Creating GPD Files  QUESTION: A number of tools at the National
Snow and Ice Data Center (NSIDC) are used to grid satellite data
into a particular map region (e.g., mapx or ms2gt). They all
seem to require input of a "GPD" file. I can see that this file
blends map projection information with information about the
grid I am trying to create, but it is not obvious to me where
the grid is located on the map. Or, even more important, how to
go about creating a GPD file for a study area of my own. Can you
help?  ANSWER: Typically, when you start a project using
satellite data, you start with a portion of the Earth you want
to study. It could be the Amazon basin or the Arctic ocean, or
even a local county. Wherever it is, the first order of business
is to find satellite data that passes over the region and covers
it in some way. Sometimes the data only partially covers the
area you are interested in. Often you wish to use data from two
or more different satellite sensors. (See the last portion of
this article for a