Wednesday, May 31, 2006

Vertical Photo Collage

Vertical photography is a venerable documentation technique in field archaeology. Instead of drawing plans of your site, you put clearly visible coordinate markers on the ground and photograph it from above, using a huge camera tripod or the scoop of a mechanical excavator (don't tell the health and safety people!). Then you develop the photographs and glue them together in one big collage. The beauty of this is that it's fast, accurate, hi-resolution and that you needn't have everything cleaned and ready for photography at one go. As long as you keep reasonable accuracy on your coordinate markers, you can shoot the site a few square meters at a time. But with analog chemical photography, you have to have cheap overnight photo development.

Swedish archaelogists who developed this technique were Otto Frödin in Östergötland, Björn Ambrosiani in the Lake Mälaren area and Erik Nylén on Gotland. Nylén's team produced excellent documentation in the 60s and 70s. They were technology optimists, very scientifically minded, and it's kind of sad that the glue they used back then doesn't age well. In the 90s many of their archived photo collages were falling apart and had to be restored.

Here's a vertical shot that Gustaf Trotzig took in the 60s at Barshalder, the cemetery on Gotland that I wrote my thesis about. It shows an 11th century inhumation grave. Only the lower half of the skeleton was preserved, the rest had been destroyed by looters.

Vertical photography has received a boost in recent years thanks to the adoption of digital cameras and image processing software. These days, you need no photo lab and you can start removing stones seconds after shooting the site. Also, you no longer need to take exactly vertical photographs. If you make sure there are four coordinate markers in each shot, the computer can rectify them for you. If they're really good, the machine can even do the stitching for you.

As often mentioned here, my friend Howard Williams and myself dug a 9th century boat grave last summer. We saved enormous amounts of time by not planning the site by hand. Instead, intrepid Howard scaled the land owner's aluminium ladder and took oblique digital photographs of everything. Each shot covers a bit more than four square meters. They have been sitting on my hard disk ever since, and in the last couple of weeks I've processed them and put them together using AirPhoto and PhotoShop. Here's what came out. The coordinate crosses are one meter apart. Any PhotoShop wizards around who can help me get the colour more uniform?

I'm really thrilled to be using the methods of my scholarly fathers with modern equipment. The question now is, how do we get an archive longevity comparable to that of monochrome photographs? I should probably try to find a lab willing to print the collage on the same Ilford stuff that they used in the 60s.

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Anonymous Anonymous said...

Depending on your version of PhotoShop (assuming that it is in english and that each photo is in a separate layer!) you should do the following:

1) Select the layer you want to manipulate in the "layers" toolbox

2) Go to: Image > Adjustments > Color Balance... This (most surprisingly!) brings up the Color Balance dialogue.

3) Set the colour balance for the R, G & B channels using the sliders in the dialogue. Experiment...

4) Alternatively: Go to Image > Adjustments > Variations... which yields another dialogue that works in a more intuitive (but less accurate) way.

All this of course must normally be done before you stitch the photos together in whichever software you use (If you have any questions just leave a comment on wy website)


01 June, 2006 15:14  
Blogger Martin said...

Thanks! The multilayer file kept growing huge and unwieldy, so I unfortunately only have a single bitmap for the entire plan. But Iäll try do get the effect you describe on a selected portion at a time.

BTW, Howard told me you had some pretty weird discussions in Sheffield regarding the gendered use of toilets and the history of mullet haircuts...

01 June, 2006 17:04  
Anonymous Anonymous said...

He must be speaking of my upcoming paper "Engendering Toilets - multidisciplinary approaches to ritual behaviour and gender aspects concerning the use and reuse of the 20th century lavatory". I will present the paper at a future conference (the actual oral presentation will be given in the mens room).

About the "mullet" haricut survey thing.. well that was just a joke ;-)

01 June, 2006 23:15  
Anonymous Anonymous said...

Oh, and here's the link.

02 June, 2006 00:22  
Anonymous Anonymous said...

För att få bra lodfoton så kan man även använda en heliumgasfylld ballong. Detta är en metod som bl a används i mellanöstern och Kina. Fördelen är att man kan variera den höjd varifrån fotona tas ganska mycket. Ballongen styrs med tre linor varigenom man uppnår bra precision.

12 April, 2007 10:46  
Blogger Martin said...

Using a helium balloon for vertical photography sounds good! Then maybe you could get a wireless remote control for the camera as well.

14 April, 2007 11:42  

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