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OCR for page 242
Photogrammetnc Measurement
and Mon~tonng of
Histonc and Prehistonc Structures
THOMAS R. LYONS and NAMES I. EBERT
Photogrammetry is measurement using photographs or other images of re-
motely sensed electromagnetic data. It provides~architects, archeologists, and
others concerned with prehistoric and historic cultural resources with cost-
effective means of recording and monitoring structures, monuments, and other
cultural sites. This presentation sets forth the basic principles of photograrn-
metry, concentrating on the use of vertical aerial photographs and terrestrial
recording of structures with a ground-based camera. Controlled stereo pho-
tographs can be used as the basis of a number of photogrammetric products,
including photogrammetric maps, orthophotos, and combinations thereof. Re-
petitive aerial and terrestrial photographs of a site, and products derived from
them, can be compared to allow the monitoring and documentation of inev-
itable change in cultural resources. Methods by which this is accomplished
are illustrated by reference to a number of ongoing monitoring experiments.
Considerations dictating frequency of monitoring and precision of measure-
ment are discussed, as is the multiuse nature of photogrammetric products.
A wide-spectrum approach to monitoring cultural resources that integrates
the use of past data sources such as historic photographs and maps, present-
day photogrammetric products and data, and possible future techniques such
as microwave scanning and holography, is necessary to ensure the conservation
and integrity of our historic cultural resources.
-Thomas R. Lyons is Chief, Remote Sens~ng Division, Southwest Cultural Resources
Center, National Park Service, Albuquerque, New Mexico; Jazzes I. Ebert is Archeol-
ogist, Southwest Cultural Resources Center.
242
OCR for page 243
Photogrammetric Measurement and Monitoring of Structures 243
This paper is concerned with the methods of aerial and terrestrial
photogrammetry and their application to the measurement and mon-
itoring of historic and prehistoric architectural structures. The purpose
of these procedures for documenting the condition of building fabric
is to provide a basis for quantitative measures of alteration or deteri-
oration over time and thus to provide data for informed decisions on
maintenance, preservation, or restoration.
The photographic products of calibrated aerial and terrestrial cam-
eras are historic documents of value to historical architects and/or
archeologists. Under certain circumstances of acquisition they provide
a data base that can be used for comparative purposes, for faithful
restoration or reproductions.)
- Repetitive controlled imagery has the added function of establishing
a measure of the rate of change in building fabric and consequently a
foundation for predicting future deterioration. It therefore allows the
foreknowledge necessary to prevent or mitigate harmful effects.
INTEGRITY OP HISTORIC STRUCTURES
The value as heritage and the scientific significance of historic and
prehistoric architectural structures depend to a great extent on the
integrity of the fabric of which they are composed. The term "integrity"
as used here does not imply the absence of change. Change in materials
over time is inevitable, and humanly induced changes in structures
occur regularly and can add interest and value to historic properties.
A historic structure has integrity when the process of change through
time can be documented and when the nature of the changes and their
causes have been determined. Photogrammetry, a remote sensing tech-
nique involving accurate measurement from photographs or other im-
ages of electromagnetic data, is a powerful too! for the historian, his-
torical architect, or archeologist wishing to document the history and
causes of change in architectural resources.
BASIC PRINCIPLES OF PHOTOGRAMMETRY
Photogrammetry
Photogrammetry, in its strictest sense, is measurement using photo-
graphs. A single aerial photograph for instance, is a representation of
a three-dimensional scene reduced to a two-dimensional format. While
it fairly presents scaled measurements for each plane in the scene
parallel to the film plane, it also contains a number of distortions. One
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244
CONSERVATION OF HISTORIC STONE BUILDINGS
of these is occasioned by the fact that the relationship between the
actual size of an object photographed and its size on the film depends
on its distance from the camera (focal length of the lens being held
equal). Even in a vertical aerial photograph, this causes differences in
scale between an object on the top of a mountain and an object at the
bottom of a valley since they are different distances from the camera.2
Radial or relief displacement is another effect of forcing a three-di-
mensional scene onto a planar film. For example, if a tall object such
as a masonry obelisk appears at the exact center of a vertical aerial
photograph, only its top con be seen; but if it appears anywhere else
in the image, not only its top but also one of its sides may be seen
(Figure 11. A single, monoscopic photograph containing-such distor-
tions obviously is not an accurate representation of reality.
Cameras used in photograrnmetry are classified by the platform on
which they are placed. Space or airborne cam eras are aerial cameras,
while those resting on the earth and pointed horizontally rather than
vertically are terrestrial cameras. The difference in the orientation of
these two types of cameras allows the architect and archeologist to
derive accurate representations of structural elevations and plans from
both a vertical and horizontal perspective.
Stereophotography
The use of stereophotography turns the distortions inherent in mon-
oscopic photos into advantageous information.2 Two sorts of mea-
surements that can be made from a stereo pair of aerial photographs
are parallax measurement and radial line plotting.
Parallax measurement makes use of the radial displacement inherent
in each of the photographs in a stereo pair. The difference in distance
between the base of an object and the top of the same object in each
pair is measured; if the scale of the photographs is known, the height
of the object measured can be determined. The principle allows the
measurement of the relative heights (z coordinates) of each point in a
photograph and is the basis of topographic mapping.
The accuracy of the x, y, and z dimensions derived through parallax
measurement and radial line plotting from a stereo pair depends on
the quality of the photographs themselves. The film plane must be
extremely flat to minimize uncontrolled scale variations within the
image, and the lens axis must be accurately perpendicular to and cen-
tered over the film plane. As a consequence, photographs taken with
standard cameras are not generally suitable for mapping, and metric
cameras are used instead. Unlike "snapshot" or even higher-priced StR
OCR for page 245
Photogrammetr~c Measurement and Mon~tonng of Structures
I..: ~ -
r..~
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FIGURE 1 Relief displacement illustrated by an aerial photograph showing an obelisk
at Chalmette National Historical Park in Louisiana (upper left coiners. If this obelisk
had appeared at the exact center of the photograph, only its top would have been visible.
Since it was at the edge of the photograph instead, the sides are clearly visible.
cameras, metric cameras are designed to take extremely accurate pho-
tographs. They are collimated to insure that the axis of their lens is
perfectly perpendicular to the film plane, their platens bear fiducial
marks so the exact center of the lens axis can be determined on the
photograph, and they are precisely calibrated in a laboratory prior to
sale. While metric cameras range in price from $10,000 to $60,000 or
more, a number of firms maintain and use them regularly and will
enter into contracts for terrestrial and aerial photogrammetry.
True horizontal mapping of structures or features from vertical aerial
photographs is accomplished through radial line plotting. Radial line
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246
CONSERVATION OF HISTORIC STONE BUILDINGS
plotting makes use of the geometric fact that any point in space can
be defined by sighting it from two known reference points or stations
and recording the angles to the unknown point. In mapping, this pro-
cecture is referred to as triangulation. Radial line plotting using a stereo
pair of photographs is usually done with a radial plotter or photogram-
metric plotter. The instrument uses the center of each vertical photo
as a known station and allows the interpreter to position a line from
this point through each point of interest on the photograph. Such points
might trace the outline of a structure or site feature. A mechanical
linkage provides a means of moving a tracing pencil, which creates,
in effect, a matrix of points of intersection that planimetrically rep-
resents the location of features on the ground.3 Although such data
can be obtained through a field survey using a transit or alidade, radial
line plotting from aerial photos is many times faster and more eco-
nomical.
Accuracy, Precision, and Scale
Accuracy is the closeness of a measurement to the real world; it de-
pends on the scale at which photographs are taken and interpreted and
the scale at which these measurements are plotted. In general, the
larger the scale of a photo negative, the more accurate a final photo-
grammetric product will be. Precision refers to the distribution of the
values of a number of samples of a single parameter about the mean
value in other words, the replicability of a measurement. Precision
in photogrammetry depends primarily on distortions or errors asso-
ciated with the photography, the plotting device used, and the plotter
operator. These errors in turn depend to a certain extent on the scale
of imagery end map plotting, especially the variation from one inter-
pretation of a stereo pair to the next by the plotter operator. Accuracy
is important in photogrammetric mapping; accuracy and precision are
vitally important in the sorts of comparisons that constitute photo-
grammetric monitoring of architectural targets.
The scale at which controlled photographs of a site or structure are
taken must be decided on the basis of a balance between the constraints
imposed by reality t economies and the capabilities of equipment) and
the needs of the researcher (problem orientation). It is not practical,
nor is it very definitive, to ask for "as large a scale as possible," and
the scientist or manager who does so probably has not thought seri-
ously about his need.
If photogrammetric products and data are to be used for accurate
reconstruction of a structure, the accuracy to which building materials
can be fabricated will be a criterion for detemi~ning scale. When a
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Photogrammetr~c Measurement and Monitoring of Structures 247
structure is being recorded for monitoring purposes, questions of how
much its fabric changes cyclically {diurnally, for instanced and what
sort of change would signal impending collapse or irreparable damage
must be asked.- Answering these questions may require relatively high
accuracy. Theoretical or scientific questions, on the other hand, may
not be so demanding. If one were to survey the ethnoarcheological
literature for predictive formulae for determining the population of
structures, for instance, one would find that the variance in estimates
and observations is high enough to make the use of any photogram-
metry at all questionable.
Photogrammetry need be no more accurate than the problems to
which it is to be applied warrant. In applying this rule to the deter-
mination of scales, it should be remembered that to double the ac-
curacy at which a map can be plotted, one must double the photo-
graphic scale and therefore quadruple the area that must be photographed
and interpreted. Necessarily, there is a corresponding increase in cost.
Precision- or replicability of photogrammetric results, which is nec-
essary for comparing periodic data from a specific site or structure, is
a problem. By permanently- marking and reusing control points and
employing the same metric cameras, camera positions (difficult from
the air), and plotting equipment, a certain amount of comparability
between maps can be ensured.
Another problematic element is inconsistency on the part of the
plotter operator. Photogrammetric maps are interpretations of photo-
graphs, accurate ones to be sure, but including different amounts of
detail depending on the inclination of the human interpreter. This is
especially apparent with contour lines drawn on two maps of the same
target. Topographic contour lines are very difficult for the same op-
erator, using the same imagery, to duplicate exactly. Point locations,
on the other hand, can be duplicated by different interpreters with
surprising accuracy. For this reason, it is advantageous to locate specific
points to be used as the basis of monitoring a structure rather than
attempt to use a contour map of, say, the face of a wall as monitoring
data. This requires the selection of conspicuous, relocatable points
(along the top of a wall, at the corners of windows, etc.) and may
require that permanent markers be placed at these points for some
structures.
Orthophotography
An orthophoto is essentially a photographic planimetric map. It is
produced by a machine that not only does radial line plotting, but also
uses parallax measurement as the basis of changing the scale of very
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248
CONSERVATION OF HISTORIC STONE BUILDINGS
small segments of a monoscopic photograph as these small segments
are exposed on a film. This corrects for scale differences throughout
the photograph and creates a planimetrically correct print of a scene.
Orthophotos are more useful than planimetric maps for many purposes
because there has been no selection by an interpreter of what points
in the scene are of interest; all points Within the limits of resolution
of the photograph) have been shown, and details not available on a
map can be distinguished at a later date.
A combination of an orthophoto and a photogrammetric line map
can be made by first compiling an orthophoto and then using the same
controlled stereo pairts) on a photogrammetric plotter to produce a
topographic contour map at a desirable scale. The line map is then
photographically overlain on the orthophoto (in negative form white
map lines against the darker orthophoto, Figure 21. Such a product
combines the advantages of both of its parts: Lan~narks and other
details can be found and used for measurement on the orthophoto,
while quantitative, three-dimensional data are available from the top-
ographic map lines. An orthophoto/topographic map combination is
especially useful as an aid to plarlIiing survey, excavation, or conser-
vation activities in the field.
Ground Control
To relate the three-dimensional coordinate values of each point pho-
togrammetrically measured in a stereo mode! to other measurements
from the real world, it is necessary to mark and measure a few control
points on the ground or target to be photographed before the photos
are taken. Ideally, three or more horizontal and four or more vertical
control points are set and marked so that they will be visible in both
of the photos forming a stereo pair (Figure 31. The distance between
the horizontal control points is measured, and the difference in ele-
vation of the vertical control points is established. This allows the
operator of the photogrammetric plotter to determine the precise scale
of the photos and insert them (printed on glass plates rather than paper,
and called diapositives) in the plotting machine. If control is not set
prior to taking stereo photographs, certain conspicuous points on the
ground can often be "photoidentified" in the image and then located
and measured on the ground, serving as control points after the fact
(Figure 41.
OCR for page 249
Photogrammetric Measurement and Monitoring of Structures 249
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FIGURE 2 An orthophotograph of a Pueblo-m Period masonry site in San Juan County
New Mexico, with superimposed topographic contours. The orthophotograph is essen-
tially an aerial photo corrected to show a planimetric view of the site while also con-
veying details inherent in a photograph. Topographic contours were compiled using a
stereo plotter and photographically superimposed over the orthophoto, thus supplying
metric information as well. Such renditions are especially useful as base maps for plan-
ning and executing fieldwork. SOURCE: Koogle and Pouls Engineering, Inc., Albuquerque,
New Mexico.
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250
CONSERVATION OF HISTORIC STONE BUILDINGS
_
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Stereo Coverage
FIGURE 3 Minimal horizontal {triangles) and vertical ;circlesJ control points required
to be marked and measured in a stereo model. When flight lines are being used as the
basis of mapping, it is sometimes possible to use fewer control points and still maintain
photogrammetric control by "bridging.". Control points are located in the field prior to
flying and are marked with stakes and plastic panels so that they are visible in the
photographs taken later. The distances between horizontal control points, and the dif-
ferences in elevation between vertical control points, are then measured. Control points
can also be tied to known data points if desired.
Aenal (Far-Range, vs. Terrestnal (Close-Range, Photogrammetry
Aerial photographs are the staple for far-range photogrammetry. -They
are used every day by engineers, planners, geologists, cartographers,
and many others for the development of topographic and planimetric
data. Aerial photographs are taken in scales ranging from about 1:200,000
(small-scale photography) to 1:500 (large-scale photography). Equipped
OCR for page 251
Photogrammetr~c Measurement and Monitoring of Structures 251
_.
FIGURE 4 An aerial photograph of the Barbourville Mansion in Virginia, a brick struc-
ture designed by Thomas Jefferson and destroyed by fire in 1844. The L-shaped white
markers in the corners of this scene are ground control panels, set prior to overflight.
If these panels had not been set, it would have been necessary to photoidentify points
and measure the distances between them later. Examples of points that are well defined
and could be used for "subsequent control" are shown at A and B.
with the most commonly used aerial cameras, airplanes cannot fly
slowly and low enough to take larger images. Using a first-order plotter,
a photogrammetric map scale of 1:100 (1 in. = 8.33 it) can be obtained
using 1:1,000 scale imagery at a contour interval of as little as 0.1 ft
t3 cm). The use of aerial photography for photogramrnetry offers the
possibility of mapping very large areas inexpensively but at low res-
olution and smaller areas more precisely up to this limit. Because of
the design of most aerial plotters, oblique aerial photography is of little
utility in photogrammetry.
OCR for page 252
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historic stone
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264
CONSERVATION OF HISTORIC STONE BUILDINGS
to be considered. In general the larger the target or the number of
targets covered during a single mission, the more cost effective the
mapping method.
It is much more difficult to determine the cost effectiveness of close-
range photogrammetry. Again, however, the more the required detail,
the more apt the method is to provide savings over manual or standard
surveying and recording techniques.
APPLICATIONS RESEARCH
Monitoring
Photogrammetry, as illustrated in this paper, has proved an efficient
means of accurately recording architectural data and is being used
experimentally for monitoring historic and prehistoric sites and struc-
tures. We would like to suggest, however, that photogrammetry in its
strictest sense—the use of controlled photography taken with metric
cameras and converted maps and elevation drawings or other photo-
grammetric products—will be supplemented in the future by a number
of other remote-sensing measurement methods. Because structural
monitoring is a historic process, entailing the use of previously col-
lected baseline data and comparison with subsequent data sets, it is
vitally important that future means of recording and monitoring be
compatible with past photogrammetric data bases. The first step in
this process was initiated at Pueblo Alto in Chaco Canyon National
Monument, a ruin of the Pueblo-III period occupied during the eleventh
and twelfth centuries A.D.
After the masonry walls in Pueblo Alto were stripped of their
overburden of sand and building debris, controlled aerial photographs
were taken and a detailed map of the exposed structure was plotted
Figure 121. Following excavation and the exposure of additional ar-
chitectural features and after reidentifying and panelling of the original
ground control, the site was again overflown. This imagery was placed
into the plotter and projected onto the original map. It was not nec-
essary to remap totally the target but simply to add new details or
modify old ones (Figure 131.
This additive process suggested the feasibility of projecting past base-
line data onto the original photogrammetric map or drawing and check-
ing for any changes or modifications to the original draft. Experiments
have begin to determine the type, degree, and frequency of change
due to natural or human impact on fragile masonry structures. Plans
and elevations of historical Barbours Mansion, designed by Thomas
Jefferson and burned in the mid-1800s, and of the prehistoric Tower
Photogrammetric Measurement and Monitoring of Structures 265
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FIGURE 14 The west elevation of the Kin Ya'a tower kiva. The ruin of Kin Ya'a,
originally built of native sandstone, lies within Chaco Canyon National Monument in
New Mexico aIld probably dates to A.D. 100~1175. Variations in the wall surfaces are
measured against a vertical datum plane, and edge details are carefully drawn in by the
plotter operator. SOURCE: Koogle and Fouls Engineering, Inc., Albuquerque, New Mexico.
266
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Photogrammetr~c Measurement and Monitoring of Structures 267
Kiva at the Kin Ya'a site in New Mexico have been produced and will
serve as the baseline data for future monitoring Figures 14 and 151.
Both controlled line drawings and digitized point data derived by pho-
togramrnetric means can be used in this method of monitoring struc-
ture change.
The frequency of monitoring these features by photogrammetric
means is dependent on rate of change. To determine this, aerial and
terrestrial photography will be reacquired (for example, every 6 to 12
months). Further, it is anticipated that planned mining activities in
the Kin Ya'a area will have a negative impact, and during this activity
the periodic rechecks wfl] be more frequent. It is obvious, of course,
that effects of earthquakes, fires, or other disasters will be checked as
soon as possible.
One monitoring method that is practicable today is the use of elec-
tronic distance measurement (EDM) equipment for the repetitive mea-
surement of points on a structure. EDM devices make use of either a
laser beam or microwave emitted from a transmitter, reflected from a
prism or a point on the object to be measured, and then received at
the transmission point. The time taken for the beam to reflect and
return is accurately measured and, when atmospheric pressure, tem-
perature, and humidity are corrected for, reveals the exact distance
from the EDM to the target. Such equipment is used widely for sur-
veying today, and typical accuracies are +0.5 cm in 1 km.
Such devices could be used to measure the distance from a fixed
station to a number of fixed points on a structure. Points to be measured
would be located with respect to their importance in the integrity of
the structure that is, points where a dimensional change would fore-
tell significant alteration or collapse. Such points could be unobtru-
sively provided with sockets into which a standardized prism would
be fitted. A measurement would then be taken from the permanent
instrument station (perhaps a socket into which the instrument would
always be fitted for measurement) and the prism moved manually to
the next point. While more field time would be required for such
measurement than with aerial or terrestnal photogrammetry, far less
laboratory time Photo developing, instrument setup and plotting, etc. )
would be expended.
Many monitoring situations may not require the measurement of
the potentially large number of three-dimensional point coordinates
available in stereo photos, and EDM monitoring of a relatively small
number of highly critical points on a structure or site could greatly
speed the monitoring process. It would be feasible, for instance, to
monitor a small number of points on a structure each day, or con-
268 CONSERVATION OF HISTORIC STONE BUILDINGS
ceivably even several times a day, if this were deemed necessary. In
addition, data collected with EDMs WOUi] be compatible with point
data plotted photogramrnetrically.
ZVlicrowave Scoping
A microwave scanner, such as side-Iooking airborne radar or synthetic
aperture radar, is essentially an EDM that scans the entire surface of a
target area systematically and compiles a matrix of data on the distance
from the scanner to the target. Although one commonly sees this
matrix represented as a pseudophotographic product, it is recordable
in digital form as well. If properly controlled, the digital output pro-
duced by a ground-based microwave scanner could have certain ad-
vantages over camera photogrammetry. A computer could produce
almost instantly a comparison map of two digital point matrices re-
corded from the same site or structure at different times; it is possible
that pattern recognition programs could be developed that would reg-
ister the two data matrices automatically with manual recourse to
control points.
Ground-based microwave scanners that would allow the scanning
of specific structures with sufficient accuracy and resolution have not
been developed, and a prototype program probably would cost millions
of dollars. But this does not negate the possible use of microwave
scanning to monitor structures and sites. If we are serious about de-
veloping more efficient and effective methods for clearing with our
historic heritage, a preliminary feasibility study might well be in order.
Holography
An intriguing process using laser technology for the measurement of
physical objects, and the three-dimensional reproduction of their form,
has appeared recently in the guise of holography. This technique entails
focusing a split laser beam at an object from two directions; reflected
light is then exposed on a photographic plate. Projecting a laser beam
or other coherent light back through this plate, which contains no
obvious image when viewed in normal light, reconstructs a "wave
front" image of the object in three dimensions. The hologram can be
viewed from different directions, and parts of the object that are hidden
from one angle appear from others.
Holography has been used to record historic artifacts at the Smith-
sonian Institution and might offer a means of recording and monitoring
historic structures as well. Before this could be accomplished, however,
some startling jumps in laser technology would have to occur. To the
Photogrammetr~c Measurement and Monitoring of Structures 269
present, holograms have been made only of relatively small objects,
the largest being humans. The only lasers with enough power to record
larger things are those currently being experimented with by the mil-
itary for purposes far removed from historic preservation. Another
problem with holography for the measurement of large structures is
that during the exposure of the hologram any movement within the
scene (even on the order of the size of a wavelength of light) will disturb
or ruin the plate. Ever-changing historic structures might be difficult
to stabilize this perfectly for even an instant. Nonetheless, we predict
that holography will become a useful aid to recording and monitoring
structures and cultural resources in general.
Reconstruction
Digitized three-dimensional photogrammetric data can be used for
more than simple representation of a structure or comparison with
baseline data for monitoring purposes. In many cases the archeologist
or cultural resource manager is concemed with structures that already
have suffered considerable destruction or fabric change. Many sorts of
theoretical statements are made on the basis of the forms of structures.
These include population estimates, assumptions of the importance
of communities in a trade network, and guesses about the degree of
affluence or "refinement" of occupants. When making such assump-
tions it is important that one know the original form of the structure,
which is difficult to divine in certain instances. At Chaco Canyon
National Monument in northwestern New Mexico, for instance, a
matter of conjecture is the number of stories that pueblo ruins origi-
nally had. An experiment under way at the Remote Sensing Division
may help to solve this problem.
Pueblo Alto Figures 12 and 13), will serve to illustrate this approach.
As previously mentioned, the aerial imagery obtained served as the
basis of photogrammetric mapping. During the course of this work,
some 7,000 points were digitized in three dimensions and recorded on
computer cards. Points along the tops of walls were digitized at each
significant change in wall height, and other points were chosen within
exposed rooms to establish the depth of rubble fill. Using only the
wall-top points, a model of the ruin can be reconstructed by the com-
puter.
These wall-top points are being combined with data on rubble fill
and information derived during test excavation in an attempt to re-
construct the configuration of the original structure. While computer
reconstruction cannot produce a single "true" picture of the original
configuration of a pueblo, it can help suggest plausible alternative
270
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CONSERVATION OF HISTORIC STONE BUILDINGS
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FIGURE 16 Computer perspective drawing of a small portion of the Pueblo Alto ruin.
This drawing, constructed with a 10 x 10 mesh data resolution, shows little detail.
SOURCE: Joe McCharen, Civil Engineering Research Facility, University of New Mexico.
designs upon which theoretical statements can be based (Figures 16,
17, and 181.
SUMMARY
Both aerial and terrestrial photogrammetric techniques are invaluable
for documenting historic and prehistoric masonry or other forms of
architecture in plans and elevations with a range of detail and accuracy.
As with any procedure, of course, there are limitations to the usefulness
to
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FIGURE 17 A more detailed, 50 x 50 mesh resolution computer perspective drawing
of the sense portion of Pueblo Alto appearing in Figure 16. Note that "hidden lines" are
actually hidden in this rendition. Wall thickness and the irregular profiles of the partially
ruined walls are apparent.
Photogrammetric Measurement and Monitoring of Structures 271
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FIGURE 18 Employing digitized wall-top profiles such as those appearing in Figures
16 and 17, as well as room-fill data, the computer has reconstructed the four-room
section of Pueblo Alto as it may have appeared when in use. VVEile this experiment is
obviously on a small scale, reconstructions of entire prehistoric or historic structures
could be postulated using similarly handled digitized photogrammetric information.
Of available technological methods for assessing conditions and changes
and for establishing baseline or quantifiable data upon which to for-
mulate decisions for preservation efforts.
Digitized photogrammetric point data in conjunction with computer
programming gives the architect or archeologist a three-dimensional
too] for analysis of structures. Digitized structural data also are useful
for monitoring change or the effects of Herman or natural forces on
fabric.
Research in the application of microwave scanning, holography, and
three-dimensional, computer-aided structural reconstruction also may
offer the architect new tools for ensuring that the integrity of historic
and prehistoric structures is protected.
Finally, the methods described in this paper are nondestructive or
ninimally hat to architectural materials. The techniques are at-
tractive, of course, because one method of conservation especially nec-
essary in archeology is the hands-off approach.
REFERENCES
1. Borchers, P.E. 1977. Photogrammetric Recording of Cultural Resources. National
Park Service: Washington, D.C.
2. Lyons, T.R., and T.E. Avery. 1977. Remote Sensing: A Handbook for archeologists
and Cultural Resource Managers. National Park Service: Washington, D.C.
3. Wolf, P.R. 1974. Elements of Photogrammetry. McGraw-Hill: New York.
