# If I was stranded on a desert island, how could I get a perfectly flat surface? **URL:** \ **Category:** Factual Questions\ **Created:** [October 2, 2013, 11:12pm UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223 "2013-10-02T23:12:04Z")\ **Posts on this page:** 19\ **Page:** 2
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [October 3, 2013, 12:24pm UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223/21 "2013-10-03T12:24:25Z")
Ice from a frozen pond? Supersaturate water with sugar, maybe salt, let it cool and the whole thing may solidify (I did this once accidentally trying to make rock candy, the sugar was cemented to the pot as one soild block. But I disagree with the premise that a straight line is needed to start building anything, anyone who spent time in the backwoods of Appalachia knows that moonshine is a workable, and sometimes preferred, substitute for straight lines in building projects. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [October 3, 2013, 12:37pm UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223/22 "2013-10-03T12:37:00Z")
> [@Chronos](#): > > If you want solid flat surfaces, get three rocks, as close to flat as you can find. Rub rocks 1 and 2 against each other, then 2 and 3, then 1 and 3, and repeat. With enough rubbing, they’ll wear down to flat. > > With just two rocks, you’ll generally end up with spherical surfaces on both, one concave and one convex. But this can’t happen with three. > [@j\_sum1](#): > > Love this! > > Must add that the orientation of the rocks in question needs to be changed continually during grinding. That is, one is rotated with respect to the other on the axis perpendicular to the surface being ground. Otherwise two rocks are as likely to form a cylinder or ellipsoid or even a saddle. > > I guess that it would be good practice to do the same with the three rocks. Although my brain is currently doing flips trying to determine the effect of different grinding modes. You probably also have to plan to gradually decrease the grind time for each pairing - otherwise there will be a point where you spend some time grinding the pair flat, then spend the rest of the time (for that specific pairing) grinding them away from flat. ---
**Author:** ![Colophon](https://avatars.discourse-cdn.com/v4/letter/c/f05b48/32.png) [@Colophon](https://boards.straightdope.com/u/Colophon)\ **Post date:** [October 3, 2013, 1:02pm UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223/23 "2013-10-03T13:02:13Z")
> [@Bisected8](#): > > Basically I was running under the assumption that having access to a flat surface and something to use as a ruler would be a prerequisite to getting anything else built. I vaguely remember reading something interesting, almost certainly linked from the SDMB, about the progression of tooling complex objects, a sort of evolution that enabled us to get from banging rocks together to building the internets. But I can’t remember any details about it and have therefore never been able to find it again. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [October 3, 2013, 1:23pm UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223/24 "2013-10-03T13:23:49Z")
> [@Colophon](#): > > I vaguely remember reading something interesting, almost certainly linked from the SDMB, about the progression of tooling complex objects, a sort of evolution that enabled us to get from banging rocks together to building the internets. But I can’t remember any details about it and have therefore never been able to find it again. ‘Bootstrap’ may be a useful keyword for (possibly) finding it again. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [October 3, 2013, 3:04pm UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223/25 "2013-10-03T15:04:24Z")
wind coconut fibers into a cord. tie to a coconut and you have a straight line. coat the cord with starch from taro root and let dry. ---
**Author:** ![Dax-ion](https://avatars.discourse-cdn.com/v4/letter/d/dec6dc/32.png) [@Dax-ion](https://boards.straightdope.com/u/Dax-ion)\ **Post date:** [October 3, 2013, 3:15pm UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223/26 "2013-10-03T15:15:32Z")
Use string with knots to in it to make a 3:4:5 right-angle triangle and run that off a plumb-line? Isn’t that how the Egyptians did it? ---
**Author:** ![Nava](https://avatars.discourse-cdn.com/v4/letter/n/da6949/32.png) [@Nava](https://boards.straightdope.com/u/Nava)\ **Post date:** [October 3, 2013, 3:32pm UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223/27 "2013-10-03T15:32:21Z")
> [@Bisected8](#): > > Basically I was running under the assumption that having access to a flat surface and something to use as a ruler would be a prerequisite to getting anything else built. Heh, no; the house my brother is rebuilding is made of insufficiently-dried adobes and one of his problems is that there wasn’t a straight line to be found in the whole place; grass huts don’t look like they’re in need of a lot of straight lines. ---
**Author:** ![Dax-ion](https://avatars.discourse-cdn.com/v4/letter/d/dec6dc/32.png) [@Dax-ion](https://boards.straightdope.com/u/Dax-ion)\ **Post date:** [October 3, 2013, 4:18pm UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223/28 "2013-10-03T16:18:28Z")
I believe OP was thinking more on the order of building a bamboo helicopter to fly off the island? ---
**Author:** ![Baffle](https://avatars.discourse-cdn.com/v4/letter/b/dec6dc/32.png) [@Baffle](https://boards.straightdope.com/u/Baffle)\ **Post date:** [October 3, 2013, 7:14pm UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223/29 "2013-10-03T19:14:55Z")
> [@Chronos](#): > > If you want solid flat surfaces, get three rocks, as close to flat as you can find. Rub rocks 1 and 2 against each other, then 2 and 3, then 1 and 3, and repeat. With enough rubbing, they’ll wear down to flat. > > With just two rocks, you’ll generally end up with spherical surfaces on both, one concave and one convex. But this can’t happen with three. This, by the way, is called ‘lapping’ and as stated, needs three rocks (or pieces of wood) to work. ---
**Author:** ![dstarfire](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dstarfire/32/5762_2.png) [@dstarfire](https://boards.straightdope.com/u/dstarfire)\ **Post date:** [October 3, 2013, 7:46pm UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223/30 "2013-10-03T19:46:48Z")
You can make a level out of a plum bob (weight and length of string) and an equilateral or isosceles triangle (3 or 2 congruent (same length) sides). Tie the plum bob to one corner and mark the middle of the piece opposite it (cut a grass stem to the same length as the side, fold the stem in half, mark the piece where fold occurs) To make the triangle: just take a (relatively straight) stick and cut of a piece that looks like a bit less then 1/3 the total length, then cut another piece to that same length, using the piece you just cut as a guide. Repeat as needed for the third piece. This will give you a relatively equal pieces, though each will probably be slightly longer than the guide piece (by the distance between the face (perpendicular to the cutting edge) and the actual cutting point)\* \*If you’re having trouble visualizing it, this bit of ascii art should help ```auto I ___V___ I ``` **I** represent the face of your cutting tool you’d place against the guide piece **V** represents the point where it actually makes the cut. This assumes a primitive cutting tool where the face sticks out a bit from the cutting edge, like an axe, knife, or the primitive obsidian arrowhead I pictured using for this. ---
**Author:** ![Dax-ion](https://avatars.discourse-cdn.com/v4/letter/d/dec6dc/32.png) [@Dax-ion](https://boards.straightdope.com/u/Dax-ion)\ **Post date:** [October 3, 2013, 8:28pm UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223/31 "2013-10-03T20:28:26Z")
Knot a great example: [![](https://nrich.maths.org/content/00/01/penta4/tri.jpg) ](https://nrich.maths.org/content/00/01/penta4/tri.jpg) ---
**Author:** ![aNewLeaf](https://avatars.discourse-cdn.com/v4/letter/a/ac91a4/32.png) [@aNewLeaf](https://boards.straightdope.com/u/aNewLeaf)\ **Post date:** [October 3, 2013, 8:43pm UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223/32 "2013-10-03T20:43:11Z")
Straight Dopers should apply as a team to one of those survival shows. ---
**Author:** ![corkboard](https://avatars.discourse-cdn.com/v4/letter/c/e9bcb4/32.png) [@corkboard](https://boards.straightdope.com/u/corkboard)\ **Post date:** [October 3, 2013, 9:21pm UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223/33 "2013-10-03T21:21:47Z")
Just grab the largest FedEx box that washed up on shore. Boom- flat surface. ---
**Author:** ![Bisected8](https://avatars.discourse-cdn.com/v4/letter/b/a4c791/32.png) [@Bisected8](https://boards.straightdope.com/u/Bisected8)\ **Post date:** [October 3, 2013, 11:16pm UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223/34 "2013-10-03T23:16:34Z")
> [@Dax-ion](#): > > I believe OP was thinking more on the order of building a bamboo helicopter to fly off the island? I’m sure a microlight would be more practical. ¬\_¬ But seriously, I was thinking along the lines of something like drawing up maps or making sure my giant letter shaped fires were legible to passing aircraft (it would be rude to ask for help with sloppy presentation, now, wouldn’t it). ---
**Author:** ![Nava](https://avatars.discourse-cdn.com/v4/letter/n/da6949/32.png) [@Nava](https://boards.straightdope.com/u/Nava)\ **Post date:** [October 4, 2013, 1:32am UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223/35 "2013-10-04T01:32:49Z")
The map doesn’t need to be to scale; you only need as much detail as you need, which isn’t as redundant as it sounds; there is a river that goes to the big bay, the river snakes up the hills, there is a lake halfway up; you don’t need to know whether “halfway up” has a height of 20 feet or of 22, or whether it’s 2 miles from the beach or 2.1; you need to know whether it’s a go-and-come-back-before-lunch journey or a two-day journey. Look at old navigational maps, or medieval road maps. The scale for the second kind was along the lines of “towns placed about one day’s travel apart are always named”; bridges and fords are represented, but there is no attempt to represent every curve on the road; navigational maps were very careful about drawing capes and bays but only represented those inland features which could be seen from sea. You’re thinking in 21st century terms: lower your centuries. Think about what you would need, not about what you’re used to. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [October 4, 2013, 1:58am UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223/36 "2013-10-04T01:58:40Z")
> [@Colophon](#): > > I vaguely remember reading something interesting, almost certainly linked from the SDMB, about the progression of tooling complex objects, a sort of evolution that enabled us to get from banging rocks together to building the internets. But I can’t remember any details about it and have therefore never been able to find it again. It wasn’t [How do they shape things perfectly?](http://boards.straightdope.com/sdmb/showthread.php?t=35264), was it? ---
**Author:** ![ducati](https://avatars.discourse-cdn.com/v4/letter/d/cc9497/32.png) [@ducati](https://boards.straightdope.com/u/ducati)\ **Post date:** [October 4, 2013, 5:18pm UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223/37 "2013-10-04T17:18:52Z")
Look around you – can you form some sort of rudimentary lathe? ---
**Author:** ![jharvey963](https://avatars.discourse-cdn.com/v4/letter/j/54ee81/32.png) [@jharvey963](https://boards.straightdope.com/u/jharvey963)\ **Post date:** [October 4, 2013, 6:14pm UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223/38 "2013-10-04T18:14:13Z")
I didn’t read all of the replies, but OP asks for a FLAT surface. He doesn’t say that that surface has to be LEVEL. J. ---
**Author:** ![Colophon](https://avatars.discourse-cdn.com/v4/letter/c/f05b48/32.png) [@Colophon](https://boards.straightdope.com/u/Colophon)\ **Post date:** [October 5, 2013, 4:18pm UTC](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223/39 "2013-10-05T16:18:35Z")
> [@Lumpy](#): > > It wasn’t [How do they shape things perfectly?](http://boards.straightdope.com/sdmb/showthread.php?t=35264), was it? I think it might have been - nice find! Shame the external links don’t work any more. [Previous page](https://boards.straightdope.com/t/if-i-was-stranded-on-a-desert-island-how-could-i-get-a-perfectly-flat-surface/670223.md?page=1)