Data Set Overview : The FMAP is a full-resolution (75 meters/pixel) global mosaic produced by the U.S. Geological Survey from Magellan F-BIDR data. The complete dataset consists of 340 quadrangles in Sinusoidal equal-area projection. Quadrangles extend approximately 12 degrees in latitude, except for those between 84 and 90 degrees North and South. Quadrangles near the equator extend 12 degrees in longitude; at higer latitudes, the longitudinal extent is increased to maintain a roughly constant number of samples. The data between 84 and 90 degrees North is also available in Polar Stereographic projection on a separate volume.   Parameters : N/A   Processing : Cosmetic techniques (spatial filtration and blending) have been used to remove seams between adjacent F-BIDRs caused by radiometric artifacts.   Ancillary Data : N/A   Coordinate System : N/A   Software : Refer to the SOFTINFO.TXT file in the SOFTWARE directory on this CD-ROM volume for information on the software provided with this data set.   Media/Format : The FMAP dataset will be delivered to the Magellan Project (or its sucessor) using CD-ROM media. Formats will be based on standards for such products established by the Planetary Data System (PDS) [PDSSR1992].

Overview : These image mosaics should be considered a preliminary product. They were assembled by the U.S. Geological Survey from Magellan Full-resolution Basic Image Data Records (F-BIDRs) [Pettengill et al. 1992, Saunders et al. 1992] processed and provided by the Jet Propulsion Laboratory. There are several known problems with the Magellan image data that either have led to reprocessing of a subset of the FBIDRs or that may be of sufficient importance to warrant future processing. The mosaics on this disk were created on a 'best effort' basis, using the most current versions of the constituent FBIDRs available to the USGS at the time of processing (1993-1994). Given the limitations of the available data, no other quality control was possible or was attempted. The present FMAP may be superseded by later versions utilizing reprocessed and improved data. The major known problems with the Magellan image data are the following:   The SAR Image Data ------------------ The synthetic-aperture radar (SAR) image data were reprojected from the radar coordinates (delay-doppler) onto the surface of Venus by using information about the spacecraft location based on a priori predictions and earth-based tracking; self-consistency of the image data from multiple orbits was not used to refine the ephemeris accuracy as is commonly done with optical imaging data. The FBIDR data contain known positional errors of up to several kilometers, and, in particular, discontinuities of this magnitude between adjacent BIDRs whose ephimerides were processed separately. Reprocessing of the Magellan SAR data with ephimerides refined by matching features in multiple BIDRs is technically feasible and might be carried out at some future date.  For the orientation of the Magellan spacecraft, only a priori information was available. The positional accuracy of SAR imaging is not dependent upon knowledge of the spacecraft orientation and hence was not affected. Orientation information is needed, however, to correct for the varying sensitivity of the instrument across the width of the radar beam. There are radiometric (tonal) mismatches between almost all neighboring F-BIDRS because of errors in the correction for beam sensitivity. These mismatches were suppressed in the FMAP by the use of purely cosmetic processing techniques: highpass spatial filtering of each BIDR with a boxcar elongated in the direction of the flight path, followed by linear blending of the data from adjacent BIDRs in the area of overlap. The effect of this processing on absolute radiometric accuracy is not known.   Venus Topographical Model ------------------------- A topographic model of the surface of Venus is needed in order to project the SAR data into map coordinates. Most BIDRs were processed with a topographic model based on pre-Magellan (Pioneer Venus and Venera 15/16) data; the limited resolution of this dataset leads to planimetric inaccuracies in the BIDRs and FMAP mosaics. During the Magellan mission, an attempt was made to process the BIDRs with a higher resolution topographic model based on Magellan data. This topographic model proved to have a higher resolution than could be handled by the Magellan SAR processor, leading to severe interpolation errors and distortion of the data. The affected BIDRs (revs ??--??, longitudes ??--??) were reprocessed with the earlier topographic model, and the reprocessed versions were used in this mosaic when available, but the FMAP may contain some data with this defect.   Magellan SAR Processor ---------------------- An error in the Magellan SAR processor resulted in the production of BIDRs with limited dynamic range (specifically, all data values less than about 70 DN were set to this value). This problem affected revs ??--??, of which only revs ??--?? (longitudes ??--??) were obtained with illumination from the west ('left looking SAR') and were used in the FMAP. Again, reprocessed versions of these BIDRs were incorporated in the mosaic as available, but some erroneously processed data may be present. It was not feasible to track the version of each F-BIDR incorporated in the FMAP.   Review : The FMAP will be reviewed internally by the Magellan Project prior to release to the planetary community. The FMAP will also be reviewed by the PDS.   Data Coverage and Quality : The FMAP incorporates all Magellan images of Venus obtained with the nominal incidence angle profile for left-looking (illuminated from the west) observations. Data obtained during the first 243-day cycle of operations cover approximately 84% of the surface; some additional left-look data filling gaps in this coverage were obtained in the second cycle [Saunders et al. 1992]. The division of the mosaicked data into quadrangles covering 12 degrees of latitude and longitude at the equator, and the division of the quadrangles into tiles 2 by 2 degrees at the equator is described under the keyword DATA_SET_DESC and in more detail by Kirk et al. 1993b. For a variety of reasons, data may be unavailable for portions of the FMAP ranging from a few kilometers in north-south extent to entire F-BIDRs. Regions of missing data are flagged by a data value of 0; valid data are in the range from 1 to 255. The F-BIDRs contain information flagging certain pixels as 'substandard' because they contain information from less than four statistically independent 'looks' at the surface [Petengill et al. 1991]. Both standard and substandard pixels were used identically in producing the FMAP and are not flagged in it.   Limitations : N/A