فنون

الجمال والعناية

حِرَف

الثقافة والتاريخ

ترفيه

البيئة

الطعام والمشروبات

الهندسة العكسية

العلوم

رياضة

التقنية

الأجهزة القابلة للارتداء

أنشأه

Mark

23. سبتمبر 2026

Setting Out a Right Angle on the Ground

A carpenter's square is 300 millimetres long. A building is twenty metres long. Put the square on a corner and extend the line, and an error of half a degree you cannot even see becomes 170 millimetres of skew at the far end. So a right angle on the ground is never set with a square. It is set with a measurement, using the 3-4-5 triangle the Egyptian rope-stretchers used and Pythagoras later explained — and the longer you make the triangle, the more accurate the corner. This rung is short, it uses only a tape and some pegs, and getting it wrong is the reason a great many sheds have a trapezoid floor. It is also the foundation the setting-out and traverse rungs later in this batch both stand on.

مبتدئ

3 hours

التعليمات

Set the base line first, and make it the long one

Drive two pegs and stretch a taut line between them. This is your base, and everything else is measured from it. Choose the LONGEST edge of your layout as the base, because every angular error you make will be amplified by whatever you build off it. Tap a nail into the top of each peg and work to the nail, not the peg — a peg is 40 millimetres across and 'the middle of the peg' is a 40-millimetre uncertainty at the very start of the job.

المواد لهذه الخطوة:

أوتاد خشبية8 قطع

خيط شاقول للبناء50 metre

مسامير مُجَلْفَنة20 قطع

الأدوات المطلوبة:

مطرقة بمخلب

شريط قياس

Lay out 3-4-5 as large as the site allows

From the corner, measure 3 units along the base and 4 units along the line you want at right angles. Adjust the second until the diagonal between the two marks reads exactly 5 units. Use metres, not the tape's own numbers: 3-4-5 metres is good, 6-8-10 is better, 9-12-15 better still. The triangle's accuracy depends on how large it is relative to your measuring error, and a 30-metre tape costs nothing to use fully. This is the single highest-leverage choice in the whole layout.

المواد لهذه الخطوة:

أوتاد خشبية8 قطع

الأدوات المطلوبة:

شريط قياس

مطرقة بمخلب

Check the diagonals, always

Once all four corners are pegged, measure both diagonals of the rectangle. They must be equal. Equal diagonals with equal opposite sides is the definitive test that a quadrilateral is a rectangle — it checks all four corners at once, including errors that individual corner checks can hide. If they differ, move two adjacent corners by half the difference each, in opposite directions along their own lines, and re-measure. Two iterations usually converge.

الأدوات المطلوبة:

شريط قياس

Deal with the tape itself

Pull the tape with a consistent, firm tension, keep it horizontal, and do not let it sag or drag through grass. A steel tape stretches measurably under hand tension and sags into a catenary under its own weight, both of which make the distance you read LONGER than the straight line you wanted. Over 30 metres, careless sag is easily 30 millimetres. Consistency matters more than correctness: if you pull the same way every time, the error cancels in the diagonal comparison.

الأدوات المطلوبة:

شريط قياس

How big to make the triangle

جارٍ تحميل دفتر Jupyter…

الأدوات المطلوبة:

شريط قياس

Make a large fixed square for repeat work

If you are setting out many corners, build a permanent 3-4-5 frame from three light battens, braced and screwed, with the hypotenuse checked before every session. A 1.5-2-2.5 metre frame is portable and five times more accurate than a carpenter's square. Check it by flipping it over and comparing the line it gives against the line it gave the other way up: any error DOUBLES between the two, which makes it easy to see and easy to halve out.

المواد لهذه الخطوة: