MESSENGER MDIS Map-projected Low-incidence Angle Basemap (MAPLOI)

MAPLOI – Map-projected Low-incidence Angle Basemap

Instrument: MESSENGER Mercury Dual Imaging System

PDS Data Set ID: MESS-H-MDIS-5-RDR-LOI-V1.0DOI: 10.17189/1520239

For more information about MDIS MAPLOI products, see the CDR/RDR Product SIS.

The Mercury Dual Imaging System (MDIS) Map-projected Low-incidence Angle Basemap (MAPLOI) data products comprise a global map of I/F measured by the Narrow-angle camera (NAC) or Wide-angle camera (WAC) filter 7 (both centered near 750 nm) during the MESSENGER Extended Mission. The images are illuminated at low solar incidence angles to accentuate albedo and albedo variations instead of topography. They have been photometrically normalized to a solar incidence angle, i=30°, emission angle, e=0°, and phase angle, g=30°, at a spatial sampling of 256 pixels per degree.

The MAPLOI dataset is a companion to the Map-projected Basemap Reduced Data Record (RDRBDR) dataset, Map-projected High-incidence East-illumination Basemap (MAPHIE) dataset, and the Map-projected High-incidence West-illumination Basemap (MAPHIW) dataset, all of which are also composed of WAC filter 7 and NAC images, except acquired at higher solar incidence angles centered at 68 to 78 degrees to highlight topography.

The MAPLOI dataset is also complementary to the global 8-color map contained in the Map-projected Multispectral Reduced Data Records (RDRMDR) dataset. The RDRMDR data were acquired at nearly the same photometric geometry as this MAPLOI dataset, but the MAPLOI data are at four times higher spatial resolution.

The map is divided into 54 “tiles”, each representing the NW, NE, SW, or SE quadrant of one of the 13 non-polar or one of the 2 polar quadrangles or “Mercury charts” already defined by the U.S. Geological Survey (USGS), shown in the table below. Each tile also contains 5 backplanes: observation ID; BDR metric, a metric used to determine the stacking order of component images, modified for the higher incidence angle centered near 78 degrees; solar incidence angle; emission angle; and phase angle.

Mercury Charts, as named at the end of mission delivery:


Filename Identifier

Latitude (degrees)

Longitude (degrees east)

H-1 Borealis


65 to 90

0 to 360

H-2 Victoria


22.5 to 65

270 to 360

H-3 Shakespeare


22.5 to 65

180 to 270

H-4 Raditladi


22.5 to 65

90 to 180

H-5 Hokusai


22.5 to 65

0 to 90

H-6 Kuiper


-22.5 to 22.5

288 to 360

H-7 Beethoven


-22.5 to 22.5

216 to 288

H-8 Tolstoj


-22.5 to 22.5

144 to 216

H-9 Eminsecu


-22.5 to 22.5

72 to 144

H-10 Derain


-22.5 to 22.5

0 to 72

H-11 Discovery


-65 to -22.5

270 to 360

H-12 Michelangelo


-65 to -22.5

180 to 270

H-13 Neruda


-65 to -22.5

90 to 180

H-14 Debussey


-65 to -22.5

0 to 90

H-15 Bach


-90 to -65

0 to 360

MDIS observing variables pertaining to the MAPHIWs are the following:

  • Pixel Binning: Some images are unbinned and 1024 x 1024 pixels. Some images are 2x2 pixel binned in the focal plane hardware (also known as “on-chip” binning), resulting in 512 x 512 images. Some images are further compressed 2x2 using the spacecraft Main Processor (MP), yielding 256 x 256 images. The WAC was used to acquire basemap 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 MP binning was used.

  • 12-8 bit compression: Images are read off the detector in 12-bit format. 12-bit images may be converted to 8-bit images using one of 8 lookup tables (LUTs). All images collected as part of the global basemap have been converted to 8 bits.

  • FAST/DPCM compression: All images are compressed losslessly using FAST/DPCM compression as they are read out of the Data Processing Unit (DPU), to conserve recorder space. Once the data are written to the recorder, they may have been 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 8:1 for monochrome data. The wavelet compression ratio for MAPLOI basemap imaging is 4:1 or less. 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 MAPLOIs 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 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 MAPLOI 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 nm 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.

MDIS MAPLOI products have the following file names:

MDIS_LOI_256PPD_Hxxddv.IMG (with detached PDS label)


MDIS = instrument = Mercury Dual Imaging System

LOI = product type = Low-illumination Angle Basemap

256PPD = resolution in pixels/degree

Hxx = Mercury chart designation

dd = quadrant within Mercury chart (NW, NE, SW, or SE), or a polar chart (NP, SP)

v = version number

In ODE, MAPLOI products have the following product IDs: