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 Reduced Data Records (BDRs). The LOI data set also consists of a global map of I/F measured by the NAC or WAC filter 7 (both centered near 750 nm), but it is complementary because of illumination at lower solar incidence angles that highlights albedo variations instead of topography. It is also complementary to the global 8-color map contained in the Multispectral Reduced Data Records (MDR) data set. The MDR data were acquired at nearly the same photometric geometry as this LOI data set, but the LOI data are at four times higher spatial resolution. Map tiles are named based on the quadrant of the Mercury chart they span: MDIS_ppp_rrrPPD_Hxxddv.IMG where: ppp : product type : LOI 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_LOI_256PPD_H03NE0.IMG For this image: Product type : LOI (LOI) Resolution : 256 pixels/degree (256PPD) Mercury chart : Shakespeare (H03) Quadrant : Northeast (NE) Version : 0 The LOI directory, present in the LOI archive volume, contains MDIS Map Projected Low Incidence Angle Basemap Reduced Data Records (LOIs). The LOIs are organized into subdirectories based on the Mercury Chart containing LOIs. 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 LOI: - 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 low-incidence angle basemap campaign; and - Contains 5 backplanes: (a) observation id, (b) MDR metric, modified for the difference in pixel scale, (c) solar incidence angle, (d) emission angle, and (e) phase angle. Versions : Version numbers of LOIs increment on reprocessing or addition of new data. Polar tiles are in polar stereographic projections, other tiles in equirectangular projection. Version 1 is released at PDS release 15 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 minimized incidence angle. It is controlled and projected onto a global digital elevation model. It uses a Kasseleinin-Shkuratov photometric model, whose parameters are the same for any given wavelength band across all MESSENGER end-of-mission map data products. I/F is taken from version 5 CDRs, and information for backplanes is taken from version 1 DDRs, both generated in end of mission deivery 15. Parameters : MDIS observing variable pertaining to the LOIs are as follows. Pixel Binning: Some LOI 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 LOI 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 LOI 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 LOIs 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 LOI 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 at as low an incidence angle as possible. 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 LOI 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 LOI, 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 LOIs 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 LOIs 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 MDR metric, modified for the difference in pixel scale 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. For version 1 LOIs, any image taken as part of the low incidence angle campaign is a candidate to include, as are images from any campaign with suitable illumination, 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 - In polar tiles (H01, H15) phase angle at the center of the images is < 95 degrees - In non-polar tiles images scale is > 100 m/pixel - In polar tiles (H01, H15) images scale is > 50 m/pixel A stacking order ('which image is on top') is shared by LOIs and early verions of 3-Color Map Projected Multispectral RDRs (MD3s) and 8-Color Map Projected Multispectral RDRs (MDRs). The objective is to have 'on top' images with high spatial resolution, low emission angle, and low solar incidence angle. Image sorting and stacking is as follows. An MDR metric represents both spatial resolution and image geometry is evaluated at the camera boresight; lowest values represent the 'best' image. The 'worst' complete, map- projected image with the highest value for the metric is laid into the LOI 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. At all latitudes and solar incidence angles, the MDR metric modified for LOIs 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 166 meters, so that unfavorably illuminated images with high spatial resolutions not captured at the resolution of an LOI do not overwrite more favorably illuminated images. The photometric correction applied to low-incidence images is 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 variables. The parameters for the 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, AN, mu, c_sub_l G, 749, 0.1111, 0.5628, 0.6424 Data : There is one data type associated with this volume, LOIs 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. The value for Mercury radius is 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 LOI 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) The image was taken as part of the low incidence angle basemap campaign during MESSENGER's extended mission, or as part of another campaign with photometric geometry closer to that desired. LOIs are based on version 4 CDRs which correct a number of earlier calibration artifacts. Some issues still remain. The OBSERBATION_ID for a part of a LOI is a pointer back to the WAC filter 7 or NAC image used for that part of the LOI. Those images may be subject to some or all of 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 LOI 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 5 CDRs an improved temperature correction was derived empirically, by fitting as a function of CCD temperature, the median values of images acquired throughout 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 thousands of photometrically corrected images of relatively bland field-filling images of Mercury. (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 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. 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. For version 5 WAC CDRs in delivery 15 at end of mission, an empirical 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 LOIs 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) MAP PROJECTION ERRORS. LOIs 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 are generally at 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]. Limitations : None