Saint Manchán’s Church Project: Part 2, Capturing & Processing

First arriving at the site in Lemanaghan, I noticed that the ground inside the church walls, now quite unkempt in appearance with long grass growing in points and many rocks and stones cluttered about, was quite uneven, and clearly open to the elements, judging by the mould and moss growing on the gravestone slabs and vertical gravestone markers. In total, there are three gravestone slabs and three gravestones, two of which bear cross emblems on the top. It was an overcast day, which would minimize the possibility of heavy or stark shading across the stone surface of the church walls and the ground surface. The camera I used was a Sony Handycam DCR-SR55E. f-Stop at F/4 and shutter at 1/100. I began at the the west side of the church, keeping the camera held at a 90 degree angle from the ground as much as possible and went around the four walls in that position (held at a height of about 2 and a half ft from the ground). I did this process again at about 10 inches from the ground and then once again at a about 5 and 1/2 ft, and finally at around 7ft, holding the camera above my head, so to get the top of each wall.

After processing these images at home, I realised that I had made one fatal error in my capturing process. Although the model had come out relatively well, many of the 164 images I had taken could not align, as I was remiss in keeping in mind the 60% overlap rule. Whenever I had turned a corner, from say, the north corner between the north to the east wall, I failed to gradually turn the corner by degrees. Instead I turned at a sharp 90 degree angle, which meant  the software could not stitch, say, the last photo from against the north wall capturing the south wall opposite with the next photo taken from the east wall capturing the west side, since there was no gradual overlap or incremental change from the south wall to the west side of the church. The software, confused, could not align a whole series of shots taken along the south wall because of the lack of overlap between the last image taken at the east wall and the first along the south wall.

 

Last photo taken at east wall
Last photo taken at east wall
First photo taken at south wall (no overlap)
First photo taken at south wall (no overlap)

As many photos of the north wall failed to align because of this very issue, much of that wall was left out of the final 3D model. There were also areas behind the easternmost gravestone slab not processed, as well as some of the ground behind it. Many of the ground areas failed to render also, which resulted in many areas of the church ground and parts of the north wall left as gaping, empty gaps in my model.

I decided, given the capturing process had affected the quality of my model (despite the fact that all the images themselves were above the recommended PhotoScan quality), to head out to the site once more and to capture the images again (except this time turning by small degrees around the corners). I considered just taking the ‘corner images’ again on their own, and adding them to my last sequence of photos as a way to fill in those gaps that were the cause of the software’s confusion during the alignment phase. I then considered that the difference in lighting might be too great between the two different days of shooting, and so I decided just to take another whole sequence of images.

The north and east wall were brightly lit by sunshine, while the south wall was in the shade. I knew that this lighting was not optimum for photogrammetry, yet I knew that I would just have to make do as I might not get the chance to head to the church at a later time before the project submission deadline. Preferably, it would be better to head out sometime in the evening if it’s a sunny day with clear skies. Unfortunately, I was only able to make it to the church in the early afternoon.

I took my images of the church this time from five different height positions, not four, as I felt there was much too wide a gap between the ‘medium’ height position and the highest, so I decided to go around the church with an in-between position at around 5 ft.

Lowest height postion
Lowest height postion
Medium height position
Medium height position
Highest position
Highest position

When I uploaded my photos to PhotoScan this time, I was surprised to see that I had 464 images in total, compared to the 160 making up my last batch. This was probably due to the extra photos taken when turning corners and the extra height position, plus the fact that I went around more of the church area (I decided not to cover some of the west side of the church last time as a grave in the corner made it hard for maneuverability around that area. The fact that I wanted all of the information from each photograph processed meant that I did not have to make masks for the images. I noticed that 8 images were not up to the image quality standard as recommended by PhotoScan, so I decided to deactivate these from the alignment phase. The reason for their low quality was mainly to do with the images being too blurry, and two images were taken from very low down and were obscured by small rocks on the ground, and were therefore too dark and didn’t register any information.

DSC01276
Ground level view blocked by vegetation

For the alignment phase of my processing I chose the option ‘high’ with ‘generic’ matching of images. The process took just over 2 and a half hours, and when I saw the result I was extremely impressed. It seems that for larger areas, the more photos the better, as the 460 plus photos appear to make it an easier job for the software to match all points. 164 images, it seems in hindsight, just wasn’t enough. The fact that the ground area is often obscured from different angles, because of the tomb slabs, long grass and scattered debris and rocks, the more angles and images taken around the area meant that some photos in which an area may be obscured can be registered in another image. I then decided to execute the dense point cloud in ‘ultra high’ quality mode for best results. On realising this setting would take up to 25 hours to complete, I aborted and chose the ‘high’ quality setting instead. This took 3 and a half hours to complete.

dense_point_cloud
Model After Alignment

Issues with Model

The church area was quite difficult to capture. The uneven ground ,tombstones and fallen masonry cluttering the area make it difficult to capture each and every nook and cranny. The long grass does not appear to process correctl. In the dense point cloud model , much of what should have been the rendered grass was positioned “underneath” the actual ground level where the base of the tombstones begin. If I had more time to undertake this project, I would test out different methods (maybe cut out the images taken from the lowest height position, as many of these were simply capturing just the base sides of tombstones or long leaves of grass). There are many ‘holes’ in the ground of the model as a result. I would also use MeshLab or other similar image processing tools to edit my model. I am pleased that the position of the tombstone slabs and headstones came out correctly, and that there is a good sense of the ‘space’ of the area.

Dense point cloud
Dense point cloud
Window recesses
Window recesses

Vulnerability of the Church and Conservation:

According to the Conservation Plan for the church site and wider monastic complex, the sites and their fabric will continue to deteriorate unless corrected (47).

Phases of development of the church [Margaret Quinlan Architects]
Phases of development of the church [Margaret Quinlan Architects]

Excessive ivy growth (which has now been cleared by conservation specialists) was at one stage the only thing keeping masonry in place before part of the south wall was restored and the ivy cleared.  Other preservation and conservation issues include:

•Concern for the future security of some of the higher quality carved stone associated with the site.

• The two early Christian slabs attached to the wall of the church
are at risk of theft and should be made more secure. As these are not in their original location, it would be desirable to remove them and store them with the rest of the collection of slabs from the site.

• The potential of increased visitors to the site poses the risk of additional wear and tear to the structures, and perhaps to the togher (historical tree-lined pathway to the monastic complex).

For reasons of preservation, the Plan also points out that :

local pride in the history and archaeology of Lemanaghan is strong. However, an increased awareness of the national significance of the site would be of benefit in terms of providing further protection. This is particularly true of educational projects at school levels which will ensure understanding and protection by future generations.

After rendering my three-dimensional model, I can see many benefits of photogrammetry for archaeological purposes and for the dissemination of information about these sites. If a visitor centre were ever built for the site, a three-dimensional model could be used as an edutainment tool, or a way for visitors to virtually explore and experience the site from all dimensions and vantage-points. Photographs may not give us a real sense of a significant historical site, as, especially with the case of Manchán’s church, lack of perspective means that the width and length of the church cannot be captured satisfactorily, nor the distance between the grave stone slabs and the vertical gravestones. Photographs cannot give us a true sense of the amount of space the stone slabs take up either, only two dimensional overhead survey sketches or overhead images could potentially afford us these views. Three dimensional models can also afford us accurate masonry detail including depth, as well as the depth and relative size of features such as the piscina in the wall and window recesses that splay inwards. The church site is itself quite ‘squashed’, for want of a better word, and it’s difficult to take a picture from one position without any of the fallen masonry or tomb structures blocking your view: again, a three-dimensional model manages to outstrip this issue. The model could also be thought of as a digital preservation technique. The Saint Manchán Church site, as stated by the Conservation Plan report, is in dire need of repair, conservation and preservation. Although work has been carried out for conservation, if this does not continue and if the masonry is not continually strengthened much of the facade of the church, along with the ancient stone slabs within the church, will continue to deteriorate. A three-dimensional model may preserve the state of the site at the time the photographs were taken – potentially serving in some form as a record – with the 3D model bringing that space to life.

 

Church door final model
Church door – mesh
Church west gable final model
Church west gable – mesh
Sketchfab screenshot
Sketchfab screenshot
Sketchfab Screenshot - South wall
Sketchfab Screenshot – South wall
Piscina - final model
Piscina – final model

Manchán’s Church
by alexghughes
on Sketchfab

O’Brien, Caimin. Stories from a Sacred Landscape. Offaly County Council. 2006

Quinlan,  Margaret and Rachel Moss. Lemanaghan County Offaly Preservation Plan. The Heritage Council. 2007.

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