Shooting for architectural photogrammetry.

1.Golden rules:

  1. Convergence: 2 images pointing at the same object from 2 view points creates intersections: this is convergent imagery.
  2. Overlap: pairs of convergent images must overlap by at least a third.
  3. Consistency: focal length, aperture and ISO need to be constant. The stand off (distance beween the camera and subject) of complete circuits needs to be reasonably consistent.
  4. Corners need at least 3 convergent pairs
  5. The more photos the better
Sparse cover of the corner has corrupted the geometry

2.Camera positions:

  • CONVERGENT: images must be of the same subject shot from 2 positions : not parallel to the facade. Think in triangles: a ray from a point on one photo must intersect with a ray from the same point on the operlapping one.
  • OVERLAPPING: images must overlap by at least a third.
  • OFFSET: viewpoints of the same subject must have a minimum of 2 steps (2m or 2 yds) to the left or right between them.
  • LINKED the corners of the building (and projections like porches and pediments) need cover from both sides and the corners ( from 2 positions,) Each corner should have a least 6 shots from 6 positions.
  • DIFFERENTIATED avoid shots with undefined texture. Make sure sequential shots look different, several shots of a white rendered wall without distinguishing detail will fail to align.
  • SYSTEMATIC: complete a circuit of the building at one stand-off before doing another. A series of shots at a consistent stand off is required, don’t jump from close up to full facade shots. Make a separate, complete, pass for each stand off. If a mix of wide views and close ups are required complete passes at each stand-off should be captured. An imbalance in the density of capture in one facade will skew photo alignments in software. Make sure coverage is evenly distributed.
  • COMPLETE: if details are to be shot make sure there is a wide enough view to gain an overlap between shots. Ensure the subject fills the frame, aviod unwanted objects in the shot.
  • RAISE THE CAMERA: a high viewpoint looking both up & down always improves cover of the base of facades and fills in things like cill tops.
  • AVOID DUPLICATE SHOTS: always move camera postion between shots.
Excessive paralelism has lost cover of the porch and verandah details as well as skewing the image alignment algorithm to exclude the few oblique shots of the side facades.
Removing the bias from the excessive photo cover of the front facade allows the alignment algorithm to resolve the corner geometry.

3.Camera settings:

  • LOW ISO
  • RAW format, turn off image stabilisation.
  • SMALL APPERTURE (f8-11)
  • LONG EXPOSURE (1/60th) use a tripod and a remote shutter release
  • FIXED FOCUS
  • NO ZOOMING IN A SET
  • FOCAL LENGTH: avoid very wide angle lenses
  • CONSISTENT: don’t mix portrait and landscape camera orientations

4.Lighting

  • Avoid deep shadow
  • Bright overcast is good
  • Flash needs careful use: the camera mounted flash is not the best, remote indirect light is better.
  • Mixing time of day can work to get raking light but beware the ‘dark’ (North facing) facade.

5.Useful control measurements

Photogrammetry will work without measurement but the results will be scale free unless some distances are known. This can be as basic as a single distance across a facade or as complex as 3d co-ordinate measurement by TST.

  • A simple scale: a long vertical or horizontal line on each facade
  • Facade control: TST shots to numbered and marked points
  • Wire frame trace of key features
  • Natural points
  • Fixed Targets

6. How to take a shot from a pole.

To trap the wobbly motion of the camera held aloft on a pole a faster than usual shutter speed is needed. A tripod can be held aloft to gian height if its a reasonably lightweight one.

One way to do this is to use aperture priority with a wide stop (e.g. f2.8). This will allow the ISO to be kept low and speed up the shutter.

Without a remote preview system cover will be guesswork. Live view can help with a tilted screen but estimating the cover by trial and error is fine.

Use the countdown timer (also known as the self timer) to release the shutter if there is no remote release available.

7. Roof surfaces

A drone is you friend here. It may be possible to exploit view points from neighbouring buildings or more distant cover, use a pole or kite.

Drone cover is ideal for roof survey. Mavic pro2 parallel cover at 60m and 120m AGL with obliques is adequate.

8. Use the best camera available

A good DSLR is better than a mobile phone for a number of reasons:

  • Full manual control
  • choice of lenses
  • polarising filter
  • big sensor
  • accurate view finder
  • live view
  • tripod mount
  • remote shutter release
Left: with some oblique cover the porch is rendered Right: missing oblique cover fails to capture the porch. A single position is insufficient to get the convergence needed and without reasonable separation to form a baseline between photos no intersections can be calculated.

9. Consider the output

There are a variety of photogrammetric products:

  • Ortho image
  • Mesh model
  • Point cloud
  • Textured model

One thing you don’t get out of photogrammetry is a drawing: if a good quality record is to be achieved you will need to take notes on the details that photogrammetry isn’t very good at. This is typically:

  • Anything with a sharp, projecting edge
  • Distant details such a roof ridge lines, finials etc
  • Mouldings
  • Railings
  • Tracery

If a scale drawing is the final goal key details will need close up photography, site skeches and measured drawing.

A trace from an otho-image can form the basis of an architectural record but infil data is needed from close up photos, measured drawing and site sketches.

About billboyheritagesurvey

Heritage worker
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