Photogrammetry 
Photogrammetry is a way of measuring objects by photography in order to establish their 3D dimensions. High accuracies are achieved by fitting the object with retroreflective targets that pinpoint the points of interest.
These targets have been especially designed for this measuring technique by Starmountain Survey & Consultancy and can be photographed from any direction. When all targets have been placed a series of photographs with a calibrated camera at at least 8 megapixels is taken.
 

 Dedicated photogrammetry software is used to process the photographs, resulting in a coordinate set of all measured points. This set can be rotated and shifted until the origin and axis meet the client's requirements.
On the object several distances are measured using a distomat or a high precision tape measure in order to provide a means of quality control as these distances can be compared with the results from the calculations (lines in adjacent picture). In addition to that the software also returns a set of quality indicators from the adjustment done. A high reliability is established by taking at least 5 pictures per point at different angles.


When due to the situation photogrammetry is not the suitable measuring method objects can be measured using tachymetry (see here for a comparison of both methods).
A standard deviation (1σ, 68%) of 1:10,000 or better for X,Y and 1:20,000 or better for Z coordinates is feasible (assuming that the Zaxis coincides with the vertical at the location of the object). For principal points (e.g. fixed points, antennae, transducers) this can improve up to respectively 1:20,000 and 1:40,000 or better.  

 This means that, with an object with a largest diagonal of 20 meters, the standard deviation of the principal points can be 20/20,000 = 0.001 meter or better in X, Y and 20/40,000 = 0.0005 meter (half a millimetre) or better in Z.
Apart from the higher accuracies photogrammetry has a few other advantages: What You See Is What You Get as all points are photographed. In addition to that the measuring time (read: downtime) is only half than when using tachymetry. Usually a complete vessel geometry is measured within 4 hours. 

Photogrammetry can also produce an orthophoto. With an orthophoto the effects of tilt and relief are removed from the photograph by a mathematical process called rectification. This is both possible for single images as for series of images.  

Above image is produced using 43 photo's and resembles a birdseye view of an archaeological site in The Netherlands. The area shown measures 10 x 50 meters. Orthophoto's can be produced at any resolution and to any reference surface (horizontal, vertical, sloped). The original of above image was produced at a resolution of 200 pixels per meter. 

 The archaeological finds can also be measured using photogrammetry. In this way one can measure fragile objects without touching them. The adjacent image shows a 3D wire frame model of a navigational instrument found on board of the wreck of the Kronan in Sweden. Based on this measurement Starmountain produced CAD drawings that were used to create a copy of the instrument. For more info on this project please visit www.DeHilster.info


A leaflet on photogrammetry is available for download here. Click here for a list of objects measured by Starmountain.


