Archaeology by nature is a destructive science. Archaeologists remove artifacts from their burial environment to document, conserve, and analyze the collection, in order to answer questions about our past. Before we remove anything from the site, it is important to map each object to preserve its spatial data, called provenience. Recording the provenience helps us recreate the site on paper, and it helps us preserve and interpret the relationships between artifacts long after they have been raised.
We use a grid system to plot the location of artifacts on the seabed. Each grid square, called a unit, has unique mapping coordinates to help us reference where an artifact was found. We map an artifact even further to get its exact east and north coordinates within a unit. We use three different methods to map the site: total unit recording, trilateration, and direct offset measurements. Total unit recording involves a diver drawing the unit square to scale on a slate and sketching where each artifact falls within that unit. Trilateration is measuring the distance of an object to two specific fixed points. Direct offsets are recorded as the distance from the object to a point on the baseline directly parallel to the object. The baseline is one of the most important features. It is a fixed line travelling across the site acting as a reference line for the entire grid system. You can see it in the map above as the thick black line running from north to south.
At the end of the day, we use the data we collected while underwater to draw artifacts on a scaled map. We double check measurements against our sketches, photos, and our notes from the day to make sure the map is correct before removing the artifact the next day.
It may seem redundant, but using multiple ways to map an archaeological site ensures that your data is as accurate as possible. After all, once you remove it, you can’t always put it back to check!