer USGS images are a fixed size—1,800 x 1,200 meters.

The TerraServer assigns a unique UGridID to each TerraServer image by concatenating the UTM zone with the image's Easting ID (Easting + 400/1800) number followed by a bit interleave of the Northing ID (Northing/1200). The bit interleaving causes nearby images to have a common UGridID prefix. SPIN-2 uses latitude-longitude. The SPIN-2 data is taken from 200 kilometers up. An original SPIN-2 image is a 40 x 160 km photographic swath taken by a former Russian military satellite. (These are declassified photos. A recent US-Russian treaty allows Russia to export to the United States.) Each photo has a resolution of 1.56 square meters per pixel. We have 2 trillion square meters of these images (0.7 TB). Each 40 x 160 km photograph is scanned into four separate 40 x 40 km images because it is too large to scan all at once. The digital scan is separated into four separate 20 x 20 km files because Adobe Photoshop cannot rotate an image larger than 30,000 pixels.

Aerial Images personnel geo-locate five points on one 20 x 20 km image. Photoshop is used to rotate each 20 x 20 km quadrant. This creates the appearance of a diamond-shaped photo within a square white canvas. The upper left corner point and lower right corner point of the square image are computed. The upper left and lower right corner points of the other three images are computed relative to the first image. Our image editing program reads each SPIN-2 20 x 20 km image and creates TerraServer SPIN-2 images that are 1/48 of a degree wide x 1/96 of a degree high. Pixels from adjacent 20 x 20 km images are merged, creating a single TerraServer Spin-2 image. We mapped the SPIN-2 data into a latitude-longitude reference system—each image is given a unique Z-Grid ID, which is the interleaving of the latitude and longitude of the center point of the TerraServer SPIN-2 image. On earth, there are a total of 298,598,400 unique ZgridID values.

((360 longitude degrees x 48 "cuts per degree") x (180 latitude degrees x 96 "cuts per degree")) Images are 256-level gray scale JPEG. Browsers generally reduce the number of levels displayed. The images are all stored as JPEGs compressed to 80 percent to be faithful to the original image. This typically gives a 5:1 compression. The image tile pyramid. The images are stored in the database as image pyramids so that users can zoom in and out (see Figure 9). An additional constraint is that no image should be much larger than 10 KB. This constraint comes from the need to support clients accessing the database through 28.8 KBps modems. It takes about 3 seconds to download a 10-KB image. A complete Web page is made of mosaics of these small images. Tile, thumbnail, browse, and jump images. Large images are first sliced into tiles that are about 10 kilobytes each. Each of these tiles covers a tiny area (less than a tenth of a square kilometer). The TerraServer returns a mosaic of these tiles to the user on each query.

Coarser tile resolutions are stored to support zooming. The data load processes mosaic tiles and then dithers them down to wider-panorama images. For USGS data, an 8x8 mosaic is dithered down to 8-meter resolution to produce a browse image from the original images. For the SPIN-2 1.56-meter data, a 5x5 array of tiles is dithered to form the browse image. These browse images are further dithered down to 16-meter (thumbnail image) and 32-meter (jump image) resolution images. Because these images have lower resolution, they occupy 1/64, 1/256, and 1/1024 of the space of the tile images. That is, they occupy almost no space at all. Figure 9. The fine-resolution image of the Washington Monument is used to make an 8 x 8 mosaic. It is then dithered to an image where one pixel is 8 x 8 meters. This thumbnail image is then dithered down to a 16 x 16 meter browse image and a 32 x 32 meter jump image.

The TerraServer mosaics these small images to make zoomed-out images. Tile 4 x 3 aspect ratio. Tile sizes were chosen to approximately match the 4 x 3 computer display aspect ratio. The USGS data follows the USGS coordinate system. In the USGS data set, each tile is exactly 225 x 150 meters. Based on visual experiments, we chose a browse image size of 1/48 of a degree wide and 1/96 of a degree high. This translates to an area about 1 x 2 km. Further north, the 1/48 of a degree wide shrinks to almost nothing. In the middle latitudes, it has the desired 3 x 4 ratio. Each thumbnail is sliced 5 x 5 to make the tile images. Once a tile size is chosen, all the other sizes are a pyramid derived from that basic unit. Table 1 gives the approximate sizes and cardinalities of the data sets. Table 1. Cardinalities and sizes of the USGS and SPIN-2 data sets as stored in the TerraServer.

The SPIN-2 data tiles vary in size because the thumbnail images are 1/48 x 1/96 of a degree. 700 GB Database Themes As explained so far, there are two separate data themes: USGS DOQs and SPIN-2 satellite images. Each theme has its own set of SQL tables. Each image, along with its metadata, is a record in the database. The data is indexed by its geographic coordinates. The database size parameters are summarized in Table 2. Both USGS and SPIN-2 data continue to arrive—so these numbers will have increased by the time you read this. Table 2. The TerraServer database has 790 GB of user data stored in 223 million records. About 200 GB of additional space is consumed by overhead (about 25 percent). The remaining space is used for indices, catalogs, recovery logs, and temporary storage for queries and utilities. The database has a formatted capacity of 2.2 TB. Total Disk Capacity Figure 10: The image pyramid gives a zoom-in view of a spot SPIN-2 Theme.

The raw SPIN-2 data is divided into 4 40km x 40km photographs and scanned at 1.56-meter resolution. One 40km x 40km photo is picked to be the anchor photograph. Five points are geo-located in the anchor photograph. The anchor and its 3 siblings are each quartered (a total of 16 images) and rotated the same angle. They rotate so that North is up, optical distortion is minimized, and geo-located pixels in the anchor image are accurate to 50 meters. The sibling images are geo-located such that pixels from the siblings can be aligned with the anchor image. The SOVINFORMSPUTNIK and Aerial Images do all this work. The data is then sent to Microsoft on 20 GB DLT magnetic tapes. The typical image is 300 MB. It would take three years to download such an image over a 28.8 modem. We slice and dice these images into 10-KB tiles that can be downloaded in a few seconds.

The slice-and-dice step produces four products: Jumps: JPEG compressed images covering a 1 x 1.3 km area at 32-meter resolution Thumbnails: JPEG compressed images covering a 1 x 1.3 km area at 16-meter resolution Browse: JPEG compressed images covering a 1 x 1.3 km area at 8-meter resolution Tiles: JPEG images that cover a 240 x 300 m area at 1.6-meter resolution. The key property is that these tiles can be downloaded quickly over a voice-grade telephone line. The tile images are lightly encrypted. Each image, along with its metadata (time, place, instrument, and so on), is stored in a database record. Each resolution is stored in a separate table. This data can be cross-correlated with the Gazetteer and other sources by using the Z-transform (see Endnote ). In January 1998, we had 16 million tiles, and 650 thousand browse, thumbnail, and jump images. This totals 101 gigabytes of compressed user data. It is 800 GB of uncompressed data. Loading continues as more data arrives from the SOVINFORMSPUTNIK.

USGS Theme: The USGS images are handled similarly. They arrive on DLTs from the USGS. The slice-and-dice step produces four products: Jumps: JPEG compressed images covering a 1 x 1.3 km area at 32-meter resolution Thumbnails: JPEG compressed images covering a 1 x 1.3 km area at 16-meter resolution Browse: JPEG compressed images covering a 1 x 1.3 km area at 8-meter resolution Tiles: JPEG images that cover a 150 x 225 m area at one-meter resolution. Each image, along with its metadata (time, place, instrument), is stored in a database record. Each resolution is stored in a separate table. Today we have 96 million tiles and 1.5 million browse, thumbnail, and jump images. This totals about 800 gigabytes of user data. The US is about 9.8 million square kilometers, so this is about 30 percent of the US. Important areas have not yet been digitized. The USGS will provide additional data as it becomes available. They plan to digitize the entire country by the year 2002.

Logical Database Design for Image Data The images are stored in the SQL Server, Enterprise Edition database along with their metadata. The tiles, thumbnails, browse, and jump images are kept as SQL image fields as part of relational records. The schema is shown in Figure 11. Metadata. All the original metadata for each large image is stored in the OriginalMetaData table. A user can ask the TerraServer for the lineage of a particular image and the TerraServer then returns the appropriate record from this table. This data describes the data set in detail. The original metadata table has about 100 fields. These fields describe the instrument, when the image was acquired, what format it is in, when and how it was processed, the resolution and size, and so on. The ImgSource field says USGS or SPIN-2, for now, and the image type is either JPEG or TIFF. Currently, all the TerraServer data is in JPEG3 format. The ImageMeta table stores the metadata for each jump-browse-thumbnail tile group. The meaning of most of the fields is obvious. The ImgStatus field allows us to hide sensitive places. Both the United States Government and the Russian Government want to be able to quickly hide a region in case there is a conflict.

One can hide an area by updating this field. The TerraServer will not show images that have a "false" status. Figure 11. The SQL Server database schema for the TerraServer's spatial and image database. Each image resolution has its own table. The finest resolution (tile) table has its own metadata table. Jump, Browse, Thumbnail, and Tile are separate tables. Each of the jump, browse, and thumbnail image resolutions is kept in a separate table. We could have kept them all in one table, but the programming was more convenient if they were in separate tables. SPIN-2 and USGS are separate tables. Similarly, although the schemas are identical, we segregated the SPIN-2 data from the USGS data. This was done just to simplify the programs—a program wants either SPIN-2 data or USGS data at any one time. This simplified both programming and index design. For example, we require that the UgridId values be unique in the USGS tables because we only keep one image per UgridId. This means we discard old USGS images as we get newer ones. We do not keep any Zgrid indices on the USGS tables, since all searches are by Ugrid.

We do compute a ZgridId so we can find a SPIN-2 image that overlays this image. The reverse is true for SPIN-2 tables. Here we keep Zgrid indices and do not maintain Ugrid indices. We also keep duplicate ZgridIds for SPIN-2 data since we do want to show an older image and the latest image of the same spot. Because we had two separate search schemes, two separate image retention schemes, and frankly, two separate application requirements because of the desires of two separate vendors, we decided to maintain two separate tables. This makes it fairly easy to add a third data provider with an entirely new search scheme and application. The Pick table. The Pick table is a list of recommended or interesting images. Recent or topical locations can be added to this table and they will appear in an Image Picks recommendation Web page. The log and hit tables are used by the administrators to track how the system is used. These tables record each request and record a count of requests for each grid ID and gazetteer entry. The Tile table. The bulk of the database is in the Tile table. It contains the 110 million tiles for USGS and SPIN-2.

Each tile has some metainformation giving its location in the grid system, the date the data was acquired, and the 10-KB image itself. The SPIN-2 data tiles are lightly encrypted. The encryption key is stored in the metadata. A parallel TileMeta table stores additional metadata about each tile. Look-up by GridID. The USGS and SPIN-2 databases are typically accessed by spatial location. The user points to some spot on the planet and asks for the images around that spot at some specified resolution. Thumbnail (16 m/p) is the default resolution. Suppose you live latitude 40ºN, longitude 140ºW. This translates to a ZGridID and a UGridID. Depending on the theme, the TerraServer will look up images at or near that ZGridID (SPIN-2) or UGridID (USGS). These IDs are constructed using the Z-transform so that nearby IDs are close to one another on the map. Requests for adjacent images will probably ask for data that has already been prefetched by SQL Server, Enterprise Edition into the database cache. The use of GridIDs makes spatial look-ups easy, and clusters nearby data together on disk.

Database Design for Gazetteer Using maps to navigate to images of particular areas is universal. It transcends language barriers and is intuitive. However, it often requires several steps to zoom in on a spot. Worse, if the viewer does not know the geography of a place, the graphical point-and-see metaphor may not be very useful. Someone who does not know the Washington DC area might have difficulty finding the White House or the Pentagon. Name Look-up. A name look-up application that quickly takes the user to a particular place solves this problem. We implemented an English-language name lookup system for the TerraServer. We began with Microsoft's Encarta World Atlas 97 gazetteer (also called Virtual Globe). Microsoft geographers, using many public and private sources, have been refining the Encarta gazetteer for several years. Today it has over 250 different countries, 1,083 states, and 1,089,897 places. Many countries, states, and places have alternate names. Indeed, there are about 1,448 country names, 3,776 state names.

For example, there are at least 25 different ways to spell Albania and 29 alternate names for the country of Yemen. Figure 12. The Gazetteer database snowflake schema. The core is a fact table (Place) describing over a million places. It can be indexed by state, country, feature type, or geographical proximity. Gazetteer is a snowflake-schema. We extracted the Encarta gazetteer into the schema shown in Figure 12. This is a classic snow-flake decision-support schema with the place table at the core and the attribute tables radiating from the core. The CountrySearch and StateSearch tables have the many alternate spellings for each country or state. For example, Uzbek S.S.R., Uzbek Soviet Socialist Republic, Uzbekskaya Sovetskaya Sotsialisticheskaya Respublika, Uzbekskaya SSR, Zbekiston Respublikasi, and Uzbekistan all map to the nation of Uzgekistan. Gazetteer features.

The feature-type table encodes the type of place: (1) Airport/Railroad Station, (2) Bay/Gulf, (3) Cape/Peninsula, (4) City, (5)Hill/Mountain, (6) Island, (7) Lake, (8) Other Land Feature, (9) Other Water Feature, (10) Park/Beach, (11) Point of Interest, (12 ) River. The GazetteerSource tells what source the Place data record came from (currently, Encarta is our only source). The PlaceGrid table is not yet populated, but when it is, it will map each spot on the earth (each ZGridID) to the closest point for which the TerraServer has image data. Place table links to ZGrid and UGrid. The place table is the core of the schema. It is heavily indexed by the other tables. In addition, the place table has 8 indices of its own—six for the Gazetteer look-up and two for UGrid and ZGrid look-ups. The place table is denormalized: We did not create a PlaceSearch table that factors out the AlternateName to Name mapping.

On average, there are only two names for a place, so a complete place record is stored for each instance. This redundancy speeds access. The meanings of most of the place table fields are obvious: they give the latitude, longitude, and grid ID of the place in both the USGS (Universal Transverse Mercator projection) and in TerraServer's SPIN-2 ZGrid projection. The place table record also has the place's latitude and longitude. Each place table record has the most recent timestamp for USGS and SPIN-2 images of the place, if the images exist. A boolean flag (ImageFlag), indicates whether the place has an image. Indices to support arbitrary lookups. The user interface allows users to specify any subset of place name (for example, Paris or Pentagon), state (for example, Wisconsin), country (for example, France), or type (for example, river). If state is specified, but country is not, we default to USA as the country. When searching for all places in a country, state, or type, the TerraServer shows the places that have images first in the list. If the list is more than 10 elements, the search must continue just after the previous last record.

These requirements translate to a need for the following five indices in addition to the PlaceID, UgridID and ZgridID indices: Table 3. Place table indices Indices for Place Table akplace5 ImgFlag, countryID, typeID Showing places with images first. The image flag causes all places with satellite images to sort low (early) in the answer set. The rich indexing of the place table means that all the lookups, after cascading through the small snow-flake dimension tables, go directly to an index lookup of the place table. The queries that access this table each select the first ten qualifying records. To tell the optimizer that the query will not read the entire set, each query has the hint FastFirstRow. Figure 13. An example of using the Encarta gazetteer to navigate the TerraServer by name. The user specifies any combination of a city name, state, country, or place type. The gazetteer returns matching places and Web links for all those places for which the TerraServer has image data. The name lookup process flows as follows: The client fills in some fields of the HTML form shown in Figure 2. In this particular case, the client asked for airports in California.

The form invokes and active server page on the TerraServer. The active server page invokes a SQL Server, Enterprise Edition stored procedure to find up to ten places matching the criteria. There are ten possible queries, the stored procedure uses a case statement to pick the appropriate query (for example, SELECT ImageFlag, Place.AlternateName, USGSdate, SPIN2date FROM ( (Place JOIN FeatureType ON Place.FeatureID = FeatureType. FeatureID ) JOIN StateSearch ON Place.StateID= StateSearch.StateID) WHERE FeatureType.Description = 'Airport' AND StateSearch.AlternateName = 'California' ORDER BY ImageFlag, Place.AlternateName) FASTFIRSTROW. The query opens a SQL Server cursor, and fetches 10 qualifying rows. These rows are returned to the active server page, which formats the answer into an HTML page. This page contains the URLs for each matching place. Each such URL points to the TerraServer and includes a theme (USGS or SPIN-2), and a grid ID for that place. Given this information, the TerraServer can materialize an HTML page for that spot. The user can scroll through the pages that match his query (each page has up to ten matches).

As the client scrolls forward, the SQL Server query opens the cursor where it left off, and reads the next ten qualifying places. If the user selects a link and follows it, that sends a request to the TerraServer to materialize an image page for that place (in a USGS or SPIN-2 theme.) Initially, navigating by name is one of the most popular ways to get oriented. Once the user has found a locale, zooming and panning are the most convenient ways to navigate. Gazetteer is used to show image place names. The Gazetteer provides another useful feature. When displaying an image, the TerraServer looks up the place name of the center of the image. For example, as one pans down the Washington Mall, the place names progress through the various monuments and museums. This is a valuable aid to helping users orient themselves. Physical Database Design One SQL Server, Enterprise Edition database. The TerraServer physical database design is very simple. The image and gazetteer data are all stored in one SQL Server, Enterprise Edition database. SQL Server spreads the database and its recovery log across all the logical volumes. SQL Server also manages the use of physical memory for the buffer pool. Previous sections described the indices on the various tables.

SQL Server, Enterprise Edition file groups. In SQL Server, Enterprise Edition, a Windows NT file is the basic unit of allocation, backup, and recovery. The SQL Server database is built from these files. SQL Server spreads a table across a group of files. By default, all tables go into a default file group. This group stores the master database and other system information. We stored the Gazetteer database in this default group, but defined a special file group to store the image data. This image file group contains all the image and tile files. A third group of files is dedicated to the database log. RAID5 makes four 600-GB disk volumes. The TerraServer's physical database design is very simple. The database is mapped onto 4 huge disk volumes (each volume is almost 600 GB). Hardware RAID5 provided by Digital StorageWorks converts the 324 disks into 28 large RAID5 disks. Windows NT software striping (RAID0) is used to convert these 24 disks into four huge logical volumes. Therefore, SQL Server, Enterprise Edition sees four huge disks. 20-GB files are a convenient size.

To ease tape handling, and to provide some granularity of backup and recovery, we defined thirty 20-GB files on each disk. This file size fits nicely on current tapes. Actually, SQL Server, Enterprise Edition and Windows NT grow these files on demand as the database fills. Therefore, we just described the 120 (4 x 30) files to SQL as the file group. It's that simple. That is all there is to say about the physical design. SQL Server, Enterprise Edition and Windows NT Server manage the huge disk array, place the image data on the disks, index them, and retrieve them on demand. Data Loading Input was 300 DLT tapes. The USGS sent us approximately 210 DLT tapes. Aerial Images sent us approximately 80 DLT tapes. These tapes contain over five terabytes of uncompressed images. The first step was reading these tapes to disk. Fortunately, the TerraServer was located in a demonstration area at Microsoft's Executive Briefing Center. There are many high-performance disk farms on display in this center. Consequently, we had access to almost five terabytes of disk space in order to process the images. Figure 14.

A set of active server pages provides an operations interface to the slicing, dicing, and load process. STC TimberWolf tape robot. We first processed the SPIN-2 imagery. Then we loaded the USGS images. In mid-January, we discovered a flaw in the registration of the SPIN-2 images and had to reprocess them a second time. All this tape handling taught us the virtue of a reliable tape robot. A tape robot is also essential for unattended backups and restores. We used a Storage Technology 9714 TimberWolf tape robot. It can store up to 25 TB and has six DLT7000 tape transports. We sustained over 20 MBps transfers to and from it. Merging data from adjacent areas. The SPIN-2 images have been partitioned into 40x40 km patches. Tiles from different satellite image passes often overlap. To provide a seamless mosaic of an area, these patches must be combined into a large image, and then sliced and diced into the image pyramid described earlier. Consequently, all the patches for an area need to be online when the SPIN-2 data is being processed.

Similarly, the USGS data for a particular zone is spread across all the tapes the USGS sent us. We had to read almost all the tapes to build an image of a zone. To minimize tape operations, we decided to process all the SPIN-2 data and then all the USGS data online. Figure 15. The load process takes tapes from Aerial Images and USGS, loads them to disk, cuts them into tiles, and then loads them into the SQL Server database. Automated slice-and-dice process. Once the data is online, slicing and dicing it into tiles, thumbnail, browse, and jump images is very compute-intensive. This step produces a huge number of files (20 million for some UTM zones). These files are then copied into the database. We built a simple workflow system to manage the cutting and loading process. Many steps can run in parallel. Each step is restartable. Each step is recorded in a database. An active server page (Web interface) is used to observe and manage this workflow. Automated Database Load process. Once the files have been created for an area, the SQL Loader is used to insert the images into the database.

The loader program accesses the data in Zgrid order and then inserts the images, along with their metadata, into the SQL Server, Enterprise Edition database. SQL Server now allows parallel database loads into the same table, so several streams can run in parallel. We can load at a rate of about five megabytes per second. Locally, SQL Server can load data at 15 MBps, but the load program could not produce data that quickly. Opening the many files and passing them to SQL Server limited the load program to a peak of 2 MBps. Adding image data adds metadata and updates Gazetteer. The database load also populates the metadata tables and updates the Gazetteer database to record the most recent SPIN-2 and USGS data. Incremental load. Now that the TerraServer is operating, data is being added by inserting data rather than by doing bulk data load. The data rate is lower (one MBps) but still adequate for our needs. Backup and Restore We archived the data up to tape after each processing step. Thus far, we have not had to use those backups. The backup units are Windows NT files. We use the Legato NetWorker 4.4 backup utility to manage this work.

SQL Server, Enterprise Edition incremental backups: In addition, while the system is operating we take online tape backups of the database. SQL Server, Enterprise Edition supports incremental backup (only changed pages are archived). Since the TerraServer is an insert-mostly database, these backup tapes are comparable in size to the amount of data that has been loaded since the last backup. Table 3 shows the data rates we achieved. At these rates, we can reconstruct the entire TerraServer in a day, and can recover a damaged file in an hour. Table 3: Backup and Restore data rates. Task Restore a SQL Server, Enterprise Edition file (media time) 10 minutes Microsoft TerraServer Hardware Processors. TerraServer runs on a Digital Alpha 8400 system with 10 GB of memory. This system has eight 440 MHz Digital Alpha processors. The 8400 can support up to 160 PCI slots. In our configuration, we host seven KZPBA dual-ported Ultra SCSI host bus adapters—one for each of the seven disk storage cabinets. There are also six KZPSA SCSI host bus adapters for backup and for boot disks. Figure 16.

The configuration of the StorageWorks disk array: each RAID controller manages two RAID5 arrays of 11 disks to make an 85 GB RAID5 disk. Each controller has a 64-MB cache. There are two spare disks. Disks. The system has seven storage cabinets, each holding 46 9-GB drives, for a total of 324 drives. Their total capacity is 2.9 terabytes. The drives are configured as RAID5 sets using 14 HSZ70 RAID controllers, each with 64 MB of memory. Each pair of controllers manages 46 disks on six SCSI strings. Each controller manages two RAID5 sets of 11 disks. That leaves two spare disks in case a disk fails. So the 14 HSZ70s collectively manage 28 RAID5 sets, each storing 85 GB. Windows NT file striping (RAID0) is used to collapse these 28 RAID5 disks into four logical volumes. Each logical volume is 595 GB (= 7 x 85 GB). The resulting four logical drives are each given a drive letter. SQL Server, Enterprise Edition stripes the database across these 4 logical drives.

This mapping is dictated by the fault-tolerance properties of the StorageWorks array. The design masks any single disk fault, masks many string failures, and masks some controller failures. Spare drives are configured to help availability. In the end, SQL Server has just over 2.4 TB of RAID5 protected storage. The StorageWorks array has been trouble-free. Tapes. The TerraServer has a six-station STC 9714 tape robot with a near-line capacity of over 5 terabytes. This robot is used for backup and recovery. It is also used to import data from other sources. Figure 17. The world's largest PC. Digital Equipment Corporation provides all the TerraServer hardware. This includes a 342 StorageWorks disk array, an 8-processor AlphaServer 8400, and a tape archive. Intel-based servers perform the Site Server and map server tasks. Map Server. TerraServer accesses the Expedia Map Servers. These servers are a shared resource available to Microsoft Internet applications. TerraServer shares four map servers with Microsoft Expedia Travel Services, Microsoft Sidewalk® city guide, MSN's CarPoint™ automotive service, and MSNBC.

The servers run on dedicated Compaq servers (Intel processors), each with four processors and 256 MB of memory. Site Server Commerce Edition. Aerial Images and the USGS each host an e-commerce Web site tailored to interface with Microsoft TerraServer. Both sites run Microsoft Site Server Enterprise Edition version 3.0 software on Windows NT Server version 4.0, Internet Information Server version 3.0, and SQL Server version 6.5. Aerial Images chose a Digital Alpha 800 server cluster with 100 GB of disk space. Aerial Images maintains the server in Raleigh, North Carolina. The USGS has chosen a multiprocessor Intel–based server from Gateway 2000. The USGS houses the server at the USGS EROS Data Center in Sioux Falls, South Dakota. Networking. The TerraServer is behind the Microsoft firewall. It is connected by dual 100 Mbps Ethernets to high-speed Internet ports. Slicing and Dicing. Most of the slicing and dicing of the images was done on the TerraServer with the aid of two Digital 4100s with 200 GB of disk storage and some 4-way Intel processors.

The load processes deliver new data from this node to the TerraServer over a switched 100 Mbps Ethernet. Image processing by the SOVINFORMSPUTNIK, Aerial Images, and the University of California at Santa Barbara used Windows NT Server–based Intel systems. Assessment The TerraServer is a simple application, but it involves many tools: HTML, Java, VBScript, HTTP, IIS, ASP, OLE DB, ODBC, and SQL Server, Enterprise Edition. As such, it is a showcase Web application. Once the rough design was chosen, it was fairly easy to design and configure the database. We were novices at the many data formats used in geo-spatial data, but learned quickly as we did the slicing and dicing. Working with our geo-spatial-data mentors at Aerial Images, USGS, and the UCSB Alexandria Digital Library vastly accelerated this process. Once the first user interface was built, it was clear that we needed a better one. We are now on the fourth iteration of that design. The design has converged, but designing intuitive user interfaces is one of the most difficult aspects of any system.

Once we understood the process, the slicing, dicing, and loading went very smoothly—but it was the bottleneck in bringing the real TerraServer online. It takes a huge amount of computation to process five terabytes of image data. Although we were using Alpha and Beta versions of the next SQL Server, it gave us very few problems. The Digital Alpha and StorageWorks equipment performed flawlessly. Having great tools makes it possible to experiment. The SQL Server, Enterprise Edition, IIS, and Windows NT Server management tools were a big asset. Overall, the project was relatively easy. One area that gave us difficulty was tape backup and restore speeds. That has improved dramatically since we started. We can now back up at a rate of 80 GB per hour. Microsoft Windows NT Server, Enterprise Edition supports 64-bit addressing on the Digital Alpha processors. SQL Server, Enterprise Edition has been modified to exploit the power of massive memory. Although these technologies are likely to improve TerraServer performance, they are not yet incorporated into the TerraServer. TerraServer shows that Microsoft Windows NT Server, Enterprise Edition version 4.0 and Microsoft SQL Server, Enterprise Edition can support huge databases on a single node.

If one wanted to store an atlas of the entire landmass of the planet, it would be 25 times larger. Clearly, one would have to use larger disks and would use a cluster of 20 of these huge machines. Summary Commodity-scalable servers have arrived. These scalability demonstrations show that with proper design, Windows NT Server, SQL Server, Enterprise Edition, and the other Microsoft BackOffice® products can be used to solve the most demanding problems. They demonstrate the key properties one wants of a scalable system: Scalability: growth without limits. Manageability: as easy to manage as a single system, self-tuning. Programmability: easy to build applications. Availability: tolerates hardware and software faults. Affordability: built from commodity hardware and commodity software components. The TerraServer demonstrates the extraordinary performance obtainable with commodity software components. Commodity components give these systems excellent price-performance. They are a breakthrough in the cost of doing business. The cost of serving a page onto the Internet, delivering a mail message, or transacting a bank deposit has gone nearly to zero—it costs a millionth of a dollar per transaction. One could use advertising to pay for such transactions—an advertisement pays a thousand times more per impression.

These applications were initially built by a few people in a few weeks. The TerraServer design keeps evolving. It is modular, so components are easily rewritten and plugged into the whole. The TerraServer demonstrates the incredible power of the new tools created by the Windows platform and by the Internet. The resulting applications have easy-to-use management interfaces and have the manageability and availability properties essential to operating them. Microsoft learned a great deal in building the TerraServer. We fixed many performance bugs and eliminated many system limits. There is still more to do to make these products even more self-tuning and self-managing. That process is in full swing now. Future releases of Windows 2000 Server and Microsoft BackOffice products will reflect these improvements. Acknowledgments The TerraServer project is a consortium of four major participants—Microsoft, Digital Equipment Corporation, Aerial Images, and the USGS. Each organization has committed software, personnel, equipment, and intellectual property. Numerous other organizations have made substantial donations in equipment and expertise. Without their support, the TerraServer project would not be possible. These organizations are: Aerial Images and SOVINFORMSPUTNIK.

Aerial Images is a partner of SOVINFORMSPUTNIK and is the sole distributor of SOVINFORMSPUTNIK's SPIN-2™ imagery. Aerial Images and SOVINFORMSPUTNIK have provided 350 GB of SPIN-2 imagery for use on the TerraServer project. The SPIN-2 imagery provides the world-wide coverage for TerraServer. Approximately 50 percent of the SPIN-2 imagery covers Europe, China, Australia, and other non-U.S. locations. The Aerial Images partners gave this project enthusiastic support. Mr. Mikhail Fromtchenko and Dr. Victor Lavrov of SOVINFORMSPUTNIK contributed their expertise and the Russian imagery to the project. Clarion, a subsidiary of Data General. Clarion loaned 1 TB of high speed disk storage. The Clarion disk array is connected by Fiber Channel interconnect to two Intel POCA servers. TerraServer's use of the Clarion Fiber Channel technology is a demonstration of the technology's "readiness for prime time." We have used Fiber Channel for over 8 months with only one disk failure during the period.

In general, we have found Fiber Channel to be very simple to cable together and very fast. Digital Equipment Corporation. The Digital Equipment Corporation provides the DEC Alpha 8400 and DEC StorageWorks ESA1000 complex of 2.4 TB of UltraSCSI disk capacity. The SQL Server, Enterprise Edition Database runs on the DEC 8400 and maintains the 1 TB SQL database of metadata and imagery. In addition, DEC has provided a DEC Alpha 4100 with 1.3 TB of FWD SCSI disks, a pair of DEC Alpha 4100 systems with 250 GB of disk capacity, and DEC Prioris 6200 Intel-based processor. These other systems are used to edit and load the imagery received from Aerial Images and the USGS into the DEC 8400 system. EMC Corporation. The EMC Corporation loaned 500 GB of FWD SCSI disk storage for TerraServer's use. The EMC disk array held 750GB of FWD SCSI disks. 500GB was connected to a single Intel POCA 4-PentiumPro server. Intel Corporation.

The Intel Corporation loaned six POCA servers for use as image-cutting and TerraServer demonstration equipment to Microsoft and Aerial Images. A POCA server is a four-PentiumII processor system with 256 MB of memory. They have proven themselves to be fantastic workhorses while editing 4 TB of raw imagery. To their credit, we have zero failures with all our POCA servers. Legato Networker. The TerraServer database is backed and recovered by Legato Networker. The TerraServer project uses the Networker product on Digital Alpha hardware and Windows NT–based systems and Intel processor–based Windows NT Servers. We chose Legato Networker as our primary backup/restore technology because (a) it was available on both Digital Alpha and Windows NT and (b) it supported the Digital TL894 and StorageTek 9714 tape robot at the time we started the TerraServer load in November 1997. Microsoft Research. Microsoft Scalable Servers Research group, based in San Francisco, California, conceived the TerraServer application and has led the development of the custom software.

All custom software has been developed using Microsoft development tools—Visual Studio® development system 5.0, SQL Server, Enterprise Edition, Internet Information Server 3.0 with Active Server Pages, Visual Basic Scripting Edition, and Microsoft Commerce Server, a component of Microsoft Site Server Enterprise Edition. Microsoft SQL Server Group. The Microsoft SQL Server Group supported the TerraServer development with both human and financial resources. Microsoft Geo Business Unit. Steve Smyth helped us with the Encarta Virtual Globe Gazetteer, and Microsoft's Geo Business Unit built the Java-based map control for us. Seagate Software's Backup Exec. Aerial Images delivers 20km X 20km SPIN-2 imagery on CompacTapeIV media written using Backup Exec 6.11. TerraServer processes these tapes on Backup Exec running on DEC Alpha and Intel-based Windows NT Servers. Storage Technology Corporation. The Storage Technology Corporation provided an STK 9714 tape storage system. The 9714 contains six DLT7000 tape drives and 100 tape slots for a total capacity of 4 TB of tape. The STK is connected to the DEC Alpha 8400 using FWD SCSI connections.

A single KZPSA FWD SCSI controller was connected to two DLT7000 tape drives per StorageTek. United States Geological Survey (USGS). The USGS provided 3 TB of DOQ photographs. In addition, the USGS has provided invaluable assistance in geography and image processing of orthophotographs. The USGS also provided the source data for the Coverage Map application. The USGS' goal in the TerraServer project is to evaluate the use of the Internet as a data presentation mechanism for the general public. University of California at Santa Barbara's Alexandria Digital Library project. Jim Frew, Terry Smith, and others at UC Santa Barbara helped us slice and dice the USGS imagery. UCSB has a long track record of combining the earth and computer sciences into useful research. The Alpha Server division and the Storage Works division were especially helpful. The 324 StorageWorks disks on the TerraServer worked on day one and continue to work without interruption! For More Information For the latest information on Microsoft SQL Server, check out the Microsoft SQL Server site or the Microsoft SQL Server Forum on the Microsoft Network (GO WORD: MSSQL). Endnote: Samet, H. 1994. The Design and Analysis of Spatial Data Structures.

New York: Addison Wesley. (Click here to return.) -------------------------------------------- The information contained in this document represents the current view of Microsoft Corporation on the issues discussed as of the date of publication. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information presented after the date of publication. This document is for informational purposes only. MICROSOFT MAKES NO WARRANTIES, EXPRESS OR IMPLIED, IN THIS DOCUMENT. ©1998 Microsoft Corporation. All rights reserved.

Question: A gazetteer offers indexed information mainly oriented by?
Answer: Geography