geomannie · Global Mapper User
Geologist, cyclist & geographer plus a lot more. Basically, if its part of the world, I'm interested in it :-).
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GM is a brilliant bit of cost-effective GIS software but as you are finding, does not have very sophisticated gridding tools or allow a lot of control on gridding parameters. For most grid volume applications it is fine but part of your problem is the very elongate geometry of the blast area. This means that having a 15ft difference between the shot hole area and the blast area makes a big (25%) difference.
If it were me, I would copy the outer points to just outside the balst area, use a smaller grid cell size and clip to the blast polygon. Its not elegant but would work. No grid derived from a series of points is a true representation of reallity. All you can do is to get to an accepatbel level of accuracy.