EGDR
MOLA Experiment Gridded Data Record

This is the format used for, among other things, the topographic grid data for the mare terrain databases. The files are plain ASCII with 6 columns as follows.

• Longitude
  
• Latitude
  
• Average observed radius
  
• Areoid radius
  
• Median observed topography
  
• Number of observations

     0.5    89.5  3376055.43  3378187.24  -2149.39    24
     1.5    89.5  3376056.95  3378187.27  -2153.40    22
     2.5    89.5  3376066.28  3378187.30  -2148.07    17
     3.5    89.5  3376010.75  3378187.33  -2216.51    11
     4.5    89.5  3376071.33  3378187.35  -2137.71     8
     5.5    89.5  3376128.38  3378187.38  -2070.54     9
     6.5    89.5  3376168.03  3378187.41  -2033.42     7
     7.5    89.5  3376170.55  3378187.43  -2023.11     6
     8.5    89.5  3376181.59  3378187.46  -2009.45     6
     9.5    89.5  3376189.74  3378187.48  -2002.86     7
    10.5    89.5  3376192.72  3378187.51  -1999.38     7
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The MOLA IEGDR is in the form of an ASCII table with one row for each latitude-longitude bin, from 90 to -90 degrees latitude and from 0 to 360 degrees longitude. The binned data include all MOLA nadir observations from the Orbit Insertion phase, plus Mapping Phase nadir observations, plus off-nadir observations of the north pole above 86 degrees latitude acquired during spring 1998 and of both poles taken in July 1999. Orbits 355 and 358 of the Orbit Insertion Phase and orbits 10709 through 10716, inclusive, of the Mapping Phase are excluded because solutions for these orbits are deemed to be poor. (Note: subtract 10000 from MOLA mapping phase orbit number to determine the equivalent MGS Project orbit number.) Also excluded are shots more than 1 degree off-nadir (except as noted above), channel 4 returns, and any returns not classified as ground returns, e.g. clouds or noise, according to the SHOT_CLASSIFICATION_CODE. Most observations have been crossover-corrected. The polar observations have not been fully corrected and may be revised somewhat, as the pointing of the instrument is not known as accurately as the range measurement.

The IEGDR may be released at various grid resolutions. Examples are 1 degree latitude by 1 degree longitude bins, 0.5 by 0.5 degree bins, and 0.25 by 0.25 degree bins. The IEGDR file name is in the form IEGnnn_v.TAB, where nnn represents the bin size and v the version. For example, the first release of the 0.5 by 0.5 degree IEGDR is named IEG050_A.TAB.

• The areocentric coordinate system [DAVIESETAL1994B], more generally described as planetocentric, is body-centered, using the center-of-mass as the origin. Areocentric latitude is defined by the angle between the equatorial plane and a vector extending from the origin of the coordinate system to the relevant point on the surface. Latitude is measured from -90 degrees at the south pole to +90 degrees at the north pole. Longitude extends from 0 to 360 degrees, with values increasing eastward (i.e., it is a right-handed coordinate system) from the prime meridian [DAVIESETAL1994B]. This coordinate system is preferred for use in geophysical studies in which, for example, estimates of elevation or gravitational potential are generated mathematically.

• The areographic system (more generally, the planetographic system) uses the same center-of-mass origin and coordinate axes as the areocentric coordinate system. Areographic latitudes are defined by a vector normal to a reference ellipsoid surface. Longitudes are measured from the prime meridian and increase toward the west since Mars is a prograde rotator [DAVIESETAL1994B]. This system is standard for cartography of Mars and most existing maps portray locations of surface features in areographic coordinates. For MGS, the following data have been adopted as standard for defining the reference spheroid for computing the areographic latitudes [DAVIESETAL1994B]:

  Equatorial radius = 3397 km
  Polar radius = 3375 km
  Flattening = 0.0064763
  

  Note that the flattening is computed as one minus the ratio of the polar radius to the equatorial radius. The relationship between areographic and areocentric latitudes is approximated as:

  tan(lc) = (1-f) * (1-f) * tan(lg)

  where: f = flattening
  lg = areographic latitude
  lc = areocentric latitude
  

  While the official MGS Project coordinate system is the IAU 1994 convention specified in DAVIESETAL1994B, the MOLA data are located in the IAU 1991 system, which differs only in the prime meridian W0 at J2000. To convert MOLA east longitude from IAU 1991 to IAU 1994, one must subtract 0.033 degrees; i.e., areocentric_longitude_East_94 = areocentric_longitude_East_91 - 0.033.