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BRITARCH  January 2004

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Subject:

Re: Vertical digital photographic recording

From:

Geoff Avern <[log in to unmask]>

Reply-To:

British archaeology discussion list <[log in to unmask]>

Date:

Tue, 6 Jan 2004 20:33:43 -0000

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Hi again Paul Barford and BritArch-ers,

 
Here's a fuller response to your query which may touch on issues that other BritArch-ers may find interesting.  A short list of my thoughts on the matter of "photo-real recording" are;
 
1)  Vertical Photography can be a very simple, very accurate, very quick and very cheap way to record if (I would suggest, only if) a number of conditions can first be met, the important ones being; an appropriately small degree of vertical/topographic relief (the maximum is calculable), an apparatus which permits easy and accurate positioning of the camera, and not too large an area to record (I've done 7mx5m - file sizes of mosaics become HUGE).
 
2)  We need to think about both objective and interpretive records; ideally we want both.  A photomosaic can be an "objective" record (I call it the Primary Record) but, as Paul Cripps suggests, we still need to make an Interpretive Record, most conveniently by a line drawing.  This is easily done by tracing over a computer printout of the photomosaic while you are still on site.
 
3)  The future of recording excavations (IMHO) will be a "3D version" of the above technique where an accurate 3D model of the (upper) surface of each context is captured before it is excavated.  The primary record of an excavation would be a virtual 3D space combining the surface models of all of the contexts (which can be subsequently processed to "fill in" the voids to create "solid" deposits).  But we still need to make and, ideally, to incorporate an interpretive record - a 3D line drawing which can sit in/on the 3D model as an optionally-viewed overlay.
 
4)  This 3D future is still a way off.  The impediments are in the data acquisition (which is where I'm currently working).  There are plenty of 3D data acquisition systems (more than 73 at last count) but, as I will argue at CAA in Prato this year, I don't believe any will do the job for archaeology ('though not everyone agrees with me).

 
But back to Vertical Photography.  Paul, the major impediment is the construction of an apparatus to hold the camera.  People have used tripods, long arms, etc (I can find references once I'm back at work) and while they may be light, easy to pack and transport, they don't let you accurately position the camera (and the lighter they are the more they wobble :-).  If you can accurately place and orient the camera you can shoot precise systematic grids of photos at calculated intervals.  This allows you to crop the usable part of the image (that part with calculated, tolerably-small perspective error) by batch operation and simply drop these crops into their proper place on a similarly-gridded "blank" image to create the photomosaic.  It also means that the coordinates of each centre-of-photo can be used to register vertical series of photos as the dig proceeds.  You can anticipate that all this is much easier when the area to be covered is not overly large (at least in one dimension - I've seen it used reasonably well on a long Roman street).  The system I used (a one-off device called "le Statif" conceived by Yves Gltoz of Mons, Belgium and used by Prof Pierre Bonenfant, U.L.B., Belgium) was a substantial horizontal aluminium beam (approx. 120mm x 120mm x 5m) on legs.  Along this travelled a carriage to which clamped an "inverted-L" arm, at the end of which was attached the camera at height of up to 7m.  Sliding the carriage allowed linear series of photos.  Changing the direction and length of the horizontal part of the "L-arm" gave different runs.  Finally, repositioning the whole apparatus (taking 4 or 5 people and 5-10 minutes) was neccessary to further extend the coverage.   In practical terms, my first-ever attempt at making a photomosaic with this technique took 8 "person days" - the previous year the same area had been drawn by hand taking 35 "person days" - and there was no contest as to which was more accurate and the better quality record.


Open for discussion,
 
Geoff Avern  


  ----- Original Message ----- 
  From: Paul Barford 
  To: [log in to unmask] 
  Sent: Tuesday, January 06, 2004 5:44 AM
  Subject: Vertical digital photographic recording


  Apologies for cross posting

  Is anyone on this list working on the use of on-site vertical digital
  photography coupled with graphics programs in on-site computers to create
  excavation documentation and in particular to replace hand-drawn layer and
  feat
  ure plans? Are there any published accounts of these recording systems
  and their efficiency and reliability ? What about drawbacks?

  I would be grateful for any information.

  Paul Barford

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