Data Set Overview : MDIS 3-color imaging of Mercury is mosaicked into non-overlapping, 128 pixel/degree tiles (MD3s). Each tile corresponds to the NW, NE, SW, or SE quadrant of one of the pre-existing Mercury non-polar charts, or one of the two polar charts.  Map tiles are named based on the quadrant of the Mercury chart they span:  MDIS_ccc_rrrPPD_Hxxddv.IMG  where:  ccc : product type : MD3 rrr : resolution in pixels/degree (PPD) Hxx : Mercury chart designation dd : quadrant within Mercury chart (NW, NE, SW, or SE), or a polar chart (NP, SP) v : version number  The following is an example file name with a description of the individual components:  MDIS_MD3_128PPD_H03NE0.IMG  For this image: Product type : MD3 (MD3) Resolution : 128 pixels/degree (128PPD) Mercury chart : Shakespeare (H03) Quadrant : Northeast (NE) Version : 0  The MD3 directory, present in the MD3 archive volume, contains 3-Color MDIS Map Projected Multispectral Reduced Data Records (MD3s). The MD3s are organized into subdirectories based on the Mercury Chart containing the MD3. Latitude and longitude limits of Mercury Charts, as named at the end of mission delivery, are:  Quadrangle Subdirectory Lat. (degrees) Long. (deg. east) H-1 Borealis H01 65 to 90 0 to 360 H-2 Victoria H02 22.5 to 65 270 to 360 H-3 Shakespeare H03 22.5 to 65 180 to 270 H-4 Raditladi H04 22.5 to 65 90 to 180 H-5 Hokusai H05 22.5 to 65 0 to 90 H-6 Kuiper H06 -22.5 to 22.5 288 to 360 H-7 Beethoven H07 -22.5 to 22.5 216 to 288 H-8 Tolstoj H08 -22.5 to 22.5 144 to 216 H-9 Eminescu H09 -22.5 to 22.5 72 to 144 H-10 Derain H10 -22.5 to 22.5 0 to 72 H-11 Discovery H11 -65 to -22.5 270 to 360 H-12 Michelangelo H12 -65 to -22.5 180 to 270 H-13 Neruda H13 -65 to -22.5 90 to 180 H-14 Debussey H14 -65 to -22.5 0 to 90 H-15 Bach H15 -90 to -65 0 to 360  For MD3s there is no H15 subdirectory because the 3-color mapping campaign did not include that portion of Mercury.  An MD3: - Consists of map-projected photometrically normalized I/F CDRs mosaicked into a multispectral map tile. In version 0, the value included for each band is from a single image set identified as 'best' by the MDR metric described below. In version 1, the value is an average from all images meeting a specified set of criteria; - Contains image data in I/F corrected photometrically to i:30 degrees, e:0 at a resolution of 128 pixels per degree (~332 m/pixel at the equator); - Represents one latitude-longitude bin in a global color map; - Is composed of up to 3 bands corresponding to the 3 of the 11 WAC filters; and - For version 0, contains 5 backplanes for the reference 750-nm band: (a) observation id, (b) MDR metric, (c) solar incidence angle, (d) emission angle, and (e) phase angle. - For version 1, contains 4 backplanes: (a) image count, the number of image sets averaged at a given latitude/longitude coordinate, and (b-d) standard deviations of the values used to determine average normalized I/F in each of the 3 bands.  Versions :  Version numbers of MD3s increment on reprocessing or addition of new data. Polar tiles are in polar stereographic projections, other tiles in equirectangular projection.  - Version 0 is released at PDS release 11. It is built from version 4 CDRs projected onto a sphere using version 0 DDRs. It uses a Hapke-form photometric correction, with different parameters for low- and high-incidence angle products. - Version 1 is released at PDS release 15 at end of mission. It uses version 5 CDRs projected onto a digital elevation model using version 1 DDRs. It uses a Kasseleinen-Shkuratov photometric correction with a common set of parameters among all data products.  Parameters : MDIS observing variables pertaining to the MD3s are as follows.  Pixel Binning: Some images are unbinned and 1024x1024 pixels. Some images are 2x2 pixel binned in the focal plane hardware (also known as 'on-chip' binning), resulting in 512 x 512 images. Images can be further compressed 2x2 using the MP, in addition to DPU binning, yielding 256x256 images. Images used in 3-color MD3 products are mostly not binned although some taken at the lowest altitudes are binned 2x2 'on-chip'. This less aggressive binning strategy than in the 8-color map results in nearly a 2.5x reduction in the average native pixel scale of the data.  12-8 bit compression: Images are read off the detector in 12-bit format. 12 bit images may converted to 8 bit images using one of eight lookup tables (LUTs). All WAC images collected as part of the 3-color multispectral map have been converted to 8 bits.  FAST/DPCM compression: All images are compressed losslessly using FAST/DCPM compression as they are read out of the DPU, to conserve recorder space. Once the data are written to the recorder, they can be uncompressed and recompressed more aggressively in the spacecraft main processor (MP).  Wavelet compression: Images may be integer wavelet transform- compressed in the MP, typically at 3:1 for color data. Color imaging was taken losslessly compressed when downlink volume permitted it.  Exposure Control: The exposure time of MDIS images can be set manually by command, or automatically by the software. In manual mode, exposure times from 1-989, 1001-1989, ..., to 9001-9989 ms are available. In autoexposure mode the exposure time of the next image is computed by the DPU software, and cannot exceed 989 ms in duration. If the time of the next image occurs before the calculation can be completed, and pixel binning or filter position change, then the algorithm compensates for predicted changes in scene brightness and filter transmission using an onboard data structure. All images in MD3s were acquired using automatic exposure.  Pointing: The MDIS imagers are mounted on a pivot platform, which is itself mounted to the MESSENGER spacecraft deck. The pivot platform is controlled by a stepper motor, which is controlled by the Data Processing Unit (DPU). The pivot platform can move in either direction. The total range of motion is 240 degrees, limited by mechanical 'hard' stops, and is further constrained by 'soft' stops applied by the software. The nominal pointing position for MDIS is defined as 0 degrees, aligned with the spacecraft +Z axis and the boresight for several other instruments. The range of the soft stops is set to 40 degrees in the spacecraft -Y direction (toward the MESSENGER sunshade) and +50 degrees in the +Y direction (away from the sunshade). The pivot position can be commanded in intervals of 0.01 degrees within this range. During acquisition of the global multispectral map, the pivot was used to point the WAC to low emission angles on the surface, at times when the solar incidence angle was close to the lowest values possible at that latitude.  Filter selection: The WAC imager contains a 12 position filter wheel to provide spectral imaging over the spectral range of the CCD detector. Three of the filters were chosen for the map and appear in the MD3 in order of increasing wavelength: WAC filter 6 (filter F), 430 BP 40 WAC filter 7 (filter G), 750 BP 5 WAC filter 9 (filter I), 1000 BP 15  Processing :  A sequence of processing creates an MD3 from CDRs and DDRs. A Derived Data Record (DDR) consists of multiband images whose line and sample dimensions and coordinates correspond one-for-one with those of a CDR. It has 5 bands of data used to help create an MD3, including for every image pixel: (a) latitude, (b) longitude, (c) incidence angle, (d) emission angle, and (e) phase angle. The DDRs are an intermediate product used to create MD3s and other map products, are defined as a distinct data product in the MDIS CDR/RDR Software Interface Specification, and are delivered to the PDS.  The sequence of processing is as follows:  (a) Experiment Data Records (EDRs) are assembled from raw data. (b) Radiance images are created from the EDRs and calibration files. (c) Radiance is converted to I/F CDRs by dividing by (pi * solar flux at 1 AU / heliocentricdistance_in_AU^2). (d) I/F is photometrically corrected to i : 30 degrees, e : 0 degrees. (e) Pivot positions are extracted from the spacecraft housekeeping and formatted as a pivot C kernel. (f) Using the pivot C kernel and other SPICE kernels, DDRs are created. The surface intercept on a model of Mercury's surface is calculated for each spatial pixel. The angles of this pixel relative to the equatorial plane and reference longitude constitute the latitude and longitude of the pixel. For that latitude and longitude, solar incidence, emission, and phase angles are determined. (g) I/F corrected to i : 30 degrees, e : 0 degrees is map projected into multiband MD3s using the latitude and longitude information in the DDRs. (h) For version 0, the same procedure is used on DDRs to assemble the backplanes with derived information. They are appended to the 3 image bands in the following order: OBSERVATION_ID MDR metric Solar Incidence Angle Emission Angle Phase Angle where OBSERVATION_ID taken from the CDR label, the ordinal number of the image among all MDIS images taken post-launch. The values for all backplanes are those for the filter 7 image within the color sequence. For version 1, backplanes are populated using statistics on the count and standard deviations of images that are averaged to populate each of the bands of normalized I/F.  A stacking order ('which image is on top') has been defined for MDRs and MD3s. The objective is to have 'on top' those images with high spatial resolution, low emission angle, and low solar incidence angle. Any image taken as part of the 3-color mapping campaign is a candidate to include in MD3s, as are images taken as part of the 5- or 8-color campaigns if they provide the best spatial resolution at low solar incidence angle. Rather than complete images being map-projected and laid into the mosaics, only a portion of an image is used, in which the same region of Mercury is observed in all filters of a single sequence of color images executed as part of the a color mapping campaign. The image quality metric is evaluated at the camera boresight of the middle image in that sequence; lowest values represent the 'best' image. For each color sequence, the 'worst' part of a sequence with overlapping coverage in all filters (highest value of the metric) is map-projected and laid into the MDR first; then the overlap region with the second-highest value is laid in second, overwriting the first overlap region, and so on until the 'best' overlap region with the lowest metric is on top. At all latitude and solar incidence angles, the metric for MDRs is:  PIXEL_SCALE / (cos i * cos e )  where i is solar incidence angle, e is emission angle, and the value of PIXEL_SCALE is limited not to be below approximately 332 meters, so that unfavorably illuminated images with high spatial resolutions not captured at the resolution of an MD3 do not overwrite more favorably illuminated images.  For version 1 MD3s, at any location, all images that fall within the following criteria are averaged, to reduce residuals from the time- variable instrument calibration. This results in a spatially variable number of images having been averaged:  - Controlled images are only used, with the G filter images part of the control set, and other filters in any given color set registered to the G filter images - Primarily uses images from the 3-color mapping campaign - Additional coverage is added by extracting the 3 bands from 8-color sets, when 8-color images fill gaps and meet resolution requirements - Emission angle at the center of the images is < 40 degrees - For tiles north of 43.75N latitude (N/S H02 tile boundary): - Incidence angle at the center of the images is < 88 degrees - Images scale is < 700 m/pixel - For tiles between equator and 43.75N latitude: - Incidence angle at the center of the images is < 70 degrees - Images scale is < 700 m/pixel - For tiles south of the equator: - Incidence angle at the center of the images is < 70 degrees - Images scale is < 1000 m/pixel  In version 0 MD3s, the photometric correction applied to MDIS WAC images to create MD3s is based on bi-directional reflectance equations formulated by [HAPKE1993]. The general equation for I/F is given by:  I/F:(w/4)[mu_not'/(mu' +mu_not')]{[1+B(g)]P(g)- 1+H(mu_not')H(mu')}S(i,e,g,theta)  where w is single scattering albedo, i is incidence angle, e is emission angle, g is phase angle, p(g) is the single particle scattering function, theta is a parameter representing macroscopic roughness, and mu_not' and mu' are modified versions of the cosines of the incidence and emission angle that take into account effects of theta. H(mu_not') and H(mu') describe approximations to the Chandrasehkar H-functions. The surface roughness function, S(i,e,g,theta), modifies the radiative transfer equation to account for surface roughness.  In addition, a Henyey-Greenstein function is used to describe the single particle scattering function p(g). The form of the Henyey-Greenstein function used corresponds to the form utilized in the USGS ISIS software, and is given by:  p(g):c(1-b^2)(1-2b cos(g)+b^2)^(-3/2) + (1-c)(1-b^2)(1+2b cos(g)+b^2)^(-3/2),  where g is the phase angle, b is the scattering amplitude parameter, and c is the partition parameter between forward and backward scattering.  In version 0 MD3s, no single set of Hapke parameters was found that yields close matches for corrected I/F across boundaries of images taken at different photometric geometries, for both the 3- and 8-color maps taken predominantly at low solar incidence angles (average, about 45 degrees) included in MDRs and MD3s, and monochrome maps taken predominantly at high solar incidence angles (BDRs, HIEs, HIWs). Therefore map products emphasizing low or high incidence angles initially used different sets of photometric parameters optimized for each to minimize seams between images.  The parameters for the MD3 Hapke photometric corrections were derived by modeling data acquired from multiple regions between 24 degrees and 46 degrees south latitude and 330 degrees and 353 degrees east longitude. These regions sample incidence angle (i), emission angle(e), and phase angle (alpha) coverage commensurate with the 3- and 8-color map imaging campaigns, plus additional supplementary observations intended to expand the phase angle range. The data from each of the regions was combined into a single data set and a single set of parameters derived for each filter.  The photometric measurements were modeled using a least squares grid search routine over the available parameter space. The model parameter values were individually plotted as a function of wavelength over the MDIS filter central wavelength values. A polynomial trend was fit to each parameter as a function of wavelength. The polynomial trend value at each filter central wavelength was then used as the model parameter values for determining the photometric correction. Derivation of the photometric correction involved: (1) calculating the reflectance at the observed geometry for each pixel in each image (Ro), (2) calculating the reflectance at i:30 degrees, e:0 degrees, alpha:30 degrees (R30), (3) calculating the correction factor (R30/Ro), and (4) applying the correction factor to each pixel within each image. The Hapke model parameters used are listed for each WAC filter included in the 3-color maps in MD3s in the table below, where w is the single scattering albedo, theta is the surface roughness parameter, and b and c are the Heyney-Greenstein single particle scattering function parameters defined above.  filter, wavelength, w, b, c , theta F, 433.2, 0.151360115, 0.155099698, 0.126102310, 14.60131926 G, 748.7, 0.249052388, 0.122256228, 0.072924183, 14.27068804 I, 996.2, 0.292068663, 0.133854741, 0.167852540, 14.00895011  For all wavelengths, the width of the opposition surge, h, is 0.09 and the strength of the opposition surge, B0, is 3.086.  In version 1 MD3s delivered at the end of the mission, a different photometric correction is used, a Kasseleinen-Shkuratov function described by [DOMINGUEETAL2016]. The form of the function is given as:  I/F : AN*exp[-(g*mu)]{c_sub_l[2cos i/(cos i + cos e)]+[1-c_sub_l]cos i}  where AN is normal albedo at a given wavelength, and mu and c_sub_l are wavelength-dependent parameters. Their values were fit using multiple regions between 24 degrees and 46 degrees south latitude and 330 degrees and 353 degrees east longitude. These regions sample incidence angle (i), emission angle(e), and phase angle (g) coverage commensurate with global mapping campaigns. In addition whole-disk Mercury images taken at a large number of geometries during the Mercury flybys expand the phase angle range. Parameter values were fit in the same manner as the parameters for the Hapke model. The values are given in the table below:  WAC filter, wavelength, AN, mu, c_sub_l F, 433.2, 0.0700, 0.6363, 0.6293 G, 748.7, 0.1111, 0.5628, 0.6424 I, 996.2, 0.1250, 0.5200, 0.6303  Data : There is one data type associated with this volume, MD3s consisting of mosaicked, photometrically corrected I/F from WAC CDRs acquired through 3 spectral filters. Version 0 is appended with 5 backplanes describing the component CDRs and their photometric geometries as recorded in DDRs. Version 1 is appended with 4 backplanes describing the number of image sets averaged at each latitude / longitude location, and the standard deviations in each of the spectral bands.  Ancillary Data :  There are two types of ancillary data provided with this dataset:  1. The CALIB directory contains bandpasses of the spectral filters of MDIS used to collect the images in the dataset, to facilitate comparison with other data sets. See CALINFO.TXT in that directory for more details.  2. There may be a BROWSE directory containing browse images in PNG and/or GeoTIFF format. See BROWINFO.TXT in that directory for more details.   Coordinate System :  The cartographic coordinate system used for the MDIS data products conforms to the J2000 celestial reference frame for star imaging, and the IAU planetocentric system with East longitudes being positive for planetary surfaces.  In version 0 MD3s, the IAU2000 reference system for cartographic coordinates and rotational elements is used for computing latitude and longitude coordinates of planets. However a Mercury radius of 2440.0 km is used.  In version 1 MD3s, the value for Mercury radius is updated to 2439.4 km.   Media/Format :  The MDIS archive is organized and stored in the directory structure described in the Mercury Dual Imaging System (MDIS) Calibrated Data Record (CDR) and Reduced Data Record (RDR) Software Interface Specification (SIS). The contents of the archive, along with fiduciary checksums, are compressed into a single 'zip archive' file for transmittal to the PDS Imaging node. The zip archive preserves the directory structure internally so that when it is decompressed the original directory structure is recreated at the PDS Imaging node. The zip archive is transmitted to the PDS Imaging node via FTP to the URL specified by the node for receiving it.

Confidence Level Overview : Known issues of concern are described below.   Review : This archival data set was examined by a peer review panel prior to its acceptance by the Planetary Data System (PDS). The peer review was conducted in accordance with PDS procedures.   Data Coverage and Quality :  Only a subset of raw EDR data are calibrated to CDRs and then incorporated into MD3 products. Briefly, the following criteria are met:  (a) The data represent a scene and not the instrument test pattern, as indicated by data quality index (DQI) byte 0.  (b) The exposure time is greater than zero (zero exposures occur in some images due to software features), as indicated by DQI byte 1.  (c) Less than 20 percent of the image is saturated (empirically this is a threshold dividing wholly corrupted images from everything else).  (d) The target of the image is MERCURY.  (e) In version 0 MD3s, the image was taken as part of the 3-color mapping campaign. Version 0 is based on version 4 CDRs which correct a number of earlier calibration artifacts, and on version 0 DDRs. Version 1 MD3s may use image sets from the 3-, 5-, or 8-color mapping campaigns that attain the best spatial resolution at low incidence angles. Version 1 is based on version 5 CDRs and version 1 DDRs. For version 1 MD3s, some component images may contain residuals from the following issues.  (1) COMPRESSION ARTIFACTS. Wavelet compression applied to science images is lossy. At higher compression ratios, compression artifacts will degrade data precision over spatial scales comparable to or smaller than several pixels. The degradation can be greater proportionally to the image dynamic range of brightness, if the data are converted from 12 to 8 bits in such a way that a 1 DN error occupies a greater fraction of the digital dynamic range. Wavelet compression was used minimally prior to Mercury orbit. The initial configuration in Mercury orbit was to perform 12 to 8 bit conversion using LUT0 for the WAC and LUT2 for the NAC, with a wavelet compression ratio of 4:1 for color imaging. Initial images exhibited unexpectedly visible compression artifacts. Beginning 19 April 2011, LUT0 and LUT2 were replaced with LUT1 which better preserves image dynamic range. Beginning 19 May 2011, targeted color images began to be acquired with lossless compression. Beginning 31 May 2011, the wavelet compression ratio for color images was reduced to 3:1 for global mapping. All MD3 images are taken with LUT1 and a compression ratio of 3:1 or less.  (2) RADIOMETRIC ACCURACY. The radiometric calibration of the WAC was updated several times over the mission to iteratively reduce residuals from 3 sources of error: (a) time-variable responsivity of the detector, (b) residuals in the flat-field correction, and (c) residuals in the correction to responsivity for detector temperature. For multispectral products, the residuals from time-variable responsivity initially led to distinct seams; correction of this artifact is treated in more detail below.  In version 4 CDRs, an additional update to responsivity improved temperature dependence over the full operating range. The correction was derived empirically by fitting as a function of CCD temperature the median values of images acquired from Mercury orbit at a wide range of temperatures but a narrow range of photometric geometries. The flat field was updated empirically using the median of hundreds of photometrically corrected images of relatively bland field-filling images of Mercury.  In version 5 CDRs, a new, final temperature correction used all Mercury images satisfying the illumination criteria. A new, final flat-field correction was derived similarly to the updated correction used in version 4, except using more images and a Kasseleinen-Shkuratov photometric correction  (3) SCATTERED LIGHT. The WAC is subject to scattered light originating from within the field-of-view or just outside it. One source is multiple reflections off of 13 optical surfaces (2 sides of each of 4 lenses, the spectral filter, and the CCD cover glass, as well as the CCD surface itself). The scatter becomes worse at longer wavelengths. Just off the limb of a large extended source near 1 field-of-view in size, like Venus or Mercury, measured radiance increases with wavelength from 2% to 7% of the value measured on the extended source. The value decreases with distance off the target more quickly at shorter than at longer wavelengths, but remains at 1% hundreds of pixels from the source. Conversely, light must be scattering from bright parts of an image to dark parts of an image. Averaged over sources tens of pixels in area, and away from abrupt brightness contrasts, scattered light affects shapes of spectra measured from WAC data at least at the 1-2% level, worse near brightness boundaries or for small, bright crater ejecta. The expected effect is enhanced brightness at >650 nm in dark areas, and decreased brightness at >650 nm in small bright areas.  In the end-of-mission delivery 15, a forward model of the expected WAC scatter from a given scene was derived using optical design software modeling CCD structure and hardware, with magnitudes of scatter calibrated against flight measurements. The ray trace analysis reveals an in-scene component from light diffracted by the CCD and reflected by the CCD cover glass, and an out-of-scene component from light reflected off metallic surfaces alongside the CCD and back off the cover glass. Images with the worst scattered light were excluded from multispectral map products.  (4) TIME-VARIABLE WAC RESPONSIVITY. During Mercury orbit it was recognized that filter-dependent changes in WAC responsivity on the order of +/- 15% occurred over timescales as short as several days. Because those variations were not consistent from filter to filter, they led to spurious spectral features, which were particularly conspicuous near 750 nm. The cause(s) of these variations in responsivity are not known, but they could include transient radiation effects on the detector or electronics, aging of filters, periodic deposition and burn-off of contaminants on filters, or incorrect recording of exposure time. An initial empirical correction for images during early orbital operations was developed and utilized in version 4 WAC CDRs used to create version 0 MD3s.  For version 5 WAC CDRs in delivery 15 at end of mission, an updated correction covers the full duration of the orbital phase. Overlaps between color image sets in color mapping campaigns were used to derive a multiplicative correction factor for each filter and for each Earth day (2-3 orbits). Version 1 MD3s use CDRs with this updated correction. An analysis of overlap among individual images shows that residual differences (which include errors from calibration, scattered light, and possible incomplete correction of photometric variation) average <2% for the majority of the planet.  (5) UNCONTROLLED MOSAIC PROJECTED ONTO A SPHERE. Version 0 MD3s are constructed by uncontrolled mosaicking, projecting the image data onto a sphere. At the stage where each of the 3 band images is locally map projected and joined into a cube, the bands are then spatially adjusted to match features to the 750-nm band. Systematic errors in spacecraft position and in knowledge of spacecraft and MDIS attitude, systematic errors in range to the surface due to ignoring topography, and systematic errors in latitude and longitude due projecting onto a sphere instead of a shape model will all contribute to mosaicking errors. In general these are expected to be under 1 km but locally might exceed 4 km.  Version 1 MD3s are constructed from images controlled using c-smithed kernels and a global digital elevation model (DEM), both derived using a least-squares bundle adjustment of common features, measured as tie point coordinates in overlapping NAC and WAC-G filter images of Mercury at favorable solar incidence and emission angles. Empirically, misregistration errors between images decreased generally to the pixel scale of the map, (0.2 km) in most locations. Derivation of smithed kernels and the DEM for end of mission data products is described by [BECKERETAL2016].  (6) INACCURACY IN THE PHOTOMETRIC CORRECTION. The Hapke correction applied to version 0 MD3s required the use of illumination-dependent parameters implying the possibility of systematic inaccuracy. As shown by [DOMGINUEETAL2016] the Kasseleinen-Shkuratov correction used in version 1 MD3s greatly reduced residuals between images acquired at different photometric geometries, implying reduced systematic errors.   Limitations : None