Building facade mapping
Architects are often dealing with heritage conservation projects. The fundamental need of any conservation project is understanding the object and gathering data about its physical condition prior to any action and intervention that might change the object.
Each object has its own unique characteristic and based on its physical condition, time and needed accuracy for the project, architects decide which documentation approach or tool is the most appropriate.
In the city center of Ljubljana, capital city of Slovenia, there is the Ljubljana Central open market, designed by the famous architect Jože Plečnik, built in 1942. The design of the complex reflects Renaissance influences and it is a cultural monument of national importance. For the purpose of renovation, the Architectural office Kramar had to capture all the elements of the building (walls, columns, retaining walls, wreath, windows, doors, roof, openings, …). The result of the capture is a point cloud on the basis of which BIM modeling is then performed.
Architects had 2 ways on how to approach this project most efficiently:
TLS Terrestrial Laser Scan – it gives great results, but lasers cost more than 60k EUR, plus specific software for processing has to be used which comes with extra cost
Photogrammetry – a great cost-efficient option, perfect for rocky buildings, because of the incredible texture. For this kind of project only a hand camera is needed.
Their decision was to use a hand camera and 3Dsurvey. Total investment was 500 EUR for the camera and 3000 EUR for the software.
Advantages in using photogrammetry for 3D documentation:
- Large amount of data
- High accuracy depending on the needs of the project
- Providing 3D models from small objects to large complex objects
- Providing metric and vector data of the texture of the object due to its image-based nature, which increases the understanding of the user.
- Geographical data, which can be indirectly extracted from the images at any time and according to the needs of project
- Low-cost and portable equipment
Using an ordinary digital camera, 320 photos were captured and used in 3Dsurvey for the calculations of textured 3D model. Once the textured 3D model was created, the architect was able to calculate the orthophoto from any arbitrary side in a very fast time. The latest version of 3Dsurvey allowed them to also calculate the “Custom plane” orthophoto shot from the side, the Facade Orthophoto.
With this 3D documentation the building is archived and can be used for all reconstructions in the future.
With this solution the architects were able to precisely measure the dimensions of the building and were able to make a good estimation on needed material to create an offer for the client.
How to best capture all the necessary details using hand camera:
- Images must be taken from different points of view and perspectives
- Images must be taken with a minimum horizontal and vertical overlap of 70% – this means that every detail is in at least 3 pictures (in practice it is on 10 images)
- Best results come from pictures taken perpendicular to the facade from two height levels (for example first images from eye level, then more pictures from 2,5 m, for which we can use a stick)
- In case we approach the object, not to capture photos from the same distance, we must be careful that the change of the scale between the images does not change too much! We have to photograph continuously. Example: If we made the first picture from 20m and then we would like to get closer to a certain detail at 1m (in order to have detail captured with 1mm accuracy and high detail) we should not go from 20m to 1m but 20m, approach 12m, get closer to 8, 4m, 2m, 1m.
Project tools and specs:
Equipment: Standard digital camera Olympus EP2 (12Mpix camera)
Number of GCPS: 12 (local coordinate system; GCP measured with the total station).
Area of interest: 200m x 20m
Time on the field: 4 hours
Location: Ljubljana Center
Date: August 2020
Client received printed version of the offer with Side Orthophoto and a digital version of the project, which is easily checked with 3Dsurvey Viewer (simple desktop application for viewing and sharing purposes available on 3dsurvey website).