I'd go with the 31.871 value over a general value of 30.
Also, to be on the safe side, it would be best to check the linear unit information for the image to confirm that it is meters.
In ArcMap, Right-click on the image in the Table of Contents, Properties, Source Tab, scroll down to the Spatial Reference subheading, then look at "Linear Unit" and see what the units are.
Chris Donohue, GISP