OBJECT = DATA_SET_MAP_PROJECT_CATALOG DATA_SET_ID = 'MGN-V-RDRS-5-MIDR-POLAR-STEREOGR-V1.0' OBJECT = DS_MAP_PROJECT_INFO_CATALOG MAP_PROJECTION_TYPE = 'STEREOGRAPHIC' MAP_PROJECTION_DESC = " P-MIDR images are presented in equal-area polar stereographic map projections, using the Venus body-fixed coordinate system approved by the IAU in 1985 (see Davies, et al., 1989.) For convenience, the original images, each containing of 10240 lines and 10240 samples, have been divided into arrays of framelets, each of which is 1024 lines by 1024 samples. Each framelet is projected with the same mapping parameters so that a full P-MIDR image can be reconstructed by concatenating the framelets. In this case, the values of VICAR2 header keywords defining the projection (see below) should be taken from the first (top left) framelet. Browse images (named BROWSE.IMG) follow the same rules as their accompanying framelets, except that their headers don't contain the SUBF_ROW or SUBF_COL keywords. They are reduced in size by a factor of 10, i.e. they contain 1024 lines and samples. In the polar stereographic projection, parallels of latitude are circles centered on the pole, and meridians of longitude are straight lines radiating from the pole. Images in this projection are centered on either the North or South pole of Venus, with the central longitude (specified by the 'PROJ_LON' value in the image label) running vertically from the pole to the bottom of the image for the North Polar projection, and to the top of the image for the South Polar projection. South polar P-MIDR images extend to 80S latitude and North polar P-MIDR images extend to 80N. The transformation from latitude and longitude (LAT,LON) in radians to LINE and SAMPLE is given by the following equations. SCALE = 6051000/PIXSIZ North Polar Stereographic: SAMPLE = PROJSAMP+2*SCALE*SIN(LON-PROJ_LON)*TAN(PI/4-LAT/2)+0.5 LINE = SPECLINE+2*SCALE*COS(LON-PROJ_LON)*TAN(PI/4-LAT/2)+0.5 South Polar Stereographic: SAMPLE = PROJSAMP+2*SCALE*SIN(LON-PROJ_LON)*TAN(PI/4+LAT/2)+0.5 LINE = SPECLINE-2*SCALE*COS(LON-PROJ_LON)*TAN(PI/4+LAT/2)+0.5 The north and south P-MIDR images are distinguished by the values of the PROJ_LAT keywords in their VICAR2 headers, respectively +90 for the north polar image (P-MIDR.N) and -90 for the south polar image (P-MIDR.S). In the above definitions, integral values of LINE and SAMPLE correspond to the center of a pixel, and the top left image pixel has LINE=1 and SAMPLE=1. LAT and LON are the latitude and longitude of a given spot on the surface in radians. The other parameters in these equations are the names of keywords in the VICAR2 labels associated with each image. These parameters can also be found in the PDS labels of the image, but with different names. The values of parameters in the VICAR2 and PDS labels correspond to a given framelet. Thus, the computed LINE and SAMPLE relate to the framelet. The equivalent PDS keyword name is shown in parentheses. SPECLINE (X_AXIS_PROJECTION_OFFSET) is the image line number minus one on which the map projection origin occurs. The map projection origin is the appropriate pole. PROJSAMP (Y_AXIS_PROJECTION_OFFSET) is the nearest image sample number to the left of the projection longitude PROJ_LON. PROJ_LAT (CENTER_LATITUDE) is the value of the projection latitude, i.e. +90 for the P-MIDR.N (north pole) and -90 for the P-MIDR.S (south pole). PROJ_LON (CENTER_LONGITUDE) is the value of the projection longitude. PIXSIZ (MAP_SCALE) pixel spacing at the projection origin, which is the appropriate pole for stereographic images. The value in the VICAR2 label is in units of meters/pixel. PIXSIZ is 225 meters/pixel for all P-MIDR framelets, and 2250 meters/pixel for their browse images. LAT_UC latitude of the center of the first framelet line. LAT_LC latitude of the center of the last framelet line. LON_CL the longitude of the leftmost pixel in the center framelet line. LON_CR the longitude of the rightmost pixel in the center framelet line. SUBF_COL (Y_AXIS_FRAMELET_OFFSET) column number of a framelet, which can have a value of 1 to 10. This keyword will be omitted in browse image headers. SUBF_ROW (X_AXIS_FRAMELET_OFFSET) row number of a framelet, which can have a value of 1 to 10. This keyword will be omitted in browse image headers. SCALE (MAP_RESOLUTION) number of pixels per degree at the projection origin, which is the appropriate pole for stereographic images. In addition, the following PDS keywords have no precise equivalent in the VICAR2 labels: MINIMUM_LONGITUDE the westernmost longitude of any pixel in the framelet. MAXIMUM_LONGITUDE the easternmost longitude of any pixel in the framelet. MINIMUM_LATITUDE the southernmost latitude of any pixel in the framelet. MAXIMUM_LATITUDE the northernmost latitude of any pixel in the framelet. NOTE: The PDS keywords whose names begin 'X_' and 'Y_', e.g. X_AXIS_PROJECTION_OFFSET, Y_AXIS_PROJECTION_OFFSET, etc. use 'X' to denote LINE and 'Y' to denote SAMPLE. This is reversed relative to the definitions in SNYDER1987, i.e. x(Snyder) = y(PDS), and y(Snyder) = -x(PDS)." ROTATIONAL_ELEMENT_DESC = "See DAVIESETAL1989." OBJECT = DS_MAP_PROJ_REF_INFO_CATALOG REFERENCE_KEY_ID = 'DAVIESETAL1989' END_OBJECT = DS_MAP_PROJ_REF_INFO_CATALOG OBJECT = DS_MAP_PROJ_REF_INFO_CATALOG REFERENCE_KEY_ID = 'SNYDER1987' END_OBJECT = DS_MAP_PROJ_REF_INFO_CATALOG OBJECT = DS_MAP_PROJ_REF_INFO_CATALOG REFERENCE_KEY_ID = 'LYONS1988' END_OBJECT = DS_MAP_PROJ_REF_INFO_CATALOG END_OBJECT = DS_MAP_PROJECT_INFO_CATALOG END_OBJECT = DATA_SET_MAP_PROJECT_CATALOG END