Since that time, ASTER DEM data, while not realizing its full potential due to lack of global coverage, has become more widely known and popular. Several GIS applications, including the free application MicroDem and 3DEM can handle ASTER DEM GeoTiff format with ease. I recommend that anyone interested in ASTER data investigate these applications carefully. I have recently modified GEOTIFF4 so that it will handle southern hemisphere data. This program may still be useful due to its simplicity and its ability to handle HDF format. The rest of the information in this article and the companion article that follows is current as far as I know. It will be interesting to see if ASTER remains as popular now that 90m SRTM data is finally available for much of the globe. (Release of the full resolution (30m) data outside of the United States was blocked by NIMA. However, the 90m data is a major advance over the 1km resolution DTED0 data referred to in the article.) The significantly superior resolution of ASTER data should ensure its importance as a useful GIS data source for some time to come.

At the time I wrote the article, there were only two sources of free international DEM data. The first is the 1 km DTED0 data available from NIMA (and the equivalent GTOPO30 data set available from USGS/EOS). The second was the reverse engineering approach of extracting DEMs from topo maps as described in my earlier article. Of course IKONOS and QuickBird commercial satellite imagery are (or will soon be, in the case of QuickBird) available, but although of excellent quality, this data is quite expensive. (The Digital Globe website quotes prices of $25/square kilometer. This equates to about $3,500 for a 1:24000 USGS quad.)

Since that time, two other very popular sources of DEM data have become available. In 2000, the Space Shuttle flew one of its most useful missions ever. STS 99 used a 60m boom to collect stereoscopic radar imagery for all of the earth between 56 degrees south and 60 degrees north latitude. Although intentionally degraded to 90m resolution, SRTM data has become the global standard for free DEM topographic data. However, there is another source of international DEM data that is somewhat less well known: ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer). The quality of this data is generally good, with a resolution of 30m. The ASTER GDEM product is available for free at the NASA WIST website.

ASTER is an imaging instrument that is flying on the TERRA satellite launched in December 1999 as part of NASA's Earth Observing System (EOS). ASTER represents a revolution in the remote sensing community because of the availability of its imagery and its superior resolution. ASTER resolution ranges from 15m to 90m, depending on the wavelength. The instrument records in three bands: the Visible and Near Infrared (VNIR ), the Short-wave Infrared (SWIR ), and the Thermal Infrared (TIR ), oriented on the nadir and backward looking. There are 14 spectral bands all together spanning the visible and infrared spectra, so the sensor is susceptible to cloud cover and cannot record images at night.

Because of its off-nadir sensor pointing capability, ASTER can collect the stereo pairs necessary to generate high resolution DEMS (using bands 3N and 3B). Instead of the woeful 1km resolution DTED0 data, ASTER DEMs offer a very respectable 30m resolution. The EOS ground stations archive imagery corresponding to the spectral bands of the ASTER sensors, including the Level 1A data sets. The DEMs are extracted from this data. Once produced, EOS archives the DEMs in its database so that they will be available essentially immediately upon subsequent request. If the DEM you are interested in is not in the archive, all is not lost. You can enter your request for DEM production into a queue. Your DEM will be constructed by EOS from the L1A data. This can take anywhere from two to ten weeks. Another option is to construct the DEMs yourself using the appropriate software. (It is also reportedly possible to order a fly-by acquisition, but this method is not publicized by NASA and is probably by special arrangement for government and academic researchers.)

Why does ASTER represent an advance in the world of terrain modeling? Because its 30m resolution is theoretically better than the classified NIMA DTED1 data set. NIMA has never released its DTED1 series of DEM data and in fact is doing its best to block NASA from releasing 30m SRTM data from the Space Shuttle. (Quote from the NASA SRTM website:"Following the events of Sept. 11, 2001 our project partners in the National Imagery and Mapping Agency have requested that NASA and JPL not distribute any SRTM data to the scientific community or the public, including shaded relief maps or other visualizations. We look forward to this restriction being lifted soon so data may be made publicly available." NIMA did not feel it is necessary to explain what the events of September 11 have to do with DEMs of someplace like Mt. Aconcagua in Argentina, for example, or how long "soon" might be. Meanwhile, the world waits.)

What does ASTER resolution mean to the terrain modeler? Consider the first image at top right. It is an ASTER DEM of the Jordan river valley in the occupied West Bank of Israel. The image is approximately 2500 by 2500 postings large, covering an area of about 75 square kilometers. The image immediately below is a section of NIMA DTED0 quad N32E035, covering approximately the same area. Because of the inferior resolution of the DTED0 image, it is hard to believe that the coverages coincide very closely. Before ASTER, this was effectively the highest resolution DEM available to the civilian public. This DEM contains 232 by 246 individual elevation postings. The point should be obvious. The ASTER image contains more than 100 times as much data as the DTED0 image.

Since 2000, ASTER has accumulated a relatively large archive of imagery. The DEM data is offered for free download from the NASA WIST. However, the procedure for downloading the data is not particularly easy. You have to navigate a somewhat awkward and slow user interface. After placing your order using a four-step process, you must wait anywhere from one hour to two days for your image to become available for archived images, and as long as ten weeks if you request production of a DEM not in the archive. You download the data from a designated anonymous ftp site using a ftp client like FileZilla. The DEM files are also offered in a 16-bit GeoTiff format. This does not represent much of an advantage, because most graphics applications only read the more common 24-bit tiff format. The GDEM Tutorial gives instructions on how to download the data.

Even if they could read it, very few applications know what to do with a GeoTiff DEM once they get it, since the format was really designed to accommodate 2D spatial data. (One that does due to recent upgrades is GlobalMapper). ASTER DEMs are also quite large. ASTER DEM I16 files are about 12 MB and FP32 files are 25MB in either of the two formats. This makes converting and transporting the files from one application to another a challenge.

Ten years ago I wrote the GEOTIFF4 utility to help with this problem. This program took ASTER Geotiff or HDF DEMs and converted them to USGS native DEM format. Since that time, several events have overtaken GEOTIFF4 and rendered it obsolete. The first is the abandonment of HDF as the ASTER DEM archive file format in favor of GeoTiff. While there is nothing wrong with HDF that isn't also wrong with GeoTiff (like being an overly complicated nightmare for software developers) GeoTiff has evolved to become the global standard for GIS data storage applications. Most GIS applications (including the free MicroDem and 3DEM) can read GeoTiff, and this has increased access to virtually anyone. I particularly like 3DEM as its capabilities are unmatched.

The two images at bottom right show a view of a mountainous region in the vicinity of the Khyber Pass on the Pakistan/Afghanistan border, extracted from the third DEM above. The view covers an area of approximately 75 square kilometers. (Note the moth-eaten appearance of this particular DEM. The holes represent areas of missing data, often as a result of cloud cover. ASTER imagery, unlike the carefully processed USGS data is very much raw and unprocessed. This characteristic adds to the appeal of ASTER, as you know you are exploring the frontier of this exciting technology. Unless of course the hole is directly over your area of interest, in which case some of the romance is lost.)

This DEM is subsetted from the area corresponding to the white rectangle on the image labeled 'ASTER_DEM20020104110754.TIF' above. The resulting subset DEM and terrain model are shown in the final two images. The impressive resolution of the ASTER DEMs should be apparent from the high quality of the DTM extracted from this small region of the overall DEM. Compare the terrain map of Afghanistan shown in the section on my website entitled ' Afghanistan Maps' (prepared from DTED0 data and covering thousands of square kilometers) to the ASTER subset data which covers a mere 15 by 15 kilometer square.

This article has given just a brief introduction to the exciting world of ASTER imagery. I have left out many details of how to create a DTM from ASTER DEM data. ASTER data sets represent a rich but decidedly complex source of DEM and overlay imagery for the digital cartographer. I will explore the technical challenges of using this complex data set more fully further in subsequent articles. I am not sure how NASA will respond to the increasing interest in this product. Hopefully, its response will be to recognize the considerable demand and accelerate the introduction of more such data to the mapping community.

Thanks to my good friend and ASTER expert Paul Burkhardt for introducing me to ASTER data and for his patience over many weeks of guiding me through this exciting data source. I could not have done it without his help.