Summary
1. Background
The production of digital elevation models (DEMs) is an expanding aspect of photogrammetry and a new and rapidly developing technology, radar interferometry. The growing availability of DEM data is opening new possibilities to earth scientists whether they be geologists, geomorphologists or pedologists. The land surface has long been recognized as the basis to which all geoscientific observations are tied. Only recently, however, have the technologies been developed to produce topographic data of large areas with sufficient horizontal and vertical resolution to meet the requirements of detailed spatial analysis.
Earth scientists in the past have often been constrained by using a mosaic of existing DEMs that were derived by different resolutions and datum.
The results were mostly inhomogeneous, inconsistent and, consequently, not comparable. Improved aerial photogrammetry (stereoscopy) through state-of-the-art Soft Photogrammetric Digital Workstations, and interferometric synthetic aperture radar, has meant that topography has gained a whole new meaning in spatial studies.
Large-area DEMs with consistent quality and high precision can now be produced.
NASA's recent Shuttle Radar Topography Mapping Mission (SRTM) ensured that high-resolution topographic data for the majority of the earth's surface will be available to all users within two years
(
http://www.jpl.nasa.gov/srtm/).
In the Pacific Rim, TOPographic Synthetic Aperture Radar (TOPSAR) datasets were acquired over selected sites during the PacRim1 (1996) and PacRim2 (2000) AIRSAR missions for the purposes of advancing the understanding of radar technology, in particular that of radar polarimetry and radar interferometry. Details describing sites covered during these missions can be found on the JPL AIRSAR web page
(
http://airsar.jpl.nasa.gov/).
With the advent of geographical information systems (GIS) DEMs can be used together with other spatial datasets such as geological information, airborne magnetics, gamma-ray spectroscopy and hyperspectral datasets.
The DEM provides a basic spatial reference system and images and vector data can automatically be draped over the DEM for more advanced analyses.
Software packages such as ESRI's Spatial Analyst and 3D Analyst
(
http://www.esri.com/software/arcview/extensions/spatext.html), and IDL's RiverTools
(
http://www.rsinc.com/rivertools/index.cfm) includes tools for generating geomorphometric models such as slope, and modelling of drainage basins.
Freeware packages on the Internet such as:
LandSerf (
http://www.geog.le.ac.uk/jwo/research/LandSerf/index.html),
MicroDem (
http://www.nadn.navy.mil/Users/oceano/pguth/website/microdem.htm),
GRASS
http://www.baylor.edu/~grass/index2.html and
DiGem (
http://member.aol.com/oconrad/dgm/dgm_main.htm)
can also be downloaded for visualizing and performing analysis of generic elevation datasets.
2. Current Capabilities
2.1 Stereo Photogrammetry
Aerial photography
Precision DEMs are now being acquired by commercial groups using soft photogrammetric digital workstations, such as the HELAVA system. Stereo aerial photographs used for generating the DEM are either purchased from an archive or reflown if the existing photographs are unsuitable. In addition to the DEM, digital imagery of the aerial photos can be projected or draped over the terrain data to form an orthoimage. For example, using 1:86 000 scale photography scanned at 15mm, the resultant product has a ground resolution of 1.2 m and height accuracy of 3-5 m, depending on accurate ground calibration and quality of the stereo model.
SPOT-PAN
DEMs can be produced from SPOT-PAN image stereopairs through stereoplotting and automatic correlation. The cross-track pointing capability of SPOT allows the second image of the stereo pair to be acquired as little as 2 days after the first with base-to-height ratios of 0 to 1. McMullen Nolan and Partners have shown that when using high contrast imagery with minimal time difference between the images forming the stereo pair, an optimal base:height ratio of 0.7 and suitable ground survey information, SPOT DEMs of low relief terrain typically achieve 7-12 m RMSE horizontal and vertical errors.
RADARSAT
Stereo pairs for DEM generation are created using one of RADARSAT's Fine, Standard, Wide or ScanSAR Narrow beam modes. The finest resolution of 8x8 m horizontal and 15-20 m vertical for 50 m postings is provided in the Fine beam mode but it covers the smallest area (50 km width). These products are suitable for a variety of applications at map scales smaller than 1:100,000. Further details regarding stereo accuracy and costs for each mode can be viewed at
http://www.vexcel.com/radar/stereo.dem.html
ASTER
The Advanced Spaceborne Thermal Emission and Reflection Radiometer mounted on the TERRA spacecraft launched in December 1999 obtains high-resolution (15 to 90 square metres per pixel) images of the Earth in 14 different wavelengths of the electromagnetic spectrum, ranging from visible to thermal infrared light
(
http://asterweb.jpl.nasa.gov/).
Unlike the other instruments aboard TERRA, ASTER will not collect data continuously; rather, it will collect an average of 8 minutes of data per orbit. All three ASTER telescopes (VNIR, SWIR and TIR) are pointable in the cross-track direction. Given its high resolution and its ability to change viewing angles, ASTER will produce stereoscopic images and detailed terrain height information. Stereo data have a spatial resolution of 15 m and the base:height ratio, based on the nadir and back view (at 27.6 degrees off nadir, along track), is 0.6. Standard product DEMs will come in two basic modes, relative and absolute. Both are produced using automated, correlation-based, commercial softcopy photogrammetric software. Both modes will have a X-Y resolution of 30 m and a vertical resolution (smallest Z increment measured) of 1 m. Absolute DEMs require user-supplied ground control points (GCPs). Based on pre-launch studies, with 4 or more GCPs/scene with XYZ accuracies of 5-15 m, RMSE xyz accuracies of 7-30 m for the absolute DEMs are predicted.
Standard product DEMs from ASTER are produced at no charge, but production plans are limited to only one 60x60 km scene per day. The idea is that users with their own soft copy photogrammetric software will be able to produce their own DEMs when the stereo data become available. The data are of sufficient quality to allow for 15 m posting by users with their own software, and will be available at no charge from early July 2000. The overall mission plan is to acquire and archive a global data set between 82 degrees North and South of cloud free stereo over a period of 2 and 5 years. The nominal maximum number of stereo pairs that can be acquired/day is 771, but this maximum will probably not be realized; and no more than about 350 scenes/day will be processed.
