When you are taking the photographs you should measure the distance between two points that you are photographing. You will use this distance to add scale to your 3D model. Sometimes, if you are modeling an object to produce a rendering, you may not need an accurate scale for your model in which case you need not measure a scale distance.
To get a scale, choose two points that are close to the largest dimension of the object you are measuring. In addition choose points that will appear on many photographs. Measure, or estimate by some means as accurately as possible, the distance between them. Make sure that you will be able to identify and clearly mark these points on your photographs. If you are measuring a facade, the width of the facade at its base is often a good scale choice. Record this distance for use as your scale later. The points you pick for scale should also be easily marked. That is, they should not be rounded corners nor be fuzzy features. The absolute accuracy of the dimensioned drawings depends to a large extent on how well you measure and mark the scale.
The absolute accuracy of the final 3D model will depend on the accuracy and precision of the scale. Make sure that the two points being used to measure scale are:
• easily identifiable in many photographs,
• a large distance apart,
• sharply defined by high contrast straight edges or corners (or ideally on circular targeted points).
By taking the above precautions, your scale will be easy to measure and the scale points will be easy to mark with high precision. The result will be a model with accurate scale.
See Importing and Setting Up Coordinate Systems in the Help file.