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
Example
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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More details can be found on the NASA sites, in particular, further
details on the Mars datasets is given below.
Quote
The MOLA IEGDR is in the form of an ASCII table with one row for each
latitudelongitude 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 offnadir 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 offnadir (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
crossovercorrected. 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 bodycentered, using the
centerofmass 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 righthanded
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 centerofmass 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) = (1f) * (1f) * 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.
