A question that I often get when people stop by and visit a site that I'm excavating is, "Why do you dig square holes?" I'll look around the site for a minute, observing all of the different squares in various forms of excavation and think to myself about how much archaeology is art, well designed art that is. "Imagine yourself standing on a giant sheet of graph paper", I explain.
So I will explain to the rest of you as well! In order that we know exactly where every artifact comes from when we excavate, it is important to keep control by strictly recording their position in the ground. Before an archaeological excavation, the most important tool in the archaeologist's tool box isn't a trowel or shovel, it is the datum point. The datum point is simply the starting point for the grid that will be laid out over the site, using a series of nails and string, to help us keep track of where those artifacts came from.
The datum point is arbitrary, we are archaeologist place it where we think we will get the most squares to excavate to cover a site. Often times, we use some type of survey equipment and a known point, say on the corner of something that isn't going to move, like a bridge, or USGS (United States Geologic Survey) Benchmark point and triangulate it into our site datum.
Once the site datum is established, I like to pound in a wooden stake or piece of metal re-bar, an archaeologist can start to think about his grid. Here in the US, we use the metric system for prehistoric sites, and the Imperial for historic sites (people build their houses and buildings here using feet and inches). The smallest square we can have for grid during a site excavation is 1-meter. I like to use the 2-meter square. So lets try to make a grid for a hypothetical site.
1) Establish a datum and give it a number denoting an X and Y axis.We are going to work in Northings and Eastings. So our datum will be, North 100 E100 represented as N100 E100.
2) On my grid, I use the South West corner of each square as the Unit Datum.
3) That means the next unit to the east is N100 E102 (Remember we are working in 2-meter units)
4) If we move north on our grid, it would be N102 E102
5) If we move south on our grid it would be S102 E102
Here is an example of a grid that we might use: