Error of Closure

The Heard County 100 acres, for which I have no basemap, was an interesting challenge. A few genreations ago, surveyors would have used a plane table to map this area. Basically a flat board and a heavy wooden tripod, it also had a large compass rose and rotating arm used to take bearings. The whole contraption would be carried out into the field and set up at a point of interest, with the plane table leveled and oriented to north. Then bearings would be taken to other points and the distance measured with chains. What a lot of work!

Essentially I am using a simplified plane table. My map board is the table, with the basemap sandwich of materials taped to it, along with a baseplate compass. When I want to add something, I level the board, orient it to north, use my ruler to sight to the next point, and either pace the distance or use my laser rangefinder. So it's the same technique, just simplified, lighter, and considerably more mobile.

In the Heard County 100 I first mapped the linear features, then filled in the point features, building everything gradually using what I had already done. The southernmost stream ran across the map for about 600 meters before emptying into the river. No hint of it on my blank basemap, of course, so I used my simple plane table to take bearings along the stream and pace it off. By the time I got to the river I was 10 meters too far. That's what they call the "error of closure". Imagine mapping a fence that encircles a field. How far off are you when you get back to where you started? That's the error of closure. 10 meters over 600 meters is pretty good.

I didn't do nearly as well with the contours. One part of the Heard 100 is a gently sloping hill. The contours are 100 meters apart! I could draw them in any direction and it would look correct. The easiest way is to draw an imaginary line down the spurs and the bottom of the reentrants to get a direction, then imagine a level line where the contour goes. But if there are no spurs or reentrants I have a problem.

Well, the orienteers will be more than willing to tell me how well I did, usually as soon as they finish their course while they are still hot and bothered. Probably only the upper-level runners will every see the Heard 100, which is probably a good thing.