How to square up a pole building on a hill?

I'm planning to build a cabin on poles, and build it into a hillside. At the top of the hill the posts (6x6 treated), will be about 2 feet off the ground. At the bottom of the hill they will be about 9 feet off the ground. I'm intentionally building this way to achieve a unique look, and want to build a deck over the high side. Everything is planned, except I am not sure how to get the posts in the ground so that the layout is square. Normally when building a pole structure, you use a string and make sure that the "X" between the opposite corners is the exact same length. (in other words, run string from front left to rear right, and vice versa. When the lengths are the same, it's square.
However, I'm dealing with a hill. Running the string up and down the hill is going to change the lengths. This leaves me totally puzzled how to get the posts square. Granted, I could put the posts in the ground and measure from the tops of the posts (which would be level), but I sure dont want to have to keep re-drilling the holes in the ground until I get it right, not to mention handling heavy poles over and over, and trying to keep them upright without any fill around them. There must be a better way? Does anyone have any idea how to do this? Will a laser level help in any way (other than getting the tops of the posts level)?
I've done a lot of building over the years and have built several pole buildings on flat ground, but this project has me puzzled.
Any experts on this sort of thing out there?
Thanks
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On 7/19/13 4:38 AM, snipped-for-privacy@toolshed.com wrote:

Some cut.
Would using electric fence posts or other type of stake work for you? Those can be driven in the ground and moved easily.
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Build 2 tripods and add center pole that is greater than 9 feet. Set your tripods on the low side and make sure your center pole is dead nuts plumb. Move the tripods as you create the "square" always ensuring that the center pole is dead nuts plumb. Once you have your X, mark the spots where the center poles touch the ground.
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I would start with the bottom posts. Figure out where you want your first post, then brace it so it's absolutely plumb in both directions. Measure from that first post to install and plumb the second lower post. I would use a string to ensure the faces of the two posts are parallel/coplaner. Depending on the spacing of the posts, I would install diagonal braces to connect the two posts and ensure the spacing is the same at the top as it is at the bottom (both posts exactly vertical).
Now, figure out the approximate location of an upper post and mark a line on the lower posts at that level. You could use a laser level if you have one, or a simple water level. You could even use a long straight board with a level on it if the spacing is close enough.
Then string a line between the two lower posts at the level line, and from a lower post to the approximate location of the upper post. Now use the 3- 4-5 method (Pythagorean theorem) to ensure a 90 degree corner. For example, you might measure 8 feet along the line between the two lower posts. Then measure 6 feet along the line running to the upper post. Move the line running to the upper post back and forth until you measure exactly 10 feet diagonally between the 6 and 8 foot marks on the lines. The corner is now square, so you can simply measure the distance along the line to determine the exact location of the upper post.
Repeat the process in opposite fashion for the other upper post. When you're done, diagonal measurements between posts (on the level line) should be equal.
Anthony Watson www.mountainsoftware.com/anthony.htm
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On 7/19/2013 3:38 AM, snipped-for-privacy@toolshed.com wrote:

You could rent a transit and "rod". They are used by builders to get accurate 90 degree corners. (Also used by surveyors.)
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wrote:

While the other advice I got in this thread seems sensible and both methods suggested would seem to work, I did have a thought in the back of my mind about using a transit. But I'll be honest. I've seen surveyors use them, but I dont have a clue how they work. All I know is that somehow they set them up on a tripod, and "magically" determine the edge of a piece of property that may be 500 yards away, regardless of the terain or hills, etc.......
I may have to google "how to use a transit", and see if I can find a website that shows how they work. Whether I use that, or just use one or both of these other suggestions will determine what works easiest and best. The building is only going to be 16 x 28 feet, so it's not that huge, but doing it the easiest is preferred. There will be a total of 8 posts to be set. (4 on each of the long sides, with the corner posts being 6x6s and the center ones being 4x6s treated).
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