[Therion] grid on elevation map

hello, I try to put a grid on my survey, I set the grids instructions in the thconfig file grid-size 50 50 50 m grid top it's work find for the map, but on the elevation map, Therion generate grid lines outside the survey. Explanation : my cave elavation go from level 0 to -208. I have the grid lines for level 0, -50, -100, -150, -200 and it's fine. But I have also the grid lines for level +50 and -250. So my PDF doc is very high. How can I suppress these 2 grid lines. On the map I do not have the problem, grid cross are "inside" the map rectangle, not outside. Thanks, Gilbert