True Dhar and necromancy are both forms of wind magic. Unusual or unnatural forms perhaps, but still wind magic.
Geomancy just seems to be magic that involves geography or architecture. Bel-Korhadris is described as a geomancer when Teclis thinks about the defenses of the White Tower in Sword of Caledor. The Geomantic Web is possibly different from Wind Magic, but might not be. It's never really described in huge detail AFAIK.
True Dhar usually involves crushing the winds into one screaming unnatural whole, there are no recognizable winds in it. As proof of this you can use it with warpstone that has never been Winds. Necromancy involves using the wind of death to manipulate said unnatural and non-wind magic.