Data Set Overview : The topography model of Venus was produced by a spherical harmonic analysis of the most complete set of Magellan altimetry data, augmented by Pioneer Venus and Venera data. A detailed report on this harmonic analysis and its scientific implications is in [RAPPAPORT&PLAUT1994].   Data : This dataset includes a 360th degree and order spherical harmonic topography model, and a digital map of the topography.  There are three data types for the products found on this volume: tabular, array, and image data.  The file containing the spherical harmonic coefficients of the Venusian topography field is in tabular format, with each row in the table containing the degree index m, the order index n, and the coefficients Cmn and Smn.  The digital map of topography is a binary image. There is also an ASCII 2-D gridded data array of the digital image, including a byte-scaled image of the gridded product.   Parameters : The topography of Venus was determined from altimetry data obtained by the Magellan spacecraft and augmented by Pioneer Venus and Venera data. The mean radius of Venus was found to be 6051.848 km.   Processing : The starting point for building the set of data that was analyzed to produce the topography spherical harmonic model was the GTDR (Global Topography Data Record) files which contain maps of Magellan altimetry data [FORD&PETTENGILL1992].  Gaps in the data were filled using a) Pioneer Venus Orbiter altimetry merged with Venera 15/16 altimetry data; and b) the 120 X 120 model of [KONOPLIVETAL1993], providing a complete set of data referenced to Venus body-fixed reference frame.  These data were projected on a rectangular grid of 0.25 X 0.25 degrees and the harmonic coefficients were computed using quadrature procedure [BALMINO1993].   Ancillary Data : Magellan altimetric data and documentation can be found in numerous published datasets: Global Topography Data Record (GTDR); Altimetry and Radiometry Composite Data Record (ARCDR); and Altimeter Experiment Data Record (ALT-EDR).   Coordinate System : A Venus body-fixed reference frame, as defined by [DAVIESETAL1992B], was used.   Software : N/A   Media/Format : This archive will be available electronically via the World-Wide Web and anonymous FTP transfer: http://pds-geophys.wustl.edu/pds/magellan/harmonics_maps_archive. The archive will also be delivered to the National Space Sciences Data Center (NSSDC) using compact disk write once (CD-WO) media. Formats will be based on standards established by the Planetary Data System (PDS).

Confidence Level Overview : The method used for the spherical harmonic analysis of the topography does not provide easily the uncertainties in the harmonic coefficients. A study of the uncertainty was performed in [RAPPAPORT&PLAUT1994].   Review : The volume containing the Venusian gravity and topography data sets was reviewed by Magellan mission scientists and by PDS.   Data Coverage and Quality : The coverage of the Magellan altimetry data include three main gaps, which have two separate origins associated with a) superior conjunction and b) the thermal hide strategy in which the high gain antenna was used to shadow the spacecraft, precluding altimetry observations. The two main gaps are located near the south pole, and a smaller gap is present near the north pole.  The construction of global topography maps from altimetry data requires the removal of systematic errors associated with the uncertainty in the spacecraft trajectory. The technique used by [FORD&PETTENGILL1992] dramatically reduced the systematic error in radius measurements. A conservative order of magnitude of the altimetry data error is estimated to be of order +/- 80 meters. However, the uncertainty in individual points can be very small. For example, on the smooth Venusian plains, the accuracy of individual data points is probably of the order of a few meters.   Limitations : N/A