Terrestrial Laser Scanning in the Preservation of Monuments and Historic Buildings

The use of Terrestrial laser scanning in the several phases of restoration of the St. Leonhard’s Church in Frankfurt/Main, Germany.

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History and Goal
The St. Leonhards Church was built in 1219 as a late roman basilica and later renovated in neo-Gothic style. In the church’s integrated Saint Salvator’s chapel, built between 1500 and 1515, there is a suspended stone ribbed vault which can rarely be seen today due to the difficult construction involved. It is Saint Salvator’s chapel that lends this church its unique charm.

Miraculously, Saint Leonard’s Church was only one of nine Dotationskirchen in Frankfurt that remained undestroyed during World War II.

The City of Frankfurt authorities decided to return the interior of the church to the old gothic style. In addition, the floor was to be lowered because the interior had been raised higher and higher over the centuries to protect against the flooding from the adjacent Main River. The restoration of the church started in 2011.

The restoration of the interior primarily means restoration to the early Gothic building style, modernizing the heating ducts and shifting the floor to the level that it was nearly years ago. This required completely recording the actual condition of the interior of the church, all of the side rooms, the arches, the two apses, and the towers for not just the design phase, but also for requesting bids, invoicing and for purposes of documentation.

Floor plans and views, namely in the form of measurement plans with an exact representation of the Gothic building style complete with its frescoes and details were required as measurement results. The required level of accuracy was 1 centimeter.

Steuernagel Ingenieure GmbH suggested this be done by laser scanning. Due to the complexity of the building structure, the Building Inspection Office of the city of Frankfurt accepted our recommendation for using 3D laser scanning and partially vectorising as the only cost-effective choice.

Another goal of the project was to document the different construction phases during the current excavation. Therefore each excavation phase was scanned in its entirety over a total of 15 days–to a depth of nearly 3.5 meters.

Mesasuring Concept
In order to successfully implement this project we adopted a standard workflow. We used the ScanStation C10 (and the HDS 6100 laser scanner) by Leica which can guarantee the required accuracy level of +/- 1 centimeter. We next created a high precision primary network, which has an internal accuracy of +/- 1 millimeter.

With that approach established we attached targets to the walls and columns and measured them using a tachymeter to get geographic coordinates. These targets were used as "pass (control) points" for the transformation of the point clouds. We were very careful in selecting the reference points for the laser scans in order to minimize shadowing during the scanning process.

At the same time, we took 9 pictures of reference point using a high-resolution digital camera. These photos were used to create the TruView file. Afterwards, the pass points were used to merge the individual scans into one georeferenced point cloud and to save them into an integrated database. We used the Leica Cyclone software for this purpose.

We "dyed" the pointcloud for a dimensionally-accurate, three-dimensional, photo-realistic color image of the entire interior.

The requirement was to present the main elements such as columns, windows and stairways with all the details. Similarly, all the joints had to be taken into account. For the window in the middle we subsequently added another extra point cloud of the shading to minimize the organ. For each view additional levels of representation were generated.

The measurement started in August 2009. The laser scanning was divided into 4 measurement phases.
The first measurement phase took place in 2009 and covered the church interior, the aisles, and the choir area including the altar.
The second measurement phase was commissioned later (roughly 2010) and consisted of the apses and the steeples and side rooms (sacristy, the cellar and the attic).
The third measurement phase began with the excavation and the current scanning of the individual construction phases; altogether there were 7 scan phases.
The fourth measurement phase consisted of the final scan of the entire excavation area in February / March of 2012. This final scan was used to create a 3D model of the excavated floor. This final scan was also used to create a 3D-model

The advantages of laser scanning were very impressive.

Using Pointclouds
Based on the pointcloud it was possible to create complete plans. A requirement of the architects and the involved authorities was the creation of 2D-design files of the elevation from different sides of the church. Another result of the pointcloud was the creation of the complete floor plan as a 2D-designfile, before excavation began.

After defining the detailed content of the building site measurement plan by the building inspection office, we were able to prepare the plan and make it available in rather short time.

The building site measurement plan includes all the details, even the pews and flooring, complete with all of the dimensions, elevation information and so on.

All these files were created with Bentley software using the corresponding parts of the point cloud. The generated building site measurement plan includes all the details in all dimensions and the elevation information.

In Germany, there is a profession that deals specifically with surveying and documenting old churches and excavation sites, but everything has to be done manually – by stringing up cords, and taking measurements with a measuring tape and ruler. The process for a church with the size of Saint Leonard’s used to take about a year.

For this reason, the Building Inspection Office and the Office for the Protection of Historical Monuments decided to use laser scanning instead. The experience was very good. After completion of all scanphases no additional scans were needed. The point cloud had such a good quality that we were able to generate all the required plans.

With the use of terrestrial laser scanning it didn’t need an entire year; it took only 6 weeks.

Having seen the results of the of the elevation- and floor plans, the Frankfurt authorities decided to do the documentation of the excavation as well. The whole excavation of the floor was done in 4 phases. The archaeologists needed detailed plans of the actual situation, in particular a detailed graphical design of the graves and the skeletons.

Because the entire excavation area was to be refilled to the new floor level, the office for the Protection of Historical Monuments required exact and complete documentation of this area.

Even all of the joints had to be shown in perfect detail. For that reason, it was decided to create a 3D model based on laser scan data. We measured more than 500 scanner reference points with more than 17 billion 3D points.

The above picture shows a 3D-VektorModel of the excavated floor from the 13th century as a result of the pointcloudvectoriszing and 3D modeling.

Furthermore a "tangible" 3D model and a physical 3D-Plot were created.

This column is shown on a scale of 1:20, roughly 20 to 25 centimeters high and about 15 centimeters wide. The figure that stands at the bottom of the pillar has the same scale (1:20), just to demonstrate the idea of the actual size of the pillar.

The 3D-Plot-Model consists of a plaster/plasticine mixture and it takes on increasing color during the plotting process. Also, having the point cloud, it is simple to create exactly the sectional drawings that the customer wants. Our costumer simply defines for which part of the church he needs new drawings, and by using the point cloud and the CAD-Software this can be achieved quickly and economically.

We needed 26 days for scanning the whole church including the excavation periods. We needed 520 scans (scanner positions). We had to work with nearly 600 GB (about 17 billion points) in several files.

Having the point cloud in Microstation and CloudWorx, it was easy to create exactly the sectional drawings which the customer wants. This project shows the advantages of terrestrial laser scanning as an appropriate method for documenting all phases of restoration of historic buildings.

Christof Kremer is the Project Manager for laser scanning and point cloud processing.
Udo Bastian is the Manager for IT, Data Processing and Geographic Information Systems at Steuernagel Ingenieure GmbH.

A 2.223Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE