Texture Mapping Problem

I have created a little map renderer based on heightmaps. I''m now trying to implement textures. My current problem is, if my camera get far away from the ground (ie the quads on which the textures are mapped), the texture changes to another one. I must precise that the ground texture is actually part of a texture set, in only one image file. Coordinate sets are good. I thought this was a problem linked to mip mapping but I''ve still haven''t found any info on this problem... Maybe somebody got an idea? Thanks "He who fishes in other man''s well often catches crabs." Old Chinese Proverb