Using GPS in Archaeological Field Work

Obtaining 3D data for excavation units and for artifacts and features within the excavation units is traditionally conducted by use of theodolite or total station to establish a datum point and an intersecting baseline. Locations of in situ artifacts and features are established in relationship to the datum point. During excavations, artifacts and features must be continuously measured from the baseline to establish 3D position and carefully documented prior to removal or destruction -archaeology is, by nature, a destructive process.

In recent years, several studies have been conducted to explore the use of GPS technology in the field of archaeology, which provide both innovation as well as efficiency in data collection.

GPS was used successfully in establishing site location or for creating a baseline for excavation units in several of the studies. Establishing site locations on dry land, however, comprised most of the recent archaeological work that took advantage of GPS technology. Chapman and Van de Noort (2001) used GPS experimentally to determine if the use of differential GPS (DGPS) was a viable method of surveying points in archaeological prospection of wetlands on the British Isles. By noting differential desiccation, that is, the different rates at which the ground dries, in wetland environments, manmade features can be discerned that cannot be observed by normal aerial reconnaissance. Chapman and Van de Noort demonstrated that DGPS allowed for a much quicker and efficient collection of 3D data than with traditional optical methods. Moreover, the DGPS system they used required only a single person rather than two and the data collected were more easily imported into GIS software from which excavation trenches were planned. Once the locations of the trenches were planned, they were positioned using, again, the DGPS equipment. Their conclusion was that GPS used with GIS is now a proven technique in archaeology.

In South West Turkey, Martens (2005) discussed the use of GPS in establishing an excavation grid of a Roman site in which a surface survey, an observation and evaluation of artifacts and ecofacts, was conducted. For the grid, GPS was used to establish the top row’s corners and the remaining units were laid out using triangulation based on the Pythagorean Theorem. Once GPS was used to establish the corners, a compass and tape were all that was required to lay in the remaining grid squares.

In North Kohala, Hawaii, an archaeological team used two Trimble Pathfinder GPS 8-channel Pro XR receivers to create plan maps of at least two sites. Their method was to collect the data and post-process it with correction data downloaded from the National Park Service base station though they did experiment with using one of their receivers as a base and the other as a rover. In doing so, they discovered that the base would need differential correction before the data could be considered accurate enough for their purposes. The team used the GPS to collect data points as they walked around major features such as walls, pits and terraces, creating a map that could be printed out and modified by sketching in additional information. The final result was production of plan maps of a residential structure and a religious temple called a heiau, both of which were completed four times as efficiently as would have been done with traditional methods which include tape and compass or with plane table and alidade. Ladefoged and his team also discussed the problems they encountered in their use of GPS. Among these was the failure of the equipment to obtain a signal if the sky overhead was obstructed by trees with branches more than 12 centimeters in diameter. When a signal was obtained under heavy vegetation, it wasn’t used because of the degradation due to multipath errors.

Multipath errors arise in GPS when the signal from the satellite is bounced off of a building or ground before reaching the receiver. A large tree would interfere with the most direct signal, leaving ones that bounce from the ground or other objects first. The result is a signal that is degraded slightly, skewing results.

Altai Mountains of Western Siberia
GPS has also been used successfully with satellite imagery in mapping and planning archaeological sites. In an experimental study conducted in the Altai Mountains of Western Siberia, Goossens et al (2006) tested three different GPS systems, comparing their implementation and results. The first system tested was a duel-frequency DGPS made by NavCom Technologies which used a real-time correction network transmitted to the receiver by satellite. This allowed correction of both ionospheric and tropospheric delays, giving horizontal accuracy of about a half a meter with accurate elevations to just under a meter. The other two systems Goossens et al tested were off the shelf handhelds, the 12 channel Garmin GPS 12XL and the 8 channel Motorola Oncore, which they post-processed with computer software and correction data obtained from nearby reference stations. The handheld models provided accuracy at 1-2 meters after correction, making the DGPS the most reliable and efficient of the three since corrections were real-time.

Neogene period clays
GPS has also found its way into archaeological research through necessity. Hein et al (2004) discuss the research of Neogene period clays in order to understand the geochemical makeup of these clay deposits in Crete, where Greeks of antiquity obtained their raw materials to produce ceramics. Knowing the geochemical makeup of these clays can aid in determining the provenance of ceramic artifacts. Determining which deposits pottery originated from can allow inferences to be made with regard to trade patterns in antiquity. Hein and his team obtained 61 samples from 28 different Neogene clay deposits in Crete, using a Magellan GPS-3000 XL, which offered uncertainty up to 100 meters. However, the authors chose the GPS to provide reproducibility of their results, and precise location may not have been an important consideration for sampling a clay deposit since the individual deposits were kilometers apart.

The advantages of GPS over traditional methods as variously indicated to various extents by the authors of the studies cited above include that traditional optical methods (theodolite or total station) require two people and must maintain line of sight between the instrument and the prism. With a GPS, a single operator can collect data points very quickly and the data can be transferred relatively easily to GIS software or a database for later analysis and processing. However, GPS has limitations that must be taken into consideration. For instance, a clear view of the sky is needed to conduct a quality survey and dense forest, tall buildings, or even occasional large trees can affect results. In such cases, combinations of GPS and optical methods may still yield efficient results. A datum point can be established in a location of the site where accurate GPS data can be collected and then optical methods or tape and compass used to lay out the baseline and remaining excavation grid.

The future of GPS in archaeological applications will certainly include data collection, particularly as equipment becomes readily available to excavation teams. The ease of use, increased rate o
f data collection, the quality of data, and the ability for a single surveyor to collect data will be appealing to archaeologists seeking to maximize their time. In addition, the ability to transfer data from the GPS to a laptop in the field for processing and rendering to a map further simplify the survey process and, perhaps, eliminate errors in calculation that can occur with optical methods.

References

Chapman, H., & Van de Noort. (2001, April). High-resolution wetland prospection, using GPS and GIS: Landscape Studies at Sutton Common (South Yorkshire), and Meare Village East (Somerset). Journal of Archaeological Science, 28(4), 365-375.

Goossens, R., De Wulf. (2006, June). Satellite imagery and archaeology: The example of CORONA in the Altai Mountains. Journal of Archaeological Science, 33(6), 745-755.

Hein, A., Day. (2004, August). The geochemical diversity of Neogene clay deposits in Crete and its implications for provenance studies of Minoan pottery. Archaeometry, 46(3), 357-384.

Ladefoged, T. L., Graves, M. W., O’Connor, B. V., & and Chapin, R. (1998). Integration of Global Positioning Systems into Archaeological Field Research: A Case Study from North Kohala, Hawai’i Island. Society for American Archaeology, 16(1).

Martens, F. (2005, August). The archaeological urban survey of Sagalassos (South-West Turkey): The possibilities and limitations of surveying a ‘non-typical’ classical site. Oxford Journal of Archaeology, 24(3), 229-254.