[Mars elevation data: longtitude, lattitude, elevation.]

The query tool you mentioned is for PEDRs, I'd like to look at the globally gridded MEGDR products.  

When I use the ODE data product search (as suggested at the top of the PEDR Query Tool 2 page), I can bring up a file, for example MEGT90N180GB.IMG, however, the PDS product files and Derived files do not include a simple text table (lat, lon, elevation), it only includes more images and some shapefiles. 

What is the easiest way to convert the MEGDR products to a simple text table?

 

[Mars elevation data: longtitude, lattitude, elevation.]

The MEGDRs in the link you gave are in a .img format. What is the easiest way to convert these .img files into simple text files that have lat, lon, elevation in the columns?