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Observatory build Chapter 2 of 10

Site layout

Squaring a 10-foot square with a scaled 3-4-5 triangle, and finding true north with a stick instead of a phone.

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This is one person’s build, not engineering guidance. Local codes, soils and loads vary.

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  1. Squaring the deck
  2. Finding true north

Two things have to be right before anything gets dug: the deck has to be square, and you have to know where true north is.

Squaring the deck

3-4-5 works, but at 3′-4′-5′ any misread on the tape gets multiplied across the full 10′ run. Scale it up: 6-8-10 feet is the same triangle with roughly double the accuracy.

  1. Pick an origin corner and run a taut string along the edge you care most about — parallel to a fence, the house, whatever you want the deck to line up with. Everything gets built off this line.
  2. Run a second string roughly 90° off the first.
  3. Mark 72″ along the first and 96″ along the second, both measured from the corner.
  4. Swing the second string until the distance between those marks reads exactly 120″. The corner is now square.
  5. Extend both to 120″ and mark the remaining corners.
  6. Pull both diagonals. Both must read 169-11/16″. If one is long and the other short, the deck is a parallelogram — push the two corners on the long diagonal toward each other.
  7. Re-check all four sides, then re-check the diagonals. It takes two rounds.
  8. Set batter boards outside each corner and transfer the strings onto them, so you can pull the strings for digging and put them back exactly.
Squaring the deck layout with a scaled 3-4-5 triangle Plan of the 120 inch square deck. A triangle in one corner measures 72 inches along one edge, 96 along the other, and 120 across the hypotenuse — a 3-4-5 triangle scaled up. Both diagonals of the full square must read 169 and eleven sixteenths inches. The pier center sits 52 inches in from two adjacent edges. Squaring the layout — scaled 3-4-5 72″ 96″ 120″ Pier center52″ from each edge Both diagonalsmust read 169-11/16″ origin corner — every measurement starts here 6-8-10 feet, not 3-4-5 — same triangle, twice the accuracy over a 10′ run

Pier center is at (52, 52) — measured from the origin corner along each edge. This is the single easiest thing to get wrong on the whole build. The deck is a perfect square, so nothing on site will remind you the pier is not in the middle.

Things that quietly ruin this: a fabric tape (it stretches — use steel), a sagging string, measuring from the hook on one leg and the 1″ mark on the other, and sloped ground. On a slope every corner must be measured level, not along the dirt.

Finding true north

The concrete does not care which way it points. The adapter's index rod does, because it sets where the mount's azimuth adjusters sit in their travel. The EQ6-R has only a few degrees of range, so the target is within about 2°.

The phone is fine — just not at the pier

An iPhone with "Use True North" enabled applies declination correctly. The problem is the rebar cage, four steel rods, a cast-iron rotor and buried conduit within a few feet of where you'd be standing. That is a worst case for a magnetometer.

So take the reading 20–30 feet away from the pier, the deck, and any vehicle. Drive two stakes on a north-south line 15–20 feet apart. Then sight along those stakes back toward the pier. The stakes carry north in without the phone ever going near steel.

Calibrate with the figure-eight motion first, and take the phone out of any MagSafe or magnetic case — those magnets will ruin the reading and it will still look confident. Take readings from three or four spots; if they disagree by more than a couple of degrees, there is interference.

The shadow-arc method, if you want no electronics

About 1° accurate, immune to every source of magnetic interference:

  1. Mid-morning, stand a rod plumb in the ground.
  2. Mark the tip of its shadow — point A.
  3. Tie a string to the rod's base, length equal to the distance to A. Scribe an arc on the ground.
  4. In the afternoon, watch the shadow tip shorten and then lengthen until it touches the arc again. Mark point B.
  5. The line bisecting A and B runs true north-south.

Verify the direction before you pour

Set the mount on the adapter on a bench and confirm which way the index rod needs to face. Depending on the mount's geometry the pin may sit on the north or the south side of the base. Getting it 180° out is easy and impossible to fix in concrete.