Data Set Overview : A major imaging campaign for MDIS in MESSENGER's primary mission was acquisition of a global data set at low emission angles for cartographic purposes, and moderate to high incidence angles that highlight topography. Those images were mosaicked into basemap data records (BDRs). The HIE data set is complementary in that it highlights low-relief topography that is less evident. The illumination from the east favors asymmetric topography more steeply sloped to the east than to the west. The companion HIW data set favors asymmetric topography more steeply sloped to that west than to the east. Map tiles are named based on the quadrant of the Mercury chart they span: MDIS_ppp_rrrPPD_Hxxddv.IMG where: ppp : product type : HIE 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)e limits v : version number The following is an example file name with a description of the individual components: MDIS_HIE_256PPD_H03NE0.IMG For this image: Product type : HIE (HIE) Resolution : 256 pixels/degree (256PPD) Mercury chart : Shakespeare (H03) Quadrant : Northeast (NE) Version : 0 The HIE directory, present in the HIE archive volume, contains MDIS Map Projected High Incidence Angle Basemap Illuminated from the East Reduced Data Records (HIEs). The HIEs are organized into subdirectories based on the Mercury Chart containing the HIE. 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 An HIE: - Consists of map-projected photometrically normalized I/F CDRs mosaicked into a basemap map tile; - Contains image data in I/F corrected photometrically to i:30 degrees, e:0 at a resolution of 256 pixels per degree (~166 m/pixel at the equator); - Represents one latitude-longitude bin in a global map; - Is composed of images acquired with by the NAC or by the WAC in filter 7, both centered near 750 nm; - Contains images acquired as part of the high-incidence angle basemap campaign for which SUBSOLAR_LONGITUDE is located east of the images's CENTER_LONGITUDE; and - Contains 5 backplanes: (a) observation id, (b) BDR metric, modified for the optimal incidence angle to be 78 degrees, (c) solar incidence angle, (d) emission angle, and (e) phase angle. Versions : Version numbers of HIEs increment on reprocessing or addition of new data. Polar tiles are in polar stereographic projections, other tiles in equirectangular projection. Version 0 is uncontrolled, projected onto an ellipsoidal model of Mercury, and photometrically corrected using a Hapke photometric model with parameters optimized to higher solar incidence angles (and different from parameters used in map products containing lower-incidence angle data). Version 1, released at end of mission, is compiled using NAC or WAC 750-nm images from any campaign that best fit the intended illumination geometry, i.e., low emission angle and incidence angle near 78 degrees. It is controlled and projected onto a global digital elevation model. It uses a Kasseleinin-Shkuratov photometric correction, whose parameters are the same for any given wavelength band across all MESSENGER end-of-mission map data products. Parameters : MDIS observing variable pertaining to the HIEs are as follows. Pixel Binning: Some HIE 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. The WAC was used to acquire HIE images from lower altitudes, and the NAC was used at higher altitudes. Within the altitude range for either camera, on-chip binning was used within the lower portion of the range, to control data volume and to manage data flow on the spacecraft. No further binning by the spacecraft main processor (MP) was used. 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 images collected as part of the HIE basemap 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 MP. Wavelet compression: Images may be integer wavelet transform- compressed in the MP, typically at 3:1 for color data and 4:1 for monochrome data, but any value from 1 to 32 can be used. 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 8:1 for monochrome imaging and 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, and compression ratios were decreased to 4:1 or less for monochrome data and 3:1 or less for color data where possible. Lossless compression was used when downlink allowed. 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 HIEs 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 HIE basemap, the pivot was used to point the WAC or NAC to low emission angles on the surface, at times when the solar incidence angle was close as possible to 78 degrees. Filter selection: The WAC imager contains a 12 position filter wheel to provide spectral imaging over the spectral range of the CCD detector. WAC filter 7 (750 BP 5) was chosen to complement the NAC because its bandpass within that of the NAC lessens any discontinuities that might result from regional variations in spectral slope. Processing : A sequence of processing creates an HIE 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 HIE, 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 HIEs 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 beginning with delivery 11. 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) Gimbal positions are extracted from the spacecraft housekeeping and formatted as a gimbal C kernel. (f) Using the gimbal C kernel and other SPICE kernels, DDRs are created. The surface intercept on a sphere of Mercury's radius 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 HIEs using the latitude and longitude information in the DDRs. The same procedure is used on DDRs to assemble the backplanes with derived information. They are appended to the image band in the following order: OBSERVATION_ID BDR metric Solar Incidence Angle Emission Angle Phase Angle where OBSERVATION_ID is taken from the CDR label, the ordinal number of the image among all MDIS images taken post-launch, and the photometric angles are taken from the DDR. The BDR metric or stacking order ('which image is on top') was first defined for BDRs. For HIEs, the objective is to have 'on top' those images with high spatial resolution, low emission angle, and a solar incidence angle as close as possible to 78 degrees. This incidence angle was chosen to highlight subtle topographic shading. For version 0 HIEs, any image taken as part of the high incidence angle campaign is a candidate to include. For version 1 HIEs, images from any campaign with suitable illumination can be included within the following criteria: - Controlled images are primarily used, but non-controlled images are used as needed to minimize gaps in coverage - Incidence angle at the center of the images is < 90 degrees - The north polar tile (H01) trims pixels with incidence angle > 88 degrees - The south polar tile (H15) trims pixels with incidence angle > 89.5 degrees - The image pixel scale > 100 m/pixel The stacking order is determined at the camera boresight using a metric that represents spatial resolution and image geometry; lowest values represent the 'best' image. The 'worst' complete, map- projected image with the highest value for the metric is laid into the HIE first; then the complete image with the second-highest value is laid in second, overwriting the first image where the coverage coincides, and so on until the complete 'best' image with the lowest value for the metric is on top. There are 3 expressions for the modified BDR metric depending on latitude greater or less than 65 degrees and solar incidence angle greater or less than 78 degrees. (a) Where abs(lat) <: 65 degrees and i :> 78 degrees, the metric is: PIXEL_SCALE / (cos e * ( cos ( flatten_factor * i) / cos ( flatten_factor * 78 ) ) ) where i is solar incidence angle, e is emission angle, lat is planetocentric latitude, and flatten_factor is set to 0.85 to de-emphasize low solar incidence angles. (b) Where abs(lat) <: 65 degrees and i < 78 degrees, the metric is: PIXEL_SCALE / (cos e * (cos 78 / cos i)) (c) Where abs(lat) > 65 degrees, the metric is: PIXEL_SCALE / (cos i * cos e ) where i is solar incidence angle, e is emission angle. In each case, the value of PIXEL_SCALE is limited not to be below approximately 166 meters so that unfavorably illuminated images with high spatial resolutions not captured at the resolution of an HIE do not overwrite more favorably illuminated images. In version 0 HIEs, the photometric correction applied to MDIS WAC G filter and NAC images to create HIEs 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 HIEs, 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 version 0 HIE Hapke photometric correction 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 (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. 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. The parameter values applicable to LOIs are given in the table below. Photometric behavior of Mercury in NAC images is assumed to be equivalent to that in the WAC G filter. WAC filter, wavelength, w, b, c , theta G, 749 , 0.278080114, 0.227774899, 0.714203968, 17.76662946 In version 1 HIEs 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 whose 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 G, 749, 0.1111, 0.5628, 0.6424 Data : There is one data type associated with this volume, HIEs consisting of mosaicked, photometrically corrected WAC filter 7 (G filter) CDRs and NAC CDRs, appended with 5 backplanes describing the component CDRs and their photometric geometries as recorded in DDRs. Ancillary Data : There is one type of ancillary data provided with this dataset: 1. 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 HIEs, the IAU2000 reference system for cartographic coordinates and rotational elements was used for computing latitude and longitude coordinates of planets. However a Mercury radius of 2440.0 km is used. In version 1 HIEs, 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 HIE 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 HIEs, the image was taken as part of the monochrome basemap campaign. In version 1 HIEs, the image may be taken from a different campaign if the illumination geometry more closely approaches that desired for HIE data products. Version 0 HIEs are based on version 4 CDRs which correct a number of earlier calibration artifacts, and on version 0 DDRs. Version 1 HIEs are based on version 5 CDRs and version 1 DDRs. For version 1 HIEs, some component images may contain residuals from the following issues. (1) COMPRESSION ARTIFACTS. Images may be integer wavelet transform- compressed in the MP, typically at 3:1 for color data and 4:1 for monochrome data, but any value from 1 to 32 can be used. 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 8:1 for monochrome imaging and 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, and compression ratios were decreased to 4:1 or less for monochrome data and 3:1 or less for color data where possible. Lossless compression was used when downlink allowed. Images that are part of HIE products typically were compressed 4:1 or losslessly. (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. In the NAC, scattered light from out-of-field sources is an issue. The geometry contributing most of the scatter is 1-2 fields-of-view sunward of the NAC boresight. For a very large, evenly illuminated source that overfills the field-of-view by a factor of several, ray-trace studies supported by testing during Venus flyby 2 suggest that 2-7% of the radiance measured in the field-of-view will have come from out-of-field sources. The spatial pattern of the scatter is variable, due to diffuse reflections off the internal instrument housing. 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 longer than at shorter 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 reveal 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. These analyses suggest that scattered light is present in monochrome map products but in general is not an issue in morphologic interpretations. However caution is urged in using quantitative photometric analysis in high-contrast or shadowed terrain in these 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 acquired in the first year of operations was developed and utilized in version 4 WAC CDRs used to create version 0 HIEs. 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 HIEs 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 HIEs were constructed by uncontrolled mosaicking, projecting the image data onto a sphere. 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 contributed to mosaicking errors. In general these are expected to be under 1 km but locally might exceed 4 km. Version 1 HIEs were 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 HIEs 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 HIEs greatly reduced residuals between images acquired at different photometric geometries, implying reduced systematic errors. Limitations : None