If the general ground level of any area is 10% of the flying height, the principal points may be used as the centers of radial directions for small scale mapping even in tilted photograph up to A. 1° B. 2° C. 3° D. 4°

The correct answer is $\boxed{\text{A}}$.

The principal point is the intersection of the optical axis of the camera with the image plane. It is the center of the perspective projection of the object onto the image plane. The radial direction from the principal point to any point on the image plane is the line connecting the principal point to that point.

The tilt of a photograph is the angle between the optical axis of the camera and the vertical. The tilt of a photograph can be measured in degrees.

For small scale mapping, the principal points may be used as the centers of radial directions even in tilted photographs up to a tilt of 1°. This is because the error introduced by using the principal points as the centers of radial directions is small for small scale mapping.

For large scale mapping, the principal points should not be used as the centers of radial directions in tilted photographs. This is because the error introduced by using the principal points as the centers of radial directions is large for large scale mapping.

The following is a brief explanation of each option:

• Option A: 1°. This is the correct answer. The principal points may be used as the centers of radial directions even in tilted photographs up to a tilt of 1°.
• Option B: 2°. This is not the correct answer. The principal points should not be used as the centers of radial directions in tilted photographs with a tilt of 2° or more.
• Option C: 3°. This is not the correct answer. The principal points should not be used as the centers of radial directions in tilted photographs with a tilt of 3° or more.
• Option D: 4°. This is not the correct answer. The principal points should not be used as the centers of radial directions in tilted photographs with a tilt of 4° or more.