Third, review all the points and edges that are important to you on the object or in the scene and then look at all the photographs and determine how many times each point appears in a photograph. The photographs for each point should also have good angle separation.
Good example: You are modeling a van and all the points you want in your model appear in at least two photographs and several of them appear in three or four photographs.
Bad example: You are measuring five key points on a water stain but three of the points appear on only one photograph and the other two points appear on two photos but the angle between those photos is 5 degrees.
The one case where reference counts are not important is when a single photograph project is being done to extract orthophoto or Surface Drawings. In this case the Control Points or Constrained Lines and Edges appear on only one photograph.