# How could I mark the center of a very large public grass field?October 12, 2020 7:08 PM   Subscribe

There's a neighborhood park with a very large, perfectly circular grass field with many dandelions. The circumference is 1/3 mile. The center is not marked in any way that I can see, and I would like to find a way to do this that is not disruptive/dangerous. Only I and the people I tell would know about it.

Although I'm open to all ideas, I'm most interested in doing this by planting something, so if you suggest plants, it shouldn't be invasive or large. This is in Seattle. The grass is mowed regularly so it needs to be low to the ground. What do you know about lawn daisies? Do they stay clustered?

Bonus question: what is the best way to find the center? I have already eyeballed it on the map on my phone, but is there a more accurate/clever way I can do this without hiring a surveyor?
posted by oxisos to Science & Nature (21 answers total) 3 users marked this as a favorite

A phone with decent GPS should get you within ten feet.

Another option would be a Lensatic compass, and sight on the cardinal directions. You would need to put up flags after walking one direction (say E-W), before walking N-S until you intersect.

https://coolhikinggear.com/how-to-use-a-lensatic-compass-a-guide-for-hikers
posted by nickggully at 7:15 PM on October 12, 2020

On finding the centre—let's assume the field is marked with the circumference. You need a rope or a piece of string at least the diameter of the circle, and either two people and one person and a peg.

1. Fix one point on the edge of the field, then holding the rope, walking to the very opposite edge. Keep pulling until you find the furthest distance between each other. That's the diameter.
2. Halve this to find the radius.
3. Fixing a point along the edge, walk a new circle with the found radius. This circle will go through the centre. Mark an arc.
4. Repeat that two or three times along widely separated points, the intersections of the arcs will give you the centre.
posted by Fiasco da Gama at 7:16 PM on October 12, 2020 [1 favorite]

Another alternative is to bisect chords, which is illustrated here.
posted by Fiasco da Gama at 7:18 PM on October 12, 2020 [4 favorites]

I've just seen you noted the circumference—far longer than any rope. Sorry. To go with nickggully's suggestion of flags, it would be very accurate with a known length of cord to bisect chords, mark those lines to the centre with pegs/flags, then walk to the intersection of two or three of them.
posted by Fiasco da Gama at 7:27 PM on October 12, 2020

Since the field is already full of dandelions, I assume that a small planting of another green ground cover wouldn't be noticed. My thought was clover because it is hardy, it can tolerate moving and it blends in with the grass. Over time it might spread but hopefully the center of growth will be roughly at the center of the circle.
posted by metahawk at 7:46 PM on October 12, 2020

The radius is 280 ft. You can buy ropes of that length. Perhaps that could be used to find the center?
posted by amaire at 9:42 PM on October 12, 2020

Fertilizer. My grandfather once fertilized his lawn -- he spelled out hello in big letters since he lived near an airport.
I think if you fertilized a bullseye it might work.
posted by beccaj at 9:47 PM on October 12, 2020 [3 favorites]

Another vote for clover as a marker for a few reasons:
1. It is a normal plant to find in a grass field.
2. It can be mown or walked on without problems.
3. It fixes nitrogen thus 'creating its own fertilizer'. Because clover stays green for longer than grass it will be particularly noticeable during dry weather (which I assume occasionally does happen in Seattle).
posted by rongorongo at 11:33 PM on October 12, 2020

If you have access to a metal detector, a small metal object buried in the right spot would do the job.
posted by Too-Ticky at 3:53 AM on October 13, 2020

Another way to find the centre of a large circle relies on the fact that any right-angled triangle inscribed in a circle always has a diameter for its hypotenuse.

You'll need a compass. Many phones include an inbuilt magnetometer that lets you install compass functionality as an app, if you don't already have a special-purpose compass.

Shove a stick into the ground to mark a spot on the edge of the circle. Using the compass to maintain a constant heading, walk a straight course toward some other spot on the edge. Now turn exactly 90° in a way that keeps you looking into the circle rather than away from it; then, using the compass to maintain that new direction, walk a straight course to a third spot on the edge and mark that.

You now have two markers at the ends of a diameter of the circle. Walk directly from one to the other, counting paces as you go. Then, walking back half that number of paces will put you quite close to the centre.

If you do this several times with different starting spots and different initial headings, leaving an approximate-centre marker behind each time, then the true centre will be very close to the middle of your cluster of approximate centre markers.
posted by flabdablet at 4:39 AM on October 13, 2020 [7 favorites]

Or if you want to do it without even a compass, you can get a pretty good diameter with a bit more walking just by marking a spot on the edge, then pacing out the distance around the circumference, then walking half that number of paces around the circumference again. Mark that spot and then keep counting paces as you finish walking around to your starting spot, just to check that your pace length is reasonably consistent. The two marks will be the ends of a diameter, which you can halve by pacing to find the centre as for the right-angle method.

Again, doing this multiple times and taking an average will get you as close to the centre as you're likely to be able to get in any other reasonable way.
posted by flabdablet at 4:49 AM on October 13, 2020 [1 favorite]

Or you can use a hexagon: pace out the circumference, then divide the result by six and use that to put six evenly spaced markers around the edge. Pace them out several times in both directions to make sure that they are indeed evenly spaced.

If you now count paces while walking in a straight line from one marker to the next, this will give you the radius of the circle. Walking that number of paces in a straight line from any marker to the one directly opposite will get you to the approximate centre. Do this from all six available starting markers and average the results as before.
posted by flabdablet at 4:56 AM on October 13, 2020 [1 favorite]

To mark the centre, I'd dig a hole maybe six inches deep and two feet wide, fill it with active mushroom compost, then replace the sod over the top. Scatter a bit of clover seed for good measure.

If the mushroom mycelium lives, which it probably will, it will grow outward in a "fairy ring" circle from the centre as the years go by; and it will tend to fruit and throw up edible mushrooms at the edge of that ring. Also, as the mycelium expands year on year, it will subtly modify the nutrient profile of the topsoil it's growing through, and you'll find that the mix of ground covers thriving inside the circle will be somewhat different from that outside. This will manifest as a subtle colour change that's completely obvious to anybody who knows to look for it but most likely unnoticed by anybody else.
posted by flabdablet at 5:09 AM on October 13, 2020 [14 favorites]

If you need it marked for more than two years, an inconspicuous plant won’t do. Grass and clover and chamomile and yarrow etc left to themselves swap patches, slowly, a natural crop rotation. Excellent for soil health, not permanent designs.

I’d put in lawn chamomile for a short-term marker - it isn’t very common here because it wants more sun than is reliable except in large fields. But for permanent location markers, bury metal or a mining RFID marker.
posted by clew at 9:13 AM on October 13, 2020

Depending on the field, and your location, cultivating a fairy ring might be doable, albeit a long term project.
posted by furnace.heart at 10:35 AM on October 13, 2020

Once you find the centre you could mark it with a sprinkler head. They are generally considered to be safe on play fields, are innocuous and yet readily identifiable.

You'd have to be careful with fairy rings because they can cause a ring of dead grass along side the greener grass.
posted by Mitheral at 12:02 PM on October 13, 2020

they can cause a ring of dead grass along side the greener grass

This is usually much more of an issue in a deliberate monocrop like a sadly typical HOA-mandated, meticulously weed-killered lawn than in a mix of groundcovers such as you'd find in a typical public grass field whose only regular maintenance is mowing.
posted by flabdablet at 7:25 PM on October 13, 2020

Ok, bisecting chords as mentioned above. Given any line from one part of the circle to another, a perpendicular line drawn on its midpoint will pass through the center. Given two of these, the intersection of these two line is the center.

So let’s say you get two long straight boards, say 8 feet. Mark their centers and mount a laser pointer perpendicular to each. Put the ends of each board on the circle somewhere and turn on both laser pointers. Now finding the intersection will be a little tricky. Have one person assigned to each laser. They slowly walk towards the center, making sure the laser is still on their foot, a target, whatever. When your two people end up next to each other, you can probably use a ball or something round to track each laser dot and find their intersection.

Real world challenges are elevation differences and the circle not being perfectly round. I have a laser level that projects a line, but I’ve never tried it over huge distances.
posted by advicepig at 7:28 PM on October 13, 2020

I would have thought that an 8 foot chord of ⅓ mile of circumference is going to be so sensitive to minor edge irregularities as to render the bisected chord theorem almost useless in practice.
posted by flabdablet at 7:34 PM on October 13, 2020

The real world certainly makes it difficult...
posted by advicepig at 7:47 PM on October 13, 2020

Ok, mount two laser pointers at 90° from each other. Use this to place one right angle on the circle. Find where those intersect the circle pretty far away. Mark those. The line between those passes through the center.

Repeat from somewhere else to get another line that passes the center.

Shoot some lasers and find that intersection.
posted by advicepig at 7:53 PM on October 13, 2020

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