Aerial Photography and Photogrammetric Applications at Alalakh

 

 

 

 

 

One of the most challenging tasks of archaeological documentation is to achieve high quality photographic documentation of the excavation areas. Many mechanical and electronic systems have been utilized in various projects. In most of the mounded sites, this becomes a complicated task due to the inaccessibility of the location by modern vehicles such as fire trucks with ladders--a very good solution in obtaining aerial photographs. Apart from standard methods such as the nine-step ladder, more advanced systems are often needed to better understand the contexts. Most commonly used designs, such as monopods or light camera crane systems, and slightly larger versions, such as photo towers, have been successful in getting good shots of the exposures; however, none of those systems were designed to get 90 degree angle photographs.

Further advanced systems like balloons, blimps or model helicopters with attached cameras provide high quality results but their usage is limited based on their high cost and the precariousness of various atmospheric conditions. Such systems require advanced skills and it is not always easy to find the equipment and the operators. In many cases, strong winds make these systems inoperable and projects have to resort to alternative photographing systems.

From 2003 onwards, several photographic methods were tested at Alalakh. High quality results were achieved by using fire trucks with 30 m ladders, however; these could only be applied one or two times during the excavation season. Since it is a very difficult task to have a fire truck at the site all the time, monopods were used to take photographs of excavated phases, producing a slight angle, but, nevertheless, creating an excellent visual image of the square.

The Alalakh Photo System

In the summer of the 2009 season, senior field supervisor Murat Akar designed a new photography system that can take 90 degree aerial views. Produced at the industrial district of Antakya, it was accomplished as a low cost project. The mobile system has been used successfully in the documentation of archaeological levels of 8 excavation areas.

The design was inspired by a laundry line. By having digital cameras run along a metal wire in between bars of 9 meters height, high quality images were captured with ease.

To construct the system:

- 3 aluminum bars of 6 m width and 10 mm thickness.
- One bar cut in half and welded to another two to create 9 m long bars. Spiky points were welded to their bottoms.
- A metal wire 13 m long was drilled into the bars to serve as the camera rail.
- A metal box with a loop allowed allocation of a pulley system. It was designed to hold a Nikon D200 camera with 16-70mm Nikkor Lens. A plastic pulley was used to let the camera box slide on the metal wire.
- From both sides of the bar, a guide rope system was attached to the camera box so that it can operate from both directions. This could be used to change the location of the camera.
- From the top points of the aluminum bars, 10 mm ropes were attached similar to antenna poles to provide stability to the system.
- A wireless remote shutter control system compatible with Nikon systems was used to take the photographs.

Fig 1: The camera box , pulley system and the wireless remote controller.

Fig 2: Detail of the aluminum bars. Spiky bottom.

To use the Alalakh Photo System

The system can operate with 5 experienced personnel, one as the person in charge of the remote control. During the first days 7 personnel are needed for the operation of the photo system but afterwards it can easily be operated with 5. For transportation, two personnel are enough to carry the light weight system from one excavation area to another.

- Bars are placed on the sides of the trench. In a 9 m square exposure, putting the bars at 4.5 meters from its sides will achieve a complete view of the excavation area in two shots.
- The camera box should be attached to the metal wire while the system is on the ground, so that the pulley can function as a rail system.

- The system should be raised to stand vertically on top of the excavation area at equal speed from both sides. The spiked end will hold the bars in the ground. From the back, stabilizer ropes will tighten the system to keep the structure stable and avoid bending of the aluminum bars.
- Once the system is straight, operators can change the position of the camera with secondary ropes attached to the camera box. Pulling the ropes from the sides will make the camera slide on the rail.

The squares at Alalakh are preferably photographed in the early morning or late afternoon when full shade is available. This allows control of the shadow factor and provides clean photos within a homogenous light condition. However, the low K light value in those hours of the day means a higher ISO speed. In order to prevent blurry photos, a 1/500-600 shutter speed is recommended. This makes it necessary to have an ISO boost at 400. For that reason, it is recommended to use digital cameras that can avoid noise at even high ISO settings.

 

System for Photogrammetric Illustration

90 degree photos can be utilized to create accurate architectural plans by using photogrammetric techniques.


How to use the technique

- Draw cross-marks with chalk inside the excavation area as ground control points. The more points you have, the more accuracy you will get.
- Take your photo with the Alalakh Photo System described above.
- After the photo, take coordinates by using a Total Station and digitally record them into the memory of the machine.
- To double check and to avoid problems that can occur from the distortion of the lens, get control points from features like wall corners, around pits and square bottom corners. Camera lens calibration software can also be used to fix the distortion error.
- By uploading the images and the Total Station data, GIS based programs can be used to geo-rectify images based on the available control points in order to create scaled photographic images.
- By GIS or CAD applications, the features visible in the photo can be drawn directly from the photo.

This type of drawing can only be done by high resolution cameras. By zooming into the photo, features can be drawn as vector files and can be embedded into the topography of the site as well as into GIS based databases.

An example of the use of photography techniques for architectural drawing.


Simple Stitching by Using Adobe Photoshop CS3 (and higher versions) to create single aerial photographs of large areas.

When you take several photographs of a large area, it is impossible to manually combine them to create one photo. Photoshop offers a simple, easy solution by its stitching ability.

1- Open the files you want to stitch. For that, the rule is to have some of the same elements in both pictures.
2- Choose one of your images and start dragging your other photographs onto the one selected.
3- Those will show up as layers in your menu screen.
4- Use the option to create layer from background to convert your base photograph into a layer.
5- Select all layers from the layer menu.
6- Go to EDIT menu and choose auto-align layers.
7- This will create one single image from your images.
8- Go to EDIT menu and choose auto-blend layers. This will eliminate the color differences between your photographs and will smooth any part where there is a wrong join.

Two separate photos taken with Alalakh photo system were combined in Adobe Photoshop CS3 to create a full view of the square.

 

This will create publication quality 90 degree aerial photographs of archaeological excavation areas.

 

for further information

about the system

Murat Akar

akar@alalakh.org

* We are happy to share our experience with projects. Please refer to our web page or project if you decide to use our photography system.
** In using computerized applications, it is very important to keep several backups of the same data, by making it a daily schedule during the excavation season. University or private servers should also be considered as another major back up location.