Digital archaeological drawing on the field with QGIS

I should have written this post since long, but time is always missing... The topic regards the digital archaeological (vector) drawing on the field. 

During the CAA conference of 2015, held in Siena Italy), I participate, among others, in the session 9A (Towards a Theory of Practice in Applied Digital Field Methods), moderated by +nicolò dell'unto (Lund University) and James Stuart Taylor. After my speech I was asked if we (Arc-Team), as a professional archaeological society, were really able to perform the digital documentation in real-time during an ordinary excavation. I answered that, at least in Italy, this point is very important for a professional society and that in normal conditions (but this can also happens during most of the emergency excavations) we complete the digital archaeological documentation directly on the field. The reason is simple and it is every year more evident: money are always less and less, as the time goes by, for cultural heritage matters; at least this is the trend of the last decade. For this reason, if on the one hand we have to try to counter this phenomenon, on the other we have to adapt our methodology to the current reality and this means to use the economic resources for the excavation also to produce the related documentation (without counting on a post-excavation budget).
The old video below (2014) shows how we manage the digital archaeological drawing on the field with QGIS.

The vector layers can be related to georeferenced photomosaic (bidimensional photomapping), or to georeferenced orthophoto (coming from 3D operations based on SfM/MVSR techniques). Of course orthophoto are the best solution, but currently the 3D work-flow with standard hardware is pretty slow. This is the reason why for almost all the palimpsestic documentation we still work with both the system: 2D photomapping and 3D SfM; depending on the time-table we have on the field, we choose the post-processing operations.
Within QGIS it is possible to draw vector layers in different ways. Basing on our experience (as you can see in the video), the best two best solution are:

1. to use the plugin Freehand Editing (if you want to experience something that is really similar to the old-traditional methodology, with the pencil and the paper)

2. to use the standard vector drawing tools (if you want to avoid too complex sahpes, like polygons with too many nodes)

IMHO, Freehand Editing plugin is a perfect solution for field operation, so that I am planning to add it into qgis-archeos-plugin, for ArcheOS Hypatia.

From drone-aerial pictures to DEM and ORTHOPHOTO: the case of Caldonazzo's castle

Hi all,
I would like to present the results we obtain in the Caldonazzo's castle project. Caldonazzo is a touristic village in Trentino (North Italy), famous for its lake and its mountains. Few people know about the medieval castle (XII-XIII century) whose tower is actually the arms of the town. Since 2006, the ruins are subject to a valorization project by the Soprintendenza Archeologica di Trento (dott.ssa Nicoletta Pisu). As Arc-Team we participated in the project with archaeological field work, historical study, digital documentation (SFM/IBM) and 3D modeling.
In this first post i will speak about the 3D documentation, the aerial photography campaign and the data elaboration.

1) The 3D documentation 

One of the final aims of the project will be the virtual reconstruction of the castle. To achieve that goal we need (as starting point) an accurate 3D model of the ruins and a DEM of the hill. The first model was realized in just two days of field-work and four days of computer-work (most of the time without a direct contribution of the human operator). The castle's walls were documented using Computer Vision (Structure from Motion and Image-Based Modeling); we use Pyhon Photogrammetry Toolbox to elaborate 350 pictures (Nikon D5000) divided in 12 groups (external walls, tower-inside, tower-outside, palace walls, fireplace, ...).

The different point clouds were rectified thanks to some ground control point. Using a Trimble 5700 GPS the GCPs were connected to the Universal Transverse Mercator coordinate system. The rectification process was lead by GRASS GIS using the Ply Importer Add-on.

To avoid some problems encountered using universal coordinate system in mesh editing software, we preferred, in this first step, to work just with only three numbers before the dot.

2) The aerial photography campaign 

After walls documentation we started a new campaign to acquire the data needed for modeling the surface of the hill (DEM) where the ruins lie. The best solution to take zenithal pictures was to pilot an electric drone equipped whit a video platform. Thank to Walter Gilli, an expert pilot and builder of aerial vehicles, we had the possibility to use two DIY drones (an hexacopter and a xcopter) mounting Naza DJI technology (Naza-M V2 control platform).

Both the drones had a video platform. The hexacopter mount a Sony Nex-7; the xcopter a GoPro HD Hero3. The table below shows the differences between the two cameras.

As you can see the Sony Nex-7 was the best choice: it has a big sensor size, an high image resolution and a perfect focal lenght (16mm digital = 24 mm compare to a 35mm film). The unique disadvantage is the greater weight and dimension than the GoPro, that's why we mounted the Sony on an hexacopter (more propellers = more lifting capability). The main problem of the GoPro is the ultra-wide-angle of the lens that distorts the reality in the border of the pictures.
The flight plan (image below) allowed to take zenithal pictures of the entire surface of the hill (one day of field-work).

The best 48 images were processed by Python Photogrammetry Toolbox (one day of computer-work). The image below shows the camera position in the upper part, the point cloud, the mesh and the texture in the lower part.

At first the point cloud of the hill was rectified to the same local coordinate system of the walls' point cloud. The gaps of the zenithal view were filled by the point clouds realized on the ground (image below).

After the data acquisition and data elaboration phases, we sent the final 3D model to Cicero Moraes to start the virtual reconstruction phase.


3) The Orthophoto

The orthophoto was realized using the texture of the SFM's 3D model. We exported out from MeshLab an high quality orthogonal image of the top view which we just rectified using the Georeferencer plugin of QuantumGIS.
As experiment we tried also to rectified an original picture using the same method and the same GCPs. The image below shows the difference between the two images. As you can see the orthophoto matches very well with the data of the GPS (red lines and red crosses), while the original picture has some discrepancies in the left part (the area most far away from the drone position, which was zenithal on the tower's ruin).

4) The DEM

The DEM was realized importing (and rectifying) the point cloud of the hill inside GRASS 7.0svn using the Ply Importer Add-on. The text file containing the transformation's info was built using the relatives coordinates extracted from Cloud Compare (Point list picking tool) and the UTM coordinates of the GPS' GCPs.

After data importing, we use the v.surf.rst command (Regularized spline tension) to transform the point cloud into a surface (DEM). The images below show the final result in 2D and 3D visualization.

Finally we imported the orthophoto into GRASS.

That's all.