Techniques > Surveying > Survey Methods > Photogrammetry
Techniques: Photogrammetry
Version date: 16 October 2016
Complex three-dimensional structures can be difficult to record accurately underwater but one method that has been used successfully is close-range photogrammetry.
This technique uses a computer program to process photographs of the structure and turns them in to a series of accurately positioned points. Images are taken of the structure from different places around it and at different elevations. The images are then put into the computer program and points on the structure are identified on each image, points that are the same are then identified on each image and some idea of scale is added. Using an adjustment technique similar to that used to adjust tape measurements the positions of the points are calculated in three dimensions.
The technique works on positioning points in space so is not directly applicable for positioning the often rounded and eroded shape of wreck structure. Any edges, joints and corners can be more clearly defined using white map pins or white BluTak.
This technique is effective for small areas and also requires good position control for larger areas but is rather complicated as it requires an underwater camera and a computer. The advantage of this method is that it can be used to record very complex detail very accurately., however the technique is not easy to get right and processing a large number of images can take a long time.
Suitable computer programs include Eos System's PhotoModeler, PM3 from AS Built Data Inc and ShapeCapture from ShapeQuest Inc.