Historically large-format photography is one of the tools used to document significant cultural resources. Recording a global positioning system (GPS) position and attribute data at the point each photo is taken enhances the documentation and management value of these photographs. Using this GPS base data to create a geographic information system (GIS) documents the information a manager will need to determine what photo-documentation they have, what areas are photographed, and provides a means to accurately re-photograph the site for monitoring purposes.
Background and Purpose
In it's 60+ years of existence the Historic American Buildings Survey (HABS) has used large-format photography extensively as a documentation tool. It's collection of over 175,000 photographs, along with other extensive documentation, is held by the Prints and Photographs Division of the Library of Congress. These photographs are produced under a rigorous set of standards; they must be black and white, between 4x5 and 8x10 inches in size, with 5x7 being the most accepted standard, and they must be taken with a view camera. This type of camera allows an experienced user to correct for distortions and to show a resource in its true perspective. There are also standards for the processing and handling of the photographic negatives and prints. The technical aspects of this type of photography go well beyond the actual mechanics of operating the view camera; consideration of the historic significance, construction processes, materials, and structural attributes of the resource must be considered when composing the photograph. Documentation of a historically significant site is the primary purpose of the survey, but it is also the goal of the photographer to produce an aesthetically pleasing product within the context of producing a permanent, useful, record. The management potential of this collection goes far beyond simple static documentation and inventory. Photographic documentation at various periods can show such variables as change and deterioration of the resource as well as change in the landscape surrounding the resource. In order to utilize this documentation it must be ensured that photographs are taken from the same vantage point at each occurrence. While a person might want a broad temporal variability the geographic location of the camera must remain constant. Up to this point information on camera location was held by the photographers and remained with them when they left or retired. With the advent of portable and accurate global positioning system (GPS) units and user friendly geographic information system (GIS) software the recording and display of camera positions, at least for outside shots, becomes a relatively simple matter. The GPS/GIS technology creates a legacy that remains with the photographs instead of the photographer. With this in mind Blaine Cliver and Jack Boucher, of the Historic American Buildings Survey /Historic American Engineering Record (HABS/HAER), approached Dr. John Knoerl, head of the Cultural Resources Geographic Information System (CRGIS) facility of the National Park Service (NPS), about collaborating on a project in Puerto Rico.
The NPS, in cooperation with the U.S. Army Corps of Engineers (USACOE), has undertaken work to stabilize and restore parts of the walls of the fortifications within the San Juan National Historic Site. This has been a ten-year, $45 million undertaking. Part of this work focuses on treating deterioration of the exterior stucco layer. Sandstone rubble and mortar were used to construct the walls. A sand, lime, and water stucco outer coating was then applied as a barrier to weathering and erosion. The present fortification walls are a result of over 400 years of development, which includes additions and demolitions over that period. In many areas of the fortifications the outer stucco layer is eroding or completely gone. There is archival evidence that replacement of the stucco was done as maintenance during the time of Spanish occupation. To determine the areas and extent of deterioration Jack Boucher was asked to re-photograph the walls from the precise vantage points he had used in documenting the walls and forts in 1960. In that survey photographs were taken of the forts El Morro, San Cristobal and El Canuelo and their related features. It was hoped that photographs from different periods would not only show where the deterioration was occurring but also alleviate concerns over the actual need for re-stuccoing.
Methodology of Photographic Survey Jack returned to San Juan on June 25, 1998 and, working 7 days a week, re-photographed the site, completing the survey on July 7, 1998. Copies of the 1960 photos, as well as prints taken in the 1940's and earlier, were used as guides. Before the survey the photographs were labeled with a number and then roughly located on site drawings. Once in the field the sites were toured with park staff and the original vantage point was located as closely as possible for each photograph. The cooperation of park personnel and the local knowledge they had was crucial to the success of the project; Paul Hartwig, the Superintendent, and Mark Hardgrove, Assistant Superintendent, extended every courtesy and showed real enthusiasm for the project. Without the help of the Maintenance Supervisor, Angel Diaz, many of the locations would have been very hard to find. When Angel was unable to accompany us as a guide in the initial search he arranged for one of his staff to help.
Finding the actual vantage point of the original photo was just the first step. Jack then sought to re-create, using different lenses, the extent of the structure seen in the photographs. Other factors considered in reproducing the shots were the quality of the lighting and angle and extent of shadows cast by the sun. Getting the appropriate level of lighting on the walls proved especially difficult, the original photos were taken at different times of the year with different sun angles, but was a crucial component in producing images that showed the same detail as the originals. During this survey more than 120 unique, over 240 total, photographs were taken from 88 positions on the ground, as well as numerous 4x5 aerial (helicopter) shots and photographs in which a US Coast Guard boat was used as a platform.
GPS Survey and GIS Development
During the photographic survey a GPS position and attribute data was recorded at the camera position; a feature called a 'photo point'. GPS is a combination of hardware and software, which used together with a constellation of 24 communications satellites orbiting the earth, can accurately calculate a position in map coordinates and capture attribute data related to those points at the same time. No other surveying system can produce the same accuracy combined with great speed. In addition, no other positioning methods can capture additional data about the coordinates collected, and attach this information to the physical location for use in a GIS. GPS works by triangulating a position on the earth using both the constellation of satellites and a portable receiver. Using signals broadcast from the satellites, the receiver calculates the distance from the position to at least four different satellites, allowing the receiver to locate itself, and thus the feature being mapped, on the earth's surface. James Stein used a Trimble ProXR unit, with real time differential correction, and a TDC-2 datalogger, a small hand-held computer, to collect the GPS data and record attributes. This GPS unit is accurate to within +/- one meter after the data is corrected for errors. A data dictionary was developed prior to fieldwork in order to simplify entry of attribute data for the feature being mapped. This data dictionary contained attributes that reflected items Jack would collect during a photographic survey. This will allow for the duplication of the field work by another trained photographer in the future. The attributes collected included a unique identifier for each photo, which corresponded to the photographic negative numbering system used by Jack. Other attributes were a location (fortification name), structure name, camera bearing, and a field for a text comment. The GPS unit also recorded date and time information, important for capturing the same manner and level of light and detail in future photographs. Additional ancillary feature data was collected. This ancillary data included locations of USACOE benchmarks, points for registration of aerial photos and measured drawings, fortification walls, and some roads. The collected data was viewed and edited daily and exported as an ArcView Shapefile format using Trimble Pathfinder Office 2.0 software. The photo point data collected in the field was combined with base data in ESRI ArcView 3.1 GIS software. The base data allows the photo points to be shown in the context of the fortifications and the city of San Juan. The base layers include a map of Old San Juan and environs based on CAD drawings, HABS/HAER measured drawings of the fortifications, aerial photographs of Old San Juan and the areas including forts El Morro and San Cristobal, and a USGS 7.5' digital raster graphic (DRG) of Isla de Cabras and the area of Fort El Canuelo. In the creation of this GIS the various base data layers were geo-registered to the Universal Transverse Mercator (UTM) projection, zone 19, using the collected ancillary data. The CAD (.dwg) files were registered through the use of a world (.wld) file and each layer was converted to an ArcView shapefile. The aerial photos and measured drawings, in TIFF (.tif) format, were registered using the REGISTER module in ESRI ARC/INFO 7.1 to generate a TIFF world (.tfw) file. Scanned copies of all available photographs were hot linked to the corresponding photo point in the GIS. The GIS software gives access to the location of the photo points on a base map, related attributes, and digital copies of all photos for that photo point.
The use of GPS/GIS technology to document photographic surveys provides a legacy for this valuable form of documentation and formalizes the corporate memory. Managers can use this tool to quickly determine what photo-documentation they have, what areas are documented, and to provide a means to accurately re-photograph the site for monitoring purposes. Time is saved in the planning and execution of subsequent photographic surveys, accurate repetition of camera placement is achieved, and the season and time previous photographs were taken can be repeated during these surveys. The long-term benefits for the management of resources is numerous and the precision of change and deterioration detection of and around the resource is greatly enhanced.
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