REPORT OF I.A.G. SSG 3.163: ASSESSMENT AND REFINEMENT
OF GLOBAL DIGITAL TERRAIN MODELS
Chairman: D. Arabelos
Department of Geodesy and Surveying
Aristotle University of Thessaloniki, GR-540 06 Thessaloniki

1. Introduction

The International Association of Geodesy (IAG) Special Study Group (SSG) 3.163 was established during the XXI General Assembly of the International Union of Geodesy and Geophysics (IUGG) held in Boulder, Colorado in 1995. The SSG 3.163 was composed of the 20 members and 6 corresponding members. This report outlines the SSG activities and some of the progress that has been made in this area during the period 1995-99. A SSG-3.163 page was establised (address: http://sg.topo.auth.gr/~arab/Miscellaneous/ssg3163.html) in order to share information, software and data to the members of the SSG.

2. Objectives

The main objective of the SSG are as follows: - Comparisons between the global DTMs in various test areas.

- Refinement of the global DTMs taking advantage of the local (national scale) high resolution DTMs. Detection of possible shift of coordinates and of gross errors of the global models, by comparing global DTMs with local models of the same resolution.

- Incorporation of new data to the existing models.

- Enhancement of the DTM over ice sheets using satellite and airborne altimetry, GPS, SAR interferometry, etc.

- Tests in order to assess the quality of the improved DTMs. These tests will include prediction experiments in the gravity field by taking into account the topography/ bathymetry in terms of the well known reductions (e.g., residual terrain modeling). The ground truth should be used to investigate the prediction results' quality in both cases i.e., using the original or the improved DTM.

- Prediction of bathymetry by inverting the gravity field in areas with a good coverage with gravity measurements. In case of areas that lack of satisfactory surface data, this data shall be recovered by an inversion of satellite altimetry data. Combination of other existing geophysical information is optional. The smoothing effect of the resulting model of bathymetry on other kinds of data, related to the gravity field, such as altimeter data, could be a measure to the quality of the model.

3. Membership

Members: D. Arabelos (Greece, Chairman), R. Barzaghi (Italy), P. Berry (UK), H. Denker (Germany), S. Ekholm (Denmark), Y. Fukuda (Japan), Ch. Green (UK), R. Haagmans (The Netherlands), A M. Hittelman (U.S.A.), W. Kearsley (Australia), S. Kenyon (U.S.A.), P. Knudsen (Denmark), Li Li (China), R. Salman (U.S.A.), D. Sandwell (USA), G. Sarrailh (France), H.-W. Schenke (Germany), H. SŸnkel (Austria), C. C. Tscherning (Denmark), Gwo-Chyang Tsuei (Republic of China), I.N. Tziavos (Greece)

Corresponding members: Mustapha Bouziane (Algeria), Rene Forsberg (Denmark), Scott Luthcke (USA), Nikos Pavlis (USA), Michael G Sideris (Canada), Klaus-Peter Schwarz (Canada), Martin Vermeer (Finland), H.-G. Wenzel (Germany)

4. Meetings

A business meeting was held in Rio de Janeiro, Sept. 5, 1997 during the IAG Scientific Assembly in Rio de Janeiro, September 3-9, 1997.

In the frame of the EGS XXIV General Assembly, a session was organized, entitled "Topography and Bathymetry in Geodetic and Geophysical Applications".

5. Summary of activities

5.1.New developments

NASA/GSFC-NIMA:

The global 5' elevation model JGP95E was made available to the members of the SSG by S. Kenyon and N. Pavlis. According to S. Kenyon, the updating of the model with new DTED is continued.

GETECH (Reported by Chris Green ):
i. For Central and South America: 3' 3' digital terrain model (DTM3)

ii. For Europe: 2.5' 2.5' digital terrain model (DTM2.5)

iii. 5' 5' DTM of the world (DTM5).
NGDC (Reported by Allan Hittelman)

The new global 30' DEM called GLOBE was recently released via the Web. David Hastings, a senior scientist in NOAA, chaired the international effort that developed this compilation. The relevant site at: http://www.ngdc.noaa.gov/seg/topo/globe.shtml is rich with numerous download options, documentation and images. A paper on this effort will be presented by A. Hittelman at the IUGG 99 (Symposium G3).
Hughes STX (Reported by N. Pavlis on behalf of Anita Brenner and Scott Luthcke)
i. Exploitation of satellite altimetry is to create digital elevation models of the polar ice sheets and land. 10 km digital elevations models of both Greenland and Antarctic ice sheets to 81.5o latitude will shortly be available on CD-ROM from the National Snow and Ice Data Center. A 15’ DEM of South America using ERS-1 Geodetic Mission Data has been created.
ii. Satellite laser altimetry for ice sheets and land topography

5.2. Intercomparison of the global DTMs ETOPO5, TerrainBase and JGP95E

During the business meeting in Rio de Janeiro R.-H. Rapp suggested the assessment of the global DTMs through inter comparison of them. From the inter comparison, the following conclusions could be drawn (Arabelos, 1999):

The standard deviation of the differences between the three models (90 m, 112 m and 92 m) is of the order of 100 m. Taking into account that 60% of the data (the oceanographic data), are common for the three models, we must assume that the discrepancies of the continental data resulting in standard deviation higher than 100 m.

As it was expected, the larger discrepancies were observed in areas with data characterized by poor quality and coverage (Africa, Central Asia). In areas covered with good quality data (USA, West Europe, Australia, Japan, Corea) the convergence of the models is remarkable.

The standard deviation of the differences between topographic corrections of gravity anomalies due to the differences between the three DTMs varies in Central Africa from 6 to 9 mGal, with min and max values ranging from -119 to 87 mGal.

The reliability of the oceanographic data must be considered as comparable with that of the continental data. The fact that the three models include the same data in the oceanic areas (that of ETOPO5) and the lack of reliable oceanographic data, makes the assessment impossible without taking into account the quality information of the very original oceanographic measurements.

The quality of the models considered does not correspond to the recent requirements, for geodetic and other applications. On the other hand, the resolution of the 5' is very sufficient and necessary for many applications. Therefore, significant improvements are necessary in order to be created a reliable DTM with the same accuracy for all areas around the earth, proper for regional and global geodetic, geophysical and oceanographic applications.

The differences between the three models were visualized in 20o 30o blocks. These figures are available through the home page of the SSG-3.163

(http://sg.topo.auth.gr/~arab/SSGJPEG/hague992.html).

5.3. Spherical harmonic expansions

Ultra high degree spherical harmonic models of the Earth's topography, rock equivalent topography and topographic isostatic potential were developed by H.-G. Wenzel (personal communication, 1998) This development to degree 1800, was based on the global elevation model ETOPO5. Two major points should be discussed in the next:
i.If the data in the spherical harmonic expansion of the geopotential have been computed from mean values of blocks with size larger than 180/lmax, where lmax the maximum degree of expansion, then the local variation is erroneous. It should therefore be compared to the variations of the topography: If the topography has a large variation, then an error in the gravity data may have been detected (Arabelos and Tscherning, 1999). The spherical harmonic expansion of the topography enabled this comparison.

The standard deviation of the gravity versus the standard deviation of the topography is shown in Figure 1. For the computation of the standard deviation of gravity, the GPM98A geopotential model (Wenzel, 1998) was used from degree 61 to 1800. Gravity anomalies were computed on a 5' 5' grid on 3o equal areas. Then, the standard deviation for each 3o block was computed from the 25 point values. The same procedure was followed for the computation of the standard deviation of the topography. In this case the model used was the spherical harmonic model GTM3A, complete to degree and order 1800, based on the expansion of ETOPO5 (Wenzel, personal communication 1997). In Figure there are many points showing that it is possible to have erroneous values due to errors either in gravity or in the topography.

ii.C.C. Tscherning pointed out that the three first order terms of Wenzel's rock-equivalent topography are very large. The center of mass is many hundreds meters away from the gravity center. This - in combination with the results of the assessment of the global digital DTMs- must mean that there are very large errors in the topography, probably related to the ocean bottom uncertainty and the bottom topography and the ice-masses. An estimate of the missing mass could be made.

5.4. Use of space techniques for the development and assessment of DTMs

Satellite Altimetry from ERS-1, and ERS-2 Geodetic Mission has been used in an intensive attempt to create/improve DTMs of both the Greenland and Antarctic ice sheets as well on land. The accuracy of the models based only on satellite altimeter data over the ice sheet varies from several tens meters to several hundreds meters, depending of the slop of the surface. Better results were achieved by combining a variety of digital elevation data (e.g. Ekholm, 1996).

The current accuracy estimates of the shuttle laser altimeter for low slope terrain are of the order of 1.5 to 3.0 m.(S.Luthcke)

Figure 1. Standard deviation of the gravity versus standard deviation of the topography

Intensive studies on the possibility to obtain globally topography from land altimeter data showed that it is possible to improve on the topographic knowledge of the continents at least for those regions where poor elevation data currently exists (e.g. Brenner et al., 1997, Berry 1999).

5.5. Ground and space techniques for the improvement of ocean bottom topography

Satellite altimetry has earlier been recognized as a source of information that can provide valuable information about the bathymetry. Simultaneously with the method applied by Smith and Sandwell (1994) for the bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry, least squares collocation was used for the inversion of gravity data (Barzaghi et al. 1992, Knudsen 1993). The gravity data can be observed or predicted gravity anomalies from altimeter data in areas with poor coverage with observed gravity data. Taking advantage of the flexibility of this method, Tscherning et al. (1994) performed an experiment to estimate the bottom topography in a test area in the Mediterranean by inverting gravity data. The method was further used in global (Knudsen and Andersen, 1996) or regional experiments (Arabelos, 1997; Arabelos and Tziavos, 1998). These experiments showed that the isostatic compensation of the topography, mainly at the crust/mantle interface is important to consider. The comparison between bathymetric models derived by inverting gravity data with "observed" bathymetric maps showed that the main features of the see floor are clearly described by the predicted bathymetric models. Experiments using the estimated bottom topography to smooth data related to gravity field (through e.g. RTM reductions) showed that generally the estimated bottom topography gives better results that the "observed" bathymetric models. The estimated bottom topography could be used in gravity field modeling and in other applications in areas where no high quality observed depths of the sea floor are available.

Finally, intensive experiments using SAR interferometry are described in the recent literature. Using improved techniques and exploiting all the available information, InSAR is a very promising tool for the improvement of DEMs (e.g., Rocca et al., 1997; Tscherning 1997).

SSG-3.163 Literature

Anzidei, M.: Rapid bathymetric survey and DTM of the Panarea caldera (Aeolian Islands, Southern Tyrrhenian Sea). Presented at the 24th General Assembly of the EGS, The Hague, 19-23 April, 1999.

Arabelos, D.: Validation of ETOPO5U in the Hellenic area. Bull. Geod 68, 88-99, 1994.

Arabelos, D., I.N. Tziavos: Comparisons of the global ETOPO5U model with local DTMs. Proceedings INSMAP94, pp 132-141, 1994.

Arabelos, D.: On the possibility to estimate ocean bottom topography from marine gravity and satellite altimeter data: using collocation. IAG Symposia, Vol. 119, pp 105-112, 1997.

Arabelos, D., I.N. Tziavos: Gravity field improvement in the Mediterranean Sea by estimating the bottom topography using collocation. Journal of Geodesy, 72, 136-143, 1998.

Arabelos, D.: Intercomparisons of the global DTMs ETOPO5, TerrainBase and JGP95E. Presented at the EGS XXIV General Assembly, The Hague, April 18-23, 1999.

Bamber, J., Muller., J.P., Mandanayake., A., "A Global 5 arc minute Digital Elevation Model derived From The Geodetic Phase Of ERS-1" Proc. 3rd ERS Symp. On Space at the service of our environment, Florence, March 1997.

Barzaghi, R., F. Sanso and C. Zenucchini: The collocation approach to the inversion of gravity data. Geophysical Prospecting, Vol. 40, 429-452, 1992.

Berry, P.A.M., M. Leigh: Towards validation and correction of global digital elevetion models with ERS-1 Altimetry. Proc. 3rd ERS Symp. on Space at the service of our Enviroment, Florence, Italy, 17-21 March 1997 (ESA SP-414, 3 Vols., May 1997.

Berry, P.A.M., R.F. Sanders, C. Leemans and E. Born: Generating Orthometric Heights from the ERS-1 Altimeter Geodetic Mission Dataset: Results from an Experts Systems Approach. IAG Symposia, Vol. 119, pp 113-118, 1997

Berry, P.A.M., R.A. Pinnock and J.I. Hoogerboorn: Land calibration of radar altimeter sigmao. Presented at the EGS XXIV General Assembly, The Hague, April 18-23, 1999.

Berry, P.A.M., Bron, E., Sanders, R.F. & Leenmans, C., "Use of ERS-1 Land Altimetry to Validate the GLOBE Global Digital Elevation Model", IAG Symposium, Rio de Janeiro, September 1997.

Berry, P.A.M., Sanders, R.F., Leenmans, C. & Bron, E. "Generating Orthometric Heights From The ERS-1 Altimeter Geodetic Mission Dataset: Results From An Expert Systems Approach", IAG Symposium, Rio de Janeiro, September 1997.

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