SfM for Underground Documentation

Since last October Arc-Team is working on a challenging project about World War 1 on the southwestern frontline 1915-18.

Austro-Hungarian artillery piece in wintry position 1916

In a few years we will celebrate the 100st recurrence of the Italian declaration of war against the Austro-Hungarian Empire in May 1915.
For that reason the South-Tyrolean Heritage Departement was starting an extensive archaelogical survey campaign in high alpine environment up to 3900 m.s.l.
We've had to find out a rapid, lightweight and low cost method to document the uncountable amount of structures along both front lines.
The area of our pilot project is an approximately 6 sqkm large high plane between 2000 and 2300 m.s.l. called Plätzwiese. Between 1915 and 1917 it was a strongly fortified second line artillery position of the Austro-Hungarian Army.

Crew Cavern

Because of the seclusion of this area, today there we can find some of the best preserved residues of WW1 in Europe. We expect a total amount of more than 1000 archaeological remains just in this sector.
One of the major challenges is the documentation of the numerous tunnels and caverns driven in to the rock to protect troops and material from enemy artillery fire.

View towards the exit

We apply SfM to obtain pointclouds of the interiors, dealing with difficult conditions of illumination and space. After placing GCP's around the entrance(s) and measuring them with our DGPS, we start with one or more sequences of pictures in the entrance area, advancing along the tunnels, turning at their end and coming back to the starting point.

In the dark we use of course a tripod to allow long exposures and some torches, to illuminate shadow areas.
In this way we've taken up to 300 pictures for one single structure.
Various attempts of processing them in Python Photogrammetry Toolbox divided in a few single sequences, gave us some indication of the most advantageous way of photographing.

Point Cloud in MeshLab v1.3.2.

For good results we need to follow two important rules:

1. Redundancy: The same point has to be visible in much pictures possible.
2. Long baseline: To obtain correct geometrical information, the projection centers of the sensors have not to be to close together.

• Taking a just a single photo-sequence along one way of the cavern, causes often problems with redundancy and subsequently lead to empty areas in the pointcloud, especially along the ceiling of the cavern.

Recording strategy 01

• Another of the experiences gained, is that the software has big difficulties to match features form the photo-sequence taken on the way inwards, with features from the photo-sequence taken on the way back to the entrance of the tunnel.

Recording strategy 02

• We get the best results advancing on two parallel lines, as well on the way in, as on the way out of the structure.

Final result 01

Finally we processed the parallel sequences of each direction and referenced them after in Cloudcompare in the way how Ale Bezzi described it.

Final result 02

In June 2013, after the thaw, we will resume fieldwork in Plätzwiese and we hope to  acquire further experience.