By Wan Jalil (MMA student)
Right after finding a site or an artefact, it should be in archaeologist mind-set to plot its location in any way possible. This could be done in many ways: by jotting it down on a map, putting a marker on it or by visual transit. However, with the advance in technology, plotting could be done by just a simple press of a button. Introducing the GPS device. The Global Positioning System or GPS is a satellite-based navigation system which is basically made up of a network of 24 satellites that orbit the earth around a specific time and route (Bowens 2009: 93). The GPS was originally intended for military uses, but in the 1980s, the United States government made the system accessible for public use.
How it Works
It would be too complicated for me to explain how it actually works. It involves daunting calculations, physics and sophisticated engineering methods. However, here are the basic of how the GPS works. The GPS receiver works in conjunction with 24 satellites. Those satellites circle the earth twice a day around their specific orbit route. Each satellite has their own specific signal and transmits them to a GPS receiver. The GPS receivers then take this information and use triangulation to calculate the receiver’s location (Bowens 2009: 94).
Then with the distance measurements from a few more satellites, the receiver can determine the user’s position and display it on the unit’s electronic map. The more satellites the receiver acquires, the better the accuracy and information will be given. A GPS receiver must be locked on to the signal of at least three satellites to calculate the latitude and longitude (White and King 2007: 63). And with four or more satellites, the receiver can determine a 3D position latitude, longitude and altitude. Once the position has been determined, the GPS receiver can calculate other information as well; such as bearing, track and the distance to a destination.
Using the GPS
Having used many different types of GPS receiver since my first archaeology field work in 2008, here are some basic tips to look out for in using the GPS receiver:
- Find an open space – signal disturbance can be caused by surrounding area (White and King 2007: 66). This occurs when the GPS signal is reflected off objects such as buildings, large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors.
- Give time – Some users save a point without looking into the amount of reception it receives. The lower the signal, the less accuracy/information it will give. So, give time to acquire the best signal as possible.
- The coordinate and measurement needed – This is important especially on the type of data needed. This can either be UTM (Universal Transverse Mercator), Degrees/Minute or Longitude and Latitude. There are programs that could convert the points. This is usually readily available on the internet for example GeoCalc and GPS converter.
- Datum – Different countries have different datum. For Australia, it is either WGS84 or GDA94. If this is entered wrong, a site that was supposed to be on the sea might end up somewhere in the desert!
- Record what you save – Saving way points inside the unit’s memory is not enough. Change the name of the point into a desired name to keep better track of it. Also keep note of it in a log book or a computer as a back up measure.
GPS in Maritime Archaeology.
Like watching the desert on land, it would look like there is nothing but sand. The same can be said about the ocean. Looking out to the sea, it would look like there nothing but vast open water. Trying to go back into a located shipwreck site underwater would be difficult, but not impossible. Before the GPS receiver was made public, maritime archaeologists had to use many techniques to solve the problem. An example is by compass-bearing, in which bearing points are taken of three or more identifiable object/structure and then line them up where the bearings intersect (Bowens, 2009: 88-90). This method is effective but time-consuming.
With the GPS, maritime archaeologist can easily return to a site just by typing in the coordinates. Coordinate points are also now obtainable through books and websites. An example is the shipwreck database that has been compiled by the Australian Government (Australian Government: Department of Sustainability, Environment, Water Population and Communities, 2011). Apart from it being a locator and tracker, the GPS can also be used as an indicator of artefact scatter. This is important in helping understanding site formation processes, especially in the sea, where there is dynamic water movement and animal disturbance. An experiment has been done on the glass bottlenecks of the Pearl wreck, Hawaii (Thomas, 2006: 8-9). These bottlenecks appeared to have shifted from the wreck site to a nearby lagoon. To see how the water movement and wave action affected the artefacts inside the lagoon, the bottlenecks were plotted. A year later, with the researchers taking to account the range-error of the GPS, it shows that it is practically in the same location as it was before. So what can be learn from this? With the possible improvement of GPS over the coming years, it has the potential to provide information where archaeologists could establish a more accurate search area to find artefacts.
Nowadays, maritime archaeologist turns to a much more accurate reading by using the DGPS or differential global positioning system. The DGPS works like a normal GPS, with the main advantage is the enhanced accuracy. DGPS has an average accuracy of 1 meter to 4 meter or less whereas a normal GPS have an accuracy of 2 meter to 20 meter (Bowens, 2009: 93-94). It is made possible whereby the satellite signal is augmented with designated references station on land. The reference point sends real-time correction in conjunction to the signal transmitted by the satellites improving the accuracy (Rudel et al, 2003: 40).
GPS has definitely made its mark on archaeology in general and is becoming an essential tool for maritime archaeologist. This can be seen in many recent articles where many archaeological works have been associated with use of GPS in many ways like surveying, mapping, locate, track etc. With the GPS development becoming more accessible and prominent, for example today’s smart phones could have a GPS functionality and also ROV/AOV with a built-in GPS receiver. It is safe to say due to its growing level of users, whether for archaeology or public use, this device will keep on improving and undoubtedly be used for many different kinds of archaeologial work.
- Australian Government: Australian Maritime Safety Authority, 2007. Differential Global Positioning System (DGPS) http://www.amsa.gov.au/Shipping_Safety/Navigation_Safety/Differential_Global_Postitioning_System/DGPS_Fact_Sheet.asp accessed 22 September 2011.
- Australian Government: Department of Sustainability, Environment, Water Population and Communities 2011. Australian national shipwreck database. http://www.environment.gov.au/heritage/shipwrecks/database.html accessed 22 September 2011.
- Bowens, Amanda 2009. Underwater Archaeology: The NAS Guide to Principles and Practice. 2nd ed. Blackwell Publishing, West Sussex, UK.
- Rudel, Murielle., Vidaling, Raphaele. And Wurst, Alain-Xavier 2003. Underwater Archaeology: History and Methodology. Periplus Publishing, London.
- Thomas, Lindsey. 2006. Site Formation Processes: Preliminary Study for Wooden Shipwrecks in the Northwestern Hawaiian Islands. NOAA Pacific Islands Region Maritime Heritage Program, Hawaii.
- White, Gregory G. and King, Thomas F. 2007. The Archaeological Survey Manual. Left Coast Press, Walnut Creek, California.