// step 3: get start/stop heights
    float h1 = zs[MCOORD(p1->x,p1->y)];
    float h2 = zs[MCOORD(p2->x,p2->y)];

    // step 4: set new heights of affected tiles
    float rise = h1 - h2;
    float dx = rise / (p1.x - p2.x);
    float dy = rise / (p1.y - p2.y);