tact with the object being measured. (3) A high level of accuracy may be achieved. (4) The technique is in many cases quicker than conventional methods, particularly when complex or intricate ornamentation needs ro be recorded. (5) The survey may be presented either in the form of line drawings or data for matted for use in one of the many cad systems available. (6) The photography enables the user ro extract as much or as little information as is required and further allows various scales and levels of information ro be provided from the same phorography. There are many instances when it is cheaper tO use a conventional hand survey as an alternative ro phorogrammetry; however, the benefit of phorography for record purposes cannot be emphasised roo strongly. There are many examples when phorography of a building, produced prior to damage, has been subsequently analysed photogrammetrically and drawings produced ro aid in the resroration process. For any potential user or specifier of a a phorogrammetric survey, the following questions should be asked: Docs the contraaor have the necessaryexpertise to undertake the project-· satisfactorily? Not all photogrammetric companies are capable of carrying out architectural photogrammetry despite the fact that they may have the necessary equipment for the task. What accuracyis required? Accuracies achievable in certain instances are sub-millimetre, but in architectural applications this is not usually necessary. The greater the accuracy, the longer the time required (generally) and the higher the cost. Many clients are very unsure of accuracyand tend to over specify mainly "ro be on the safe side". As a general rule, the accuracy achievable on a 1: 50 survey is ± 10 mm. Are the photographs to be used immediately? If not, they should be stored safely so that they do not deteriorate and careful records kept to assist future users who may not have been involved in their taking. Can the data be used in my CAD system? It is necessary to supply the contracror with full details of the formatting requirement for the proposed CAD system to ensure that the data are prepared correctly. PHOTARC SURVEYS What scale of drawing is required? Complex ornamentation or detail can only be shown adequately at a scale of 1:50 or larger, so much consideration should be given ro this point. Will I need any areas of detail enlarged? Although the phorogrammetric process permits great variance in the enlargement from phoro to drawing scale, there is a limit to this magnification, and careful thought should therefore be given to this matter at the outset of the project. The reader will hopefully now have a basicunderstanding of the many advantages of using photogrammetry and indeed have a guideline for the specification he will need to write to commission a photogrammetric survey. It is essential for any potential user of the technique to determine precisely what is required from the survey and specify accordingly to ensure a satisfactory conclusion to the project for both client and contractor. R M Stanbridge is managing director of Phocarc Surveys Ltd. The company, which was formed in 1974 as a specialist phocogrammetric survey organisation, has undertaken more than 200 architectural photo· grammetric projects. PBOTOGRAMMETRY: - The measuring system for all conservationists Using the latest technology, including the Intergraph IMA photogrammetric workstation, we have the experience, personnel and necessary equipment to undertake any of the following; ARCHITECTURALSERVICES Elevation SuNeys Profiles & Sections Floor Plans Site SuNeys Digitising of existing drawings Rectified Photography Computer Modelling GENERALSERVICES Aerial SuNeys Ground SuNeys Digital Mapping Engineering SuNeys Forensic Investigation • SuNey Consultancy Helicopter SuNeys PHOTARC SURVEYS LTD, Tel: 0937 64936 Compass House, 22 North Street, Wetherby, West Yorkshire, LS22 4NN Fax: 0937 580136 :::ONTEXT 27 25
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