Façade mapping of the Ljubljana Central Market building
Architects often deal 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 characteristics, and based on its physical condition, and the time and accuracy needed for the project, architects decide which documentation approach or tool is the most appropriate.
Located in the city center of Ljubljana, the capital of Slovenia, is the Ljubljana Central Market. It was designed by the famous architect Jože Plečnik, and built in 1942. The design of the complex reflects Renaissance influences and 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, etc.). The result of the facade mapping would be a point cloud, on the basis of which BIM modeling is then performed.
2 Possible Approaches
The Architects had 2 possible ways to approach this project most efficiently:
TLS Terrestrial Laser Scan – it gives great results, but lasers cost more than 60k EUR. Specific software for processing has to be used which comes with extra cost
Photogrammetry – a great cost-efficient option. Perfect for stone 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 for the facade mapping and 3Dsurvey photogrammetry software. Total investment was 500 EUR for the camera and 3000 EUR for the software.
Advantages of using photogrammetry for 3D documentation and facade mapping
- 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 side in a time effective manner. The latest version of 3Dsurvey allowed them to calculate the Custom plane orthophoto, resulting in the Facade Orthophoto.
With 3D documentation and effective facade mapping, the building can be archived and used for all future reconstructions.
The architects were able to precisely measure the dimensions of the building and make a good estimation on required material to create an offer for the client.
How to best capture all the necessary details using a handheld digital 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 of approaching the object (not capturing photos from the same distance), be careful that the change of the scale between the images does not differ too much! Try to photograph continuously while moving. Example: If you took the first picture from 20m and then wanted to get closer to a certain detail at 1m (in order to have detail captured with 1mm accuracy and high detail), you should not go from 20m to 1m, but rather start at 20m, approach and shoot at 12m, then at 8m, 4m, 2m, and finally 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
The client received a printed version of the offer with a Custom Plane Orthophoto, and a digital version of the project. This is easily checked with 3Dsurvey Viewer – A simple desktop application for viewing and sharing purposes available on 3dsurvey website.
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