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.
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.
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.
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.
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?
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.
I believe OP was thinking more on the order of building a bamboo helicopter to fly off the island?
This, by the way, is called ‘lapping’ and as stated, needs three rocks (or pieces of wood) to work.
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
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.
Knot a great example:
Straight Dopers should apply as a team to one of those survival shows.
Just grab the largest FedEx box that washed up on shore. Boom- flat surface.
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).
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.
It wasn’t How do they shape things perfectly?, was it?
Look around you – can you form some sort of rudimentary lathe?
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.
I think it might have been - nice find! Shame the external links don’t work any more.