Update: Photogrammetry at the Acropolis

Last July I stumbled across a laser scanning and photogrammetry operation during a visit to the Acropolis in Athens, Greece. As discussed here, the project involved terrestrial laser scanning coupled with aerial photogrammetry. A unique aspect of the project is that airspace restrictions over the Acropolis meant that imagery could not be collected by motorized aircraft (e.g. helicopter), so the project team rigged up a balloon system instead. Here's a picture of the balloon and camera in flight:


A paper describing the data collection and processing process is now available online here. The paper is an excellent resource for those interested in digital preservation of cultural heritage sites, and also outlines how complementary photogrammetry and laser scanning are for 3D data generation. As is the case with many photogrammetric projects, the digital orthophotos derived from the aerial photographs will ultimately reside in a GIS.

Laser Scanning Versus Photogrammetry

At the end of my previous post I asked which is more applicable for 3D cultural heritage projects, photogrammetry or laser scanning?

HDS6000 Laser Scanner

A reader pointed me in the direction of the December issue of Geoinformatics, which features an article on page 50 called "3D Laser Scanning and its 2D partners". I wanted to highlight the article in this post as it offers some interesting thoughts on the subject. In particular the article makes the following points (with my own observations):

• There are similarities between photogrammetry and laser scanning. For example, both technologies are used to capture point clouds where points have XYZ coordinates. I would add that the differentiator is that in photogrammetry we are usually capturing points to model a surface (e.g. a TIN or grid) as opposed to a true 3D point cloud (e.g. multiple points with the same XY but different Z's).
• Challenges in the adoption and acceptance of laser scanning, being the (much) more recent technology. For example, the cost and learning curve. I agree with this, however as the authors note this is changing. We face the same challenge in photogrammetry, which still carries a bit of a stigma as a dark art within the broader geospatial community. However times are changing and new technology will continue to flatten out the learning and cost curves...
  
• The all-too-common belief that the two technologies compete. The authors argue that this is a misconception and go on to outline why photogrammetry and laser scanning are complementary. I completely agree with this point: both technologies have advantage and should be implemented as needed on a case-by-case basis. I think we're seeing this in the context of the airborne mapping world as well - an increasing number of organizations are opting for optical and LIDAR systems for simultaneous collection. The Leica RCD105 Digital Camera is a good example of this, as it is typically sold alongside an ALS LIDAR system. There are a lot of advantages for such a system but that's a story for another post.
  

The article then proceeds to discuss laser scanning in the context of recording historic monuments and landscapes, along with several projects (including Cyark) and examples. Overall it is a compelling read and I'd recommend picking up a copy or checking out the online version if you are interested in this topic.

So in summary, I suppose the question above needs to be turned around. For recording cultural heritage, choose the tool set that best fit the requirements - which may mean integrating several technologies.

Angkor Wat in 3D Revisited

Several months ago I wrote about a 3D feature extraction project based on aerial photography at the temple complex of Angkor Wat in Cambodia. While photogrammetric reconstruction is one avenue of documenting historical sites, another method is terrestrial laser scanning. While Cyark has been mentioned before, I thought I'd highlight the foundation again here since it is such an excellent resource for terrestrial laser scanning in the context of archeology and documenting cultural heritage.

Aside from the slick presentation, one of the really nice features of the website is a 3D point cloud viewer, which allows you to navigate various scenes in 3D. While the point clouds are pre-cooked for the viewer (which has a 2 million point limitation), the density is still enough to provide a very realistic experience. You can even see a couple of people in the "Outer Cruciform Courtyard at Banteay Kdei". The process for digital heritage preservation is outlined quite well in this paper.

Angkor Wat 3D Model

Point Cloud: Outer Cruciform Courtyard at Banteay Kdei

One question this raises is which technology, photogrammetry or laser scanning, is the most effective (cost, quality, processing time, etcetera) for cultural heritage projects. For a study on that, check out this paper, which compares terrestrial laser scanning with "terrestrial photogrammetry" (photos are taken from the ground with an SLR camera, not aerial photogrammetry although the processing principles are similar). As one might expect, the study indicates several pros and cons of each method, as well as a look into combining methods. The cost of hardware is higher with terrestrial laser scanning, and the processing (automatic and manual) for both methods can be fairly intensive depending on the level of detail and accuracy required.

Satellite Archeology with Quickbird

Check out this article from MSNBC for an interesting application of remote sensing. The article highlights the fact that remote sensing can be an excellent tool for locating historical sites that cannot by detected by ground. The arrows in the image below show the outlines of the pyramids.

If you're interested in this topic, here is an excellent paper by Dr. Armin Gruen from ETH on "New Technologies for Efficient Large Site Modeling."

Photogrammetry at the Acropolis

After a few weeks offline I'm now back and writing from Liege, Belgium. During my time off I had the opportunity to visit the Acropolis in Athens, Greece. While walking up to the Parthenon I noticed there was a terrestrial laser scanner set-up and operational - although unfortunately I didn't get any photos. But that was enough to get me wondering what the project was about. At the top of the Acropolis I found a sign with a short description of the project (photos below). Since it is difficult to read I have reproduced the text below:

DATA ACQUISITION FOR THE PHOTOGRAMMETRIC RECORDING OF THE ACROPOLIS

The Acropolis Restoration Service carries out the project of geometric documentation of the Acropolis hill, the circuit Wall and the Erechtheion, using photogrammetric methods together with 3-dimensional scanning.

All the information to emerge is to be entered in a Geographic Information System (G.I.S) that will be available through the Acropolis Restoration Service's web site (ysma.culture.gr).

Photogrammetry at the Acropolis was also a subject of discussion at the recent ISPRS Conference in Beijing. One of the technical sessions (TS-SS19) was "Recording and Documenting the Acropolis of Athens - From Classical Ancient Greece to Modern Olympics". While I wasn't at the conference, a colleague sent me the paper for "Recording, Modeling, Visualisation and GIS Applications Development for the Acropolis of Athens", by Tsingas et al. The paper discusses the various techniques employed by the project outlined above, which include geodetic field measurements, terrestrial scanning, and photogrammetric data capture and processing. Of the many data products to come out of the project, an interesting one is a top-view orthomosaic with a 10mm resolution. A 22MP camera was used on a balloon system, as motorized vehicles such as helicopters are not permitted to fly above the Acropolis. Also of interest (and news to me) is that Leica Geosystems is a partner in the project. One of the terrestrial scanners is a Leica HD3000, while ERDAS LPS is used for parts of the photogrammetric processing. This included camera calibration, bundle adjustment, and terrain processing.

The paper describes the methodology in detail, and I will see if it is available online anywhere - it provides an excellent discussion of various techniques used in concert to fully capture a highly detailed digital version of the monument. A few other good papers on photogrammetry/mapping at the Acropolis are here and here.

Angkor Wat in 3D

Today I'd like to highlight a project from the ETH's Institute of Geodesy and Photogrammetry in Zurich, Switzerland.

The project, "Reality-based 3D modeling of the Angkorian temples using aerial images", was performed in 2006 and focused on the start to finish processing workflow for 3D modeling of the temple complex near Siam Reap, Cambodia. The project area was captured by a Leica RC20 camera in 1997 and provided to ETH by the Japan International Cooperation Agency. The project website proceeds to outline challenges (e.g. lack of ground control points within the project area) and processing methods (including LPS) employed during the creation of the 3D models of the temples.

The downloads section of the project page includes an excellent paper outlining the entire process.

If you haven't been to Angkor Wat, it is an amazing place. Here are a few photos I took there a couple of years ago: