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Applying GPS to Historic Preservation and

Architectural Surveys

Map of Ft. Washington, MD

Increasingly new technologies are expanding the horizons of historic preservationists, making their fieldwork and research more efficient and their contribution to regional planning more significant. As global positioning systems (GPS) technology grows in effectiveness as well as accessibility, it will also become a major tool for cultural resource management. At the Cultural Resources Geographic Information Services (CRGIS) program, National Park Service (NPS), we already explore the use of GPS, in combination with geographic information systems (GIS), to better manage and protect the resources within park boundaries. As part of our efforts, we engaged in this new project to determine the utility of GPS in architectural survey. Our goals included the investigation of how GPS could enhance and improve the survey process, as well as the development of methodologies which could be used by others.

For historic preservation and cultural resource management, accurate locational data is key to our success in learning about past building traditions, settlement patterns and past lifeways. Identifying and following trends on the landscape requires that we can locate resources on the ground. In addition, for management, conservation, and physical preservation of these resources, we also want to know the environmental and human influences that might pose any adverse affects to specific resources. Collecting the locational data with GPS as well as basic attribute information associated with individual features, and combining that with the power of GIS to integrate different data sources, allows preservationists to take advantage of these technologies in planning and researching cultural resources.

Map of Ft. Washington, MD

Architectural Survey and Documentation

Architectural survey and documentation form the cornerstone of historic preservation efforts. Resources identified during the survey process provide the information base from which planners, cultural resource managers and other preservation professionals make decisions, develop theories and study material culture through structures.

Surveys are conducted through State Historic Preservation Offices (SHPOs), Tribal Historic Preservation Offices (THPOs), Certified Local Governments (CLGs), and Federal Agencies who are all tasked with developing and maintaining an inventory of historic resources. For the National Park Service, this inventory would include the List of Classified Structures (LCS). The National Historic Preservation Act of 1966 (PDF), which created SHPOs/THPOs and CLGs, requires that any agency, including the National Park Service, receiving Federal monies for development conduct surveys of cultural resources, both architectural and archaeological, which may be adversely affected by the construction or disturbance. Surveys may be conducted at different scales, and collect different information about each resource, depending on the agency conducting the survey and their goals. For the most part, these surveys take the form of either a reconnaissance or intensive level of data collection. All architectural surveys collect basic information regarding structures over 50 years old including basic conditions, integrity, form of the structure and identifying features. Reconnaissance surveys take a more cursory look at a buildings collecting only the minimum of data, while an intensive level survey would involve investigating a structure's interior, associated features, as well as the background and history of a structure or group of structures.

Once resources have been identified as significant through the survey process, the documentation process insures that no information encapsulated within the resource is lost if destruction is imminent or the resource is threatened in some way. Conducted at differing levels of intensity similar to the survey itself, documentation of a structure might include a variety of activities ranging from a series of photographs capturing the significant features of the resource, to the creation of measured drawings accurately reproducing the structure on paper, to the construction of a detailed history of the property through historic documents.

In particular, measured drawings which depict the structure in floorplans, elevations, sections, or site plans, enhance our understanding of a resource. Placing these drawings with real world coordinates using GPS and associating photographs, documents, and survey information, as well as related spatial data through GIS, offers preservationists a new way to examine these resources in relationship to each other or individually, reestablishing the historic context of these structures and making them an even more informative research tool.

Ft. Washington, MD

Purpose of our Project

The purpose of our GPS architectural survey and documentation project was to investigate the utility of GPS for architectural survey and to expand our experience mapping cultural resources with GPS, but also to develop some useful methodologies which other national park units, states, organizations and researchers could apply to their own work. We approached this investigation of GPS in preservation from two different perspectives, one through architectural survey, and one through documentation.

First, we chose to add the use of GPS to the standard reconnaissance architectural survey to determine if we could effectively capture the required data and reduce the amount of time spent in the survey process. Second, we chose to add the use of GPS to standard documentation practices to determine the utility of geo-referencing site plans depicting the location of a variety of historic resources in relationship to each other. To accomplish this, we worked with the Historic American Building Survey (HABS), a branch of the National Park Service, on a project to document Fort Washington, a nineteenth century fortification outside of Washington, DC.

Fort Washington Park allowed us to pursue the use of GPS with both architectural survey and documentation. The HABS documentation team created a detailed site plan of the fort, including the ramparts themselves, as well as the various related structures within the walls of the fort. In conjunction with the HABS efforts, we collected locations on both individual resources such as gun emplacements within the fort, and significant points of reference such as the location of transits used in the documentation itself. Additionally, throughout the fort and associated grounds, we collected locational and survey data for building footprints, as well as single points on structures within the shadow of the walls.

Map of Ft. Washington, MD

Methodology Employed

Our basic methodology employed a combination of standard procedures for conducting both a GPS survey, and architectural survey and documentation. We began by assessing the probable end uses of the data for both of the projects. Once we determined this, we decided the type and the level of positional accuracy we would need for the end data and what features and attributes we would need to collect in the field. Once the features to be collected were defined and the associated attributes were determined from the National Register of Historic Places form (a national standard), a data dictionary was developed.

For both the architectural survey and the site plan geo-registration we determined that the collection of accurate vertical data would not be necessary, at this point the end use was only to look at the X,Y location of the resource in a GIS format. Other sources of data, ranging from USGS topographic maps to digital elevation models, would be used to get elevation as needed.

In the case of the architectural survey the foreseen end use was the use of a point or polygon to represent a building and place it on the X,Y plane of a USGS 7.5' topographic quadrangle map. In the context of the 7.5' topo quad, with a map accuracy standard of +/- 12 meters, the data could be collected and differentially corrected to a +/- meter, mapping grade, accuracy.

For the architectural survey we chose to use the National Register Registration Form, developed by the National Park Service to define the associated attributes to be collected. The attributes could also have easily been defined from the LCS, more familiar to park units, or any other relevant sources. Once we had determined the features to be mapped and the associated attributes we wished to collect for those features we developed a data dictionary. The specific unique features we were targeting were a point or polygon, representing either an individual building location (point) or an individual building footprint location (polygon). Only sections from the National Register registration Form that included menu picks and short answers were included in the data dictionary. Sections that called for descriptive narratives or sketches/drawings will still be completed on a hardcopy form or laptop computer. It should also be noted that developing a data dictionary is an iterative process and changes will, and should, be made to reflect knowledge gained through use.

The end use of the site plan registration was a little more complicated, the uses ranged from simply showing the real world relationship of numerous planimetric drawings of the cultural resources at a site to using the drawings to do query and analysis of the GIS data. Another consideration in choosing the accuracy of registration points is the fact that many of the plan drawings of cultural resources are at a large scale, inches representing feet, and have a high level of detail. In view of this it was decided to record carrier phase to allow the collection and differential correction of data to a decimeter, survey grade, accuracy.

Minimal associated attributes would be collected since the GPS data would be used to register the drawings only and would probably not be part of any query and analysis. This data could easily be taken using the generic point feature in the default data dictionary. We used a simple point feature with the attributes being a point identifier, simply a number, and an optional comment.

Trimble ProXL ™ and ProXR ™ 12-channel GPS units were used for all fieldwork at Fort Washington. The post-processing and editing of the data was completed in Trimble Pathfinder Office 2.01. The data was exported as a sample ArcView shapefile setup for use in ESRI 's ArcView ™ 3.1 GIS software, but the use of Pathfinder Office ™ allows for the data to be exported into various formats for use in other software package.

During the architectural survey project fieldwork we collected the locational data as both point and polygon (as a building footprint). The same attribute data was collected for both types of features, a version of the National Register Registrtion Form. We also filled out the complete hardcopy version of the survey form to compare the times for completion of both formats. In addition, to the architectural survey, we took ancillary data such as streets/roads.

Ft. Washington, MD

Results of the Survey and Documentation Projects

The results of our field trial were mixed, yet they did provide us with good information about the utility of GPS for architectural survey and documentation, as well as helping to define what we could realistically expect from GPS technology for preservationists. We did find that overall, GPS could be an extremely useful tool for architectural surveyors, however because of particular constraints on this type of survey, users must take into account some significant drawbacks to using this technology. The use of GPS for geo-registering site plans and other measured drawings, proved even more successful.

With the survey process, We found that, because of the height of buildings or in our case, the height of the solid masonry fortification walls, reception of the satellite signal was often blocked. Taking building footprints was difficult and time consuming on all the buildings we attempted in addition. We encountered physical barriers, fences, outbuildings, etc, when attempting to map building footprints as well. The blocked reception made it necessary to use the offset option to take points on a majority of the buildings, although not all buildings. Offset distance and bearing were measured using a tape and compass; the use of a laser rangefinder would have made the measurements both more accurate and quicker. The use of the point feature to map the locational data of the architectural survey worked each time attempted and the time it took to complete was minimal. The data collected lined up well after post-processing and export and when overlaid on a U.S. Geological Survey (USGS) Digital Raster Graphic 7.5 topo quad.

When taking the attribute data we found no significant time difference between entering the data in a data logger by keyboard and entering the data on the traditional paper survey form. In addition the data is already in a digital format, eliminating the manual entry into a digital database format. This data can then be exported into a database format from Pathfinder Office.

Additional time is saved by having the building located in real world coordinates with meter accuracy, something traditionally done with a measuring device off of a paper map (often a UTM counter and a 7.5' topo quad map). The accuracy issue can not be emphasized enough, eliminated is the built in map accuracy standard error allowed in the map and the human error that creeps in while taking measurements manually. Using the site plan drawings while in the field, matching points on the ground and the site plans were chosen as registration points to test the use of GPS for site plan registration. The points were chosen on the criteria that they appear very clearly on the plans and in the field and are widely separated around the site. Carrier phase mode was used to record the registration point feature in order to get high, decimeter, accuracy. To successfully record in carrier phase mode the antennae must remain stationary, mounted on a tripod, for the entire time of occupation (we used the Trimble suggested occupation time of 10 minutes), and at least four satellites must be visible during this time.

Because of these limitations, Fort Washington was a perfect site to test this project, it is a walled masonry fort on the crest of a hill and the walls are higher than surrounding vegetation and buildings. A clear line of site was available on top of the fort to maintain reception of visible satellites.

Once registration points were chosen the set-up was very straightforward, options are entered in the datalogger; phase soft key in GPS Rover Options. The antennae height must be entered to get an accurate measurement, this is only required when recording in carrier phase mode. Carrier phase mode can be made optional, with a prompt appearing before a point feature is recorded.

The registration points that are recorded are then used to geo-register the site plan drawing. We used ArcView 3.1 GIS software to register the drawings. The software you use will determine the accuracy of the geo-registering. In our case the site plans were in a CAD format (.dwg) and the registration in ArcView was done by referencing only 2 points (a .wld file was linked to the drawing); the degree of accuracy was therefore relatively low.

The accuracy of geo-registering can be improved by using software that allows for the use of numerous points. The accuracy needs of the user need to be determined before the data is collected, you may not need to record points in carrier phase mode for decimeter accuracy, and when deciding on a geo-referencing strategy.

Map of Ft. Washington, MD


This initial use of GPS in architectural survey and documentation was successful in producing meaningful results. We found that we can improve on locational accuracy in the architectural survey process, and in fact produce locational data where surveyors may not have collected this information, even if only a single point (rather than a building footprint) is collected. In addition, this process of collecting locational data does not significantly increase the amount of time spent in the field and it greatly reduces the amount of time surveyors may spend recreating their paper forms in a digital database after the survey is completed.

Working with HABS/HAER representatives, we also concluded that geo-referencing site plans added significantly to the interpretation of historic resources. Through the referencing process and the use of a site plan in a GIS, architectural historians and conservationists were able to locate features, determine treatments, and form various hypothesis' regarding the spatial relationships between elements of decay or potential threats to features of the fortifications.

Global Positioning System technology plays a role in cultural resource preservation and it will become even more important as geographic information systems become more widely used by cultural resource professionals. The strategies for the use of GPS that we present are only two of many uses for this technology in the field, and they are only models and need to be perfected and tailored for individual projects. It must also be emphasized that GPS is only a tool, one of many, and this tool is only as good as the user. Care must be taken in all aspects, from pre-planning to data collection to post-processing.

Based on our experiences with these projects, the dynamic combination of GPS and GIS can make architectural survey and documentation a more efficient and useful process where preservationists and planners can work together with other cultural resource managers. The methods we used can proactively affect the preservation of structures in addition to the ability of preservationists to understand and manage their resources. In the future, these technologies will play a critical role in capturing cultural resource data, maintaining that data, managing the resources themselves, and planning for the proper treatment of these resources.