Only if you're traveling fast relative to walking speed. You can still generally navigate by direction in the area, and it looks like it covers such a large are

Only if you're traveling fast relative to walking speed. You can still generally navigate by direction in the area, and it looks like it covers such a large area that in EG a 5 mile by 5 mile area you could still reliably set your declination and navigate with minimal error. The field lines seem to change direction in an arc the size of South Africa, for example. But it just means that if you're westerly or easterly in South Africa you need to be very cautious to set the declination on your compass and not assume it varies little, like you can in the Pacific Northwest.