rectified photography

Rectified Photography, as the simplest of photogrammetric methods, works with planar objects, like elevations of walls, floors or sections. Every feature, which is situated either in front or behind the plane, is reproduced with a perspective error, either too large or too small. This method was evaluated against traditional manual drawings during the recording process at several excavations and the advantages and limits were shown. Common GIS-packages, like open-source QGIS, include projective transformation, below the georeferencing facilities. Additionally, many commercial packages work within CAD-packages (PhoToPlan) or others including own CAD-capabilities (metigoMAP).

Application of rectified photography within archaeology can be diverse. It can be utilised for object interpretation (identification of object type, quality and quantity) and measurement (location, form and size). On my own excavations as well as at other projects, I used rectified photography to record major sections, elevations of walls, surfaces and some of the burials. With the later, photographs have to be taken strictly vertically, different parts of the body (esp. the skull) have to be recorded individually with its own set of targets. Nevertheless, still some distortions and displacements have to be taken into account. I was able to evaluate the advantages and the limits of this method on several excavations.


Setting up targets and Displacements

Setting up Targets

A minimum of four GCP (ground control points) per image is required for projective transformation. The use of more than four targets gives control and makes the results stronger and more secure, residuals can be determined. To avoid displacements, all these points have to be established on the same reference plane. This means P1 and P2 are not suitable as targets since they are located on a different plane.

If no TST is available on site, the use of the already established site grid is the appropriate way.

Points can be distributed as following:

  1. Four points as the corners of a rectangle (measure height and width of the rectangle, T1T2 and T1T5)
  2. 2 x 2 parallel lines are orthogonal to each other - T5T4T3 // T8T9, T5T6 // T4T9; measure the distance between the line pairs
  3. Trapez, measure the sides of the trapez and and its diagonals; E.g. sides: T7T9, T9T3, T3T5, T5T7: diagonals T7T3, T5T9

It is advised to sketch the layout of target points. Beside the sketch, an overview photograph of the complete area helps to reconstruct the direction the images have been taken in the office.

The accuracy of the product depends further on the quality of the lens, bended lines as a result of radial distortion of cheap optics will be the consequence. For those cameras, a determination of these parameters are advised and should be applied prior to the rectification process.


Rectified Photography - Workflow


Rectified photography can be well fitted in the standard workflow on site with few additional steps. The diagram shows all the steps involved in the pipeline. The ratio of work involved is 1:4, for one hour in the field (including setting up the targets, surveying, image acquisition and mapping of context edges) four hours in the office have to be spend. This includes the preprocessing and naming of the original image, the rectification and finally the mapping of edges and major inclusions for a cross-check on site.

After the area has been cleaned meticulously and standard photography has been taken, the next important step within the workflow is the setting up of ground control points (GCP).

The images are rectified by projective transformation and mosaicked to a photoplan in the site office. This is the base for the mapping of findings in the field and subsequent digitising in CAD and upload to GIS.

At Tell Fekheriye, a prioritisation for the mapping of findings was introduced. The individual context elements were drawn in the following order:

It has proved to be of special importance, that the rectification was carried out straight on site, as this enabled a discussion of the interpretation between trench and planning supervisor and allowed to correct some misinterpretations.

During rescue excavations, fast solutions of recording are often needed, which allow a more detailed study later during post excavation analysis. Being aware the limits of rectified photography, this method can be a powerful tool to improve and accelerate the site records. It requires a minimum of training and is easily learned by non-specialist, but still quite often its restriction to a plane is ignored and the method is used in an inappropriate way. This methodology is not suitable for some kinds of complex stratigraphic records, other photogrammetric, such as Multi Image Analysis and especially Stereo Pair Image Analysis have to be used.