To use a photographic based measurement tool such as PhotoModeler one needs to be able to photograph what one wants to measure. This statement seems obvious but if we look more closely at it we can see some limitations.
First, every point you wish to measure and model should appear in at least two and preferably three or more photographs. Secondly, most of the points that appear in one photograph should also appear in the adjacent photograph (that is, the photographs need to overlap quite a bit). Thirdly, you have to positively identify the precise position of each point as it is imaged in each photograph.