At the Conrad Observatory a permanent GPS station (Fig. 1) is installed and maintained in collaboration with the BEV. The multi-purpose station TRF2 (former TRFB, Trafelberg) meets strict requirements for the European Combined Geodetic Network (ECGN) combining various geodetic techniques like GNSS, leveling and gravity and contributes to the maintenance of the European Terrestrial Reference System 1989 (ETRS89) within the EUREF Permanent Network (EPN) as well as to the Austrian Positioning Service (APOS).

GPS reference pillar

Geodesy is the scientific discipline that provides the basis for determination of coordinates and reference systems by measuring the geometric and related physical parameters of the Earth on a global and regional scale. This includes: (1) definition of a reference ellipsoid, (2) measurement and calculation of a geometric network, (3) precision leveling and precise measurement of national heights, (4) definition of the geoid, (5) a gravity network, (6) a global satellite-determined network and its regional densification, and (7) coordinate transformation parameters for national reference systems. Since ancient times extensive surveying work has been performed for land register (Egypt) as well as regular astronomical latitude determinations for accurate positioning (Greece). Since the Middle Ages large areas were surveyed for coastal and nautical charts. Later on, in the 18th and 19th century triangulation became an established geodetic method and triangulation networks the basis of surveying. The new developments in satellite geodesy in the second half of the 20th century led to a more accurate determination and to global-scale networks. Today the International Terrestrial Reference Frame (ITRF) is the precise realization of the International Terrestrial Reference System (ITRS). The European Terrestrial Reference System 1989 serves as three-dimensional reference system, in which the Eurasian Plate as a whole was assumed as stable and was defined identical with the ITRS 1989.0 by the IAG Reference Frame Sub Commission for Europe (EUREF).

Land surveying in Austria was dominated by two major motivations: firstly, military reasons (military surveys) and secondly, tax reasons (cadastral surveys). Reliable topography maps are of vital importance during war times. For this reason, and after the Seven Year's War (1756-1763) was lost, Maria Theresa ordered a systematical land survey of the Habsburg dominions. This first Austrian land survey (scale 1:28,800), also known as “Josephinische Landesaufnahme” (1763-1787), was finished under her son Joseph II. (1741-1790). Map sheets of this survey were also used for maps (scale 1:115,200). During the second Austrian land survey, a.k.a. “Fanziszeische Landesaufnahme (1806-1869)” after Emperor Franz I. (1768-1835), triangulation was used and the results of the survey were made publicly available in contrast to the previous survey when the maps were kept secret. Land surveys mainly served military interest, whereas the cadastral survey “Franziszeische Katastralvermessung (1817-1869)” was intended to provide a uniform register for land taxes.

The “Franziszeische Kataster“ is the first complete land survey register and includes all parcels of the Austrian Empire. In 1862 the “Österreichische Gradmessungskommission” was founded. Since 1995 the “Österreichische Geodätische Kommission” (ÖGK) continues this work for higher geodesy. In 1863 the “Mitteleuropäische Gradkommission“ was founded. In order to achieve standardization of survey networks, certain principles were agreed on: the use of (1) the Bessel ellipsoid, (2) the Gauss conformal projection with 3° central meridian zones, (3) the prime meridian running through Ferro (El Hierro), and (4) the definition of a point of origin for each country. The subsequent third land survey, a.k.a. the “Franzisco-Josephinische Landesaufnahme“ (1869-1887), was conducted by the k.k. Militärgeographisches Institut (MGI) and was using the metric system (scale 1:25,000). It was replaced by the fourth land survey, a.k.a. „Präzisionsaufnahme“ (1896-1987), which is the basis of today's official topographic maps for Austria, the Austrian Map (ÖK). The maps are published by the Federal Office of Metrology and Surveying (BEV) in different scales 1:50,000 (ÖK50) or 1:25,000 (ÖK25V), 1:200,000 (ÖK200), 1:250,000 (ÖK250) as well as 1:500,000 (ÖK500). Further tasks of the BEV include the determination of a homogenous leveling network for Austria. Therefore, precision leveling are performed in the whole country.

The GNSS station TRFB started its observation in 2004, and was renamed TRF2 in 2008 due to changes in the height reference of the station. As an ECGN station TRFB/TRF2 is included in the EPN tracking station network and must fulfill strict requirements concerning the reception of GNSS satellite signals, data streaming, equipment and monumentation. The ECGN project was initiated by the International Association of Geodesy (IAG). There is a need for increased accuracy in 3D positions, and especially height is an ambiguous word in geodesy. Ellipsoidal height is purely geometrical and can be measured, e.g. by navigation satellites like GPS. Yet, physical heights, labeled as heights above sea level, refer to a boundary of the gravity potential and are derived from a combination of geometric and gravity measurements. The ECGN project is trying to use control points like the Conrad Observatory which offer the opportunity to check the different heights with geometrical and physical methods in a very accurate way. Such a permanent station can also monitor height changes in time. TRFB/TRF2 is also part of the APOS reference station network, which also uses GNSS satellite signals, and is operated by the Federal Office of Metrology and Surveying (BEV). All APOS stations represent the highest level of the ETRS98 realisation in Austria.

References
Ernst Zahn (2010) ECGN Station TRFB (Trafelberg) - its role within EPN and APOS. COBS Journal 2010. (link)
Günter Stangl (2011) Trafelberg on the move – Long-term and periodic variations of its position. COBS Journal 2011. (link)
Höggerl, Norbert (2001) Bestimmung von rezenten Höhenänderungen durch wiederholte geodätische Messungen. In: Christa Hammerl, Wolfgang Lenhardt, Reinhold Steinacker, Peter Steinhauser (Hrsg.): Die Zentralanstalt für Meteorologie und Geodynamik 1851 – 2001. Leykam, 2001.

Related Literature
Bretterbauer, Kurt (2009) Eine kurze Geschichte der österr. Landesvermessung und ihres Fundamentalpunktes Hermannskogel. Vermessung & Geoinformation 2/2009, 240-242. (pdf)
Alfred Kleusberg (2003) Satelliten im Dienst der Geodäsie und Geoinformatik. (pdf)

Suggested Literature
Geodesy (2001) W. Torge, 400 S., Walter de Gruyter.
Geophysical Continua (2008) B.L. Kennett, H.-P. Bunge, 432 S., Cambridge University Press.
Physical Geodesy (2005) B. Hofmann-Wellenhof, H. Moritz, 403 S., Springer.
The Tides of the Planet Earth (1978) P. Melchior, 609 S., Pergamon Press.
GNSS – Global Navigation Satellite Systems (2008) B. Hofmann-Wellenhof, H. Lichtenegger, E. Wasle, 516 S., Springer
Vermessungskunde (2006) H. Kahmen, 679 S., deGruyter

Suggested Online Material
Austrian Map online (BEV)
What is Geodesy? - NOAA Ocean Service Education
NASA – Looking Down a Well: A Brief History of Geodesy (YouTube, English only)
Josephinische Landesaufnahme von Niederösterreich, 1773-1781 (Wien) - ÖNB/KAR: K II 87