I split this topic into two posts. Part 1 covers taking coordinates for a pace-and-compass sketch map and Part 2 covers the mapping. Enjoy.
Many projects these days use a sub-meter GPS to record sites and nothing else. It’s actually been a little while since I had to “pace and compass” a site map. Until about June of last year I worked for a company here in the Great Basin that uses Garmin GPS Map 62s to map sites! Those GPSs are NOT sub-meter and the accuracy leaves something to be desired. I wouldn’t have minded so much, as long as we still did a pace-and-compass map as well. So, for those of you that are new in this field and only did a pace-and-compass map in field school or have never done one at all, this post is for you. First, though, why do it?
Last summer and fall I was working for a company that uses a sub-meter GPS to map sites. It was running ArcPad software which allows you to make a fairly accurate representation of the site. However, some things just aren’t intuitive if you’ve never had to draw out the site. I’m not sure why that is but it is. For example, this person I had on my crew always had to be told to record the courses of the drainages and to mark things like large rock outcrops. You wouldn’t even think to leave these details off of a hand-drawn sketch map. What is the easiest way to draw a sketch map? Pace and compass. Let’s get into it.
The first thing you need to know is what the length of your pace is. Your pace is something you probably already know from doing surveys. For those of you that don’t, here is how you do it. First, find a straight stretch of uninterrupted ground like a road or parking lot. Next, roll out 30 m from a roll-up tape measure. Now, simply walk at a normal speed, without stretching out your stride, and count the number of steps it takes to get to the end. You might want to do this several times so you can take an average. This number is your pace. I’m about 6’1” and my pace is about 43 steps. I have freakishly short legs. I know some tall people that have a pace under 30 steps! Still, some people like to shorten or lengthen their stride to make it exactly 30 steps. I would say that you should do what ever you are comfortable with as long as you are relatively accurate and consistent.
OK. Now that you have your pace, it’s time to get your map supplies together. Before we start drawing the map, however, you will need some other supplies first. To gather the data for your map you will need a notebook (or smartphone for you tech savvy people), a pencil, and a compass.
Now, the first thing you have to realize is that not every site is going to be mapped the same way. For example, if you have a large lithic scatter with no tools and no features you may just want to accurately map the boundary, a few flakes and natural features, and that’s it. On the other hand, if you have a mining site with prospect pits, shafts, refuse piles and whatever, you may want to first map the features then simply draw the boundary around them. Basically, you have to figure out what your boundary is being created by. It might be that your boundary is being created by a project APE (Area of Potential Effect), pipeline corridor, or some other artificial boundary.
Now we are ready to take some measurements. There are two ways that I can think of that are accurate and easy. First I’ll illustrate the way I was trained. You start by picking an arbitrary point in the center of the site, usually the datum (the datum isn’t always in the exact center of the site and may be a large boulder or tree). However, out here in the West (well, Nevada. Actually, northern Nevada. I don’t know what they do down south) we are no longer putting in a solid datum on the site (i.e. a wooden stake or a length of rebar). It’s being done electronically in the office. So in that case I would probably pick a natural feature, such as a large rock or a tree, or I would simply put a group of pin flags in the ground where I plan to start. Once you have your starting point it’s time to take your first measurement.
If I have clearly defined boundaries then it’s usually easier for me to take my first measurement going North. On your notepad you can start by just writing datum (or tree, pin flags, or whatever) on the top line. Using your compass (set to the proper declination) find north and pace to the boundary. If you don’t have a boundary and are simply using features, skip to the next paragraph. After you’ve paced to the boundary, write down the distance and bearing to your point from the datum on the next line in your notebook. It will look like this: “12@0°”. I usually walk counterclockwise (anti-clockwise for you folks across the pond) so I’ll sight in my next point and walk to it. Write that distance and bearing in your notebook. It might look like this: “15@275°”. Make your way around the entire site back to that first point you took North of the datum. That’s the boundary! Next you’ll want to take points on all the numbered artifacts, features, and what ever else your crew chief wants you to. On small, manageable sites (50 x 50 or so) I take all of those points as distance and bearing measurements from the datum. On larger sites you may have to just do what ever makes sense.
Mapping a site using features or numbered artifacts can be done a number of different ways. If you don’t have many features or artifacts you can simply take distance and bearing measurements from the datum for every point. Once you draw in the features and artifacts you can simply draw a line around the site. There are other ways to do it too. You just have to decide what makes sense for that site.
I’ve split this blog post up into two posts. I like to stay at or under 1,000 words for each post and this one was getting a bit long. Click here for Part 2.