Geoid Map Products
==================

This directory contains IMG, JP2 and RAW directories containing maps of 
lunar geoid heights.  There are three geoid maps in each directory, for 
which the input gravity fields are GLGM-3, produced at NASA GSFC 
[MAZARICOETAL2010], SGM100i, produced at JAXA/NAOJ [GOOSSENSETAL2011], and
LP150Q, produced at NASA JPL [KONOPLIVETAL2001].  The file names for
these maps contain the strings, correspondingly, of GLGM-3, SGM100I, and 
LP150Q.

The provided data product is a map of the lunar geoid heights. The geoid is 
defined as the equipotential surface whose mean equatorial radius is 1736.
4km. The geoid heights are calculated similarly to those embedded in the 
LOLA RDR products (see LOLARDR.FMT). The map resolution is 4 pix/deg by 4 
pix/deg.  The gravity field model is truncated at degree 60.

The IMG maps are in the form of a binary table with one row for each
0.25 degrees of latitude, pixel registered. Because the geoid height
was calculated from the gravity field truncated at degree 60, this
is sufficient to capture the long-wavelength features of the geoid
height changes. Map values are relative to a radius of 1737.4 km.
The geoid height map can be used to derive topography from the 
other LOLA elevation (or planetary radius) maps. This is done by 
subtracting the geoid height from the LOLA elevation. The use of 
topography instead of elevation can be important, e.g. for studies
which need to determine downslope direction of flow.

The geoid maps are only made available at this coarse resolution because
of the absence of information at shorter wavelengths. Interested users
are encouraged to resample the data to fit the various LOLA map
products.

The  different input gravity models are useful to give the users a sense
of the geoid height uncertainties (especially over the farside).

Other files are available, created using other gravity fields, in
order to give the users a sense of the geoid height uncertainties
(especially over the farside) by comparing models.  Each sample within 
a map represents the geoid height relative to a reference radius (OFFSET) 
and is generated from the input gravity field model (truncated at degree 60).

As with all the products in the EXTRAS directory, these files are made 
available, but are unsupported by the LOLA Science Team.