Erf, possibly derived from the word "Earth", tends to deals with places and things within the terrain - specifically a given hex. Thus, Lookamancy allows you to peer into a given hex while Findamancy might indicate the hex a target is located in. Dirtamancy can physically manipulate a hex's terrain value, as Sizemore did in page 126 when he opened a tunnel under his adversaries. Shockmancy, on the same page, appears to damage all units in a given hex. And, of course, the grand Spell effect on page 135 allowed Sizemore to remake a large number of hexes all at once.