Photogrammetric Applications for Cultural Heritage Guidance for Good Practice Summary This guidance covers the practical application of photogrammetry in recording cultural heritage, with particular reference to structure from motion (SfM) techniques. Our audience for this document includes survey contractors, archaeological contractors, voluntary organisations and specialists. Photogrammetric image acquisition and processing, until recently requiring a considerable investment in hardware and software, are now possible at a fraction of their former cost. This has led to a huge increase in the use of photogrammetry in cultural heritage recording. The skills required to apply the techniques successfully and accurately are discussed, and background information on how various parts of the process work is provided so that better results can be achieved through better understanding. Photogrammetry is characterised by its versatility, and is applicable over a wide range of scales, from landscapes to small objects. The particular requirements needed at these different scales are outlined, and both imaging techniques and useful ancillary equipment are described. The different types of outputs are discussed, including their suitability for further interrogation using a range of established analytical techniques and the presentation options available. A range of case studies illustrates the application of photogrammetry across a variety of projects that broadly reflect the areas discussed in the text. This document is one of a number of Historic England technical advice documents on how to survey historic places. This document has been prepared by Jon Bedford. This edition published by Historic England October 2017. All images © Historic England unless otherwise stated. Please refer to this document as: Historic England 2017 Photogrammetric Applications for Cultural Heritage. Guidance for Good Practice. Swindon. Historic England. HistoricEngland.org.uk/advice/technical-advice/recording-heritage/ Front cover 13th century roof boss depicting Samson wrestling a lion, Hailes Abbey, Gloucestershire. Contents Introduction .........................................1 3.4 Terrestrial photogrammetry for smaller objects .....................................73 Core skills .................................................................1 3.5 Textured 3D models ...................................77 Appropriate use of different 3.6 3D printing ..................................................78 survey techniques ....................................................2 3.7 Community involvement ...........................79 3.8 Metadata and archive ................................79 1 Basic Principles ...........................4 4 Case Studies ..............................80 1.1 Basic procedure ...........................................4 1.2 The chief ray and the principles Case Study 1: Using archive aerial imagery ........80 of intersection ..............................................6 Case Study 2: SUA imagery on single sites..........83 1.3 Collinearity ...................................................7 Case Study 3: SUA landscape survey. ..................87 1.4 Interior and exterior orientation .................7 Case Study 4: Combining SUA and 1.5 The SfM–MVS photogrammetric process ...8 terrestrial data .......................................................90 1.6 Measurement from single, paired Case Study 5: Terrestrial imaging on or multiple images .......................................9 archaeological sites ...............................................95 1.7 Sources of error..........................................13 Case Study 6: Underwater 1.8 General workflow .......................................14 photogrammetric survey .....................................102 Case Study 7: Modelling small objects ..............107 2 General Considerations .............19 5 References ...............................110 2.1 Capturing the initial data ..........................19 2.2 Image arrangement ...................................34 Software ...............................................................113 2.3 Control ........................................................42 Other web resources ............................................114 2.4 Historical imagery ......................................50 6 Glossary ..................................115 3 Scales and Applications .............53 3.1 Aerial photogrammetry for archaeology, Acknowledgements ..........................122 landscapes and buildings .........................53 3.2 Terrestrial photogrammetry for buildings Contact Historic England ....................................123 and structures ............................................66 3.3 Terrestrial photogrammetry for excavations ...........................................73 Introduction This guidance covers the application of photogrammetry in cultural heritage with particular reference to structure from motion (SfM) techniques. The aim is to raise awareness of the issues that are commonly encountered, rather than to deal with all aspects of photogrammetric processing in depth. There are many potential uses for this technique across a wide variety of scales, ranging from landscapes (for example 10km2 or more) to small objects (for example 10mm3). The different outputs are suitable for a range of established analytical techniques and offer many presentation options. Photogrammetric image acquisition and Photogrammetry processing, until recently requiring a considerable You need an understanding of the image investment in hardware, software, expertise arrangements needed to achieve the best and time, is now possible at a fraction of its coverage of the target and ensure the highest former cost. The advent of more affordable accuracy of the models produced. You will need photogrammetric software over the last few years, some knowledge of the camera distortions that in particular the use of SfM techniques, has seen can affect the quality of the results. an explosion in its use in archaeology. This has been helped by the development of relatively Survey cheap digital cameras capable of capturing You need an understanding of the appropriate high-quality imagery, and by advances in the level of detail, scale, orientation and control processing capability of personal computers. for different projects. You need to know when Additionally, the development of small unmanned it is appropriate to use photogrammetry and aircraft (SUA), which can be used to capture low- when another surveying method would be more level aerial imagery, has contributed by providing suitable to achieve the desired product. an easy method for recording individual sites. You need to be able to organise the stages of a project in an efficient way. You also need Core skills an understanding of the importance of metric accuracy, and of the methods available to In order to use photogrammetric techniques quantify and improve it. successfully in archaeology, a number of core skills are required. Software You not only need to know about Photography photogrammetric software packages, but also You need an understanding of how to use cameras those required for image processing and later and related equipment to achieve the best digitisation and analysis, such as computer-aided possible images in different circumstances. design (CAD) software and geographic information systems (GIS). < < Contents Archaeology/architecture (TSTs) and hand-drawn survey, are usually You need to be able to interpret and use the data characterised by the selection of data measured generated to answer the questions being asked. at the point of capture (Figure 1). Data presentation One survey technique on its own rarely provides You need to be able to present the data in a clear, the perfect solution for most recording and unambiguous and aesthetically pleasing format, analysis projects, and often requires augmenting using established conventions. with data derived from different sources or methods. Your focus should be on the suitability These skills can be shared across a team, but it is of the products for the task at hand and the important that all are present. required deliverables rather than the method used to derive them. Similarly, although the accuracy of photogrammetric reconstruction is affected by Appropriate use of different a number of factors, it is wise to remain focused survey techniques on the desired performance and use of the product rather than obtaining the highest possible Broadly speaking, survey techniques can be accuracy and density of reconstruction under all divided into mass capture and selective methods. circumstances. Photogrammetrically processed Mass capture methods, such as three-dimensional imagery can produce resolutions far in excess (3D) laser scanning and photogrammetry, of the required result, which often leads to time are characterised by large amounts of being spent on processing unnecessary data, undifferentiated data at the point of capture the handling of very large files and subsequently and selection from that data off-site. Selective the need to decimate the derived data in order methods, such as global navigation satellite to achieve a usable and suitable product. For systems (GNSS), use of total station theodolites example, specifying a 10mm ground sample Figure 1 Hillshaded DEM derived from mass capture techniques (left); highly selective interpretative linework derived by GNSS survey of the same area (right). < < Contents 2 distance