Data source: the archiving data is aerial photos covering mainland China during the early period.

Property: side-looking aerial photos taken from the air scout at high altitude For the detecting purpose, the flight plans will minimize the amount of photos to cover the maximum by increasing the flight altitude with oblique photos taken from both right and left sides. In order to obtain extensive information covering the surface, the period of time between two snapping photos have to prolong. The difficulty of the follow-up processing, however, has increased.

For the survey and cartography purpose, there are as below:

• Oblique-photography has geometric distortion so it is not suitable for survey and cartography.

• For the consideration of covering extensive area, it is hard to mosaic these images, since they have no end-lap and side-lap. On the other hand, they can not be measured under stereo-scope and extract 3 dimensional information like DTM data

• Considerable quantities of control points are required to register. For the region belongs to the enemy, however, the control points are hard to obtain. In addition, the old aerial photos and changing environment increase the difficulty of gathering the control points.

The problems above are the follow-up issues that we are going to face with., The relative geometric position is not the major factor for measuring purpose, so the flight state can be accordance with the continuous snapping numbers and the positions of the central lines between left and right photos to map these images to the surfaces . The followings are the examples: