How do you measure angles and curves on a drawing using a protractor or a compass?

A protractor is a semi-circular or circular tool used for measuring and constructing angles. A protractor is made of transparent plastic and has degree markings from 0 to 180 (or 0 to 360 in circular protractors). The markings allow users to measure angles in degrees. It's a fundamental instrument used in geometry, engineering, and construction to accurately gauge the size of angles between two intersecting lines. The straight edge of the protractor helps in aligning it with a baseline, while the center hole marks the vertex of the angle. This tool is vital in fields where precision in angles is essential, such as drafting, carpentry, and technical drawing.

A compass is a drawing tool used to create circles or arcs & transfer distances. It consists of two arms: one with a sharp point that anchors the compass at a fixed point and another with a pencil or lead that traces the curve. By adjusting the distance between the arms, the radius of the circle or arc can be modified. Compasses are essential in geometry and technical drawing for creating precise circular shapes. They are also used in navigation, where the term refers to magnetic compasses for determining directions. In drafting, compasses ensure accuracy in arcs and circles, making them indispensable for architects, engineers, and designers who need to depict round shapes or measure equal distances with precision.

To measure angles with a protractor, first place the protractor’s center hole over the angle's vertex, aligning the base of the protractor with one of the angle's rays. Ensure the baseline of the protractor aligns perfectly with the ray to get an accurate reading. The other ray will cross the protractor at a specific degree mark, which indicates the angle's measurement in degrees. If you're measuring an obtuse angle (greater than 90 degrees), be sure to use the upper scale, which often starts at 180 degrees on the opposite side of the baseline. If the angle is acute (less than 90 degrees), use the lower scale starting from zero. It's important to ensure the protractor is not tilted, and the vertex is exactly centered to prevent errors.

To measure curves with a compass, start by placing the pointed end of the compass at one end of the curve. Adjust the compass so that the pencil or lead tip reaches the other end of the curve or arc. Once adjusted, note the distance between the two points, which represents the radius of the curve. With this radius, you can compare the curve to standard arcs or circles, measure distances on curved surfaces, or replicate the curve elsewhere by tracing it again with the compass. This process helps in technical drawings, design, and architecture, where exact replication or understanding of curved surfaces is crucial for accurate construction or design work.

To draw angles with a protractor, begin by placing the straight edge of the protractor along the baseline where one ray of the angle will be drawn. The center of the protractor should align with the vertex, where the two rays will meet. Mark a small dot on the paper at the desired degree marking on the protractor’s arc. Then, using a ruler, draw a straight line from the vertex to the dot, forming the second ray of the angle. The two rays now enclose the desired angle. Be sure to check your markings for precision. For example, to draw a 45-degree angle, place the vertex at the protractor's center, align the first ray with the baseline, and then mark a point at the 45-degree mark.

Having to describe what a protractor and a compass are for and of what use, its mighty advanced. You might want to start with "lead holder", "architect or engineer scale", and then step it up to "circle template" and "erasing shield". After the discussion about protractor and compass, follow it up with a chat about mylar leads, and weigh the pros and cons of single or double matt mylar. As the drawing note says, drawings not to be scaled. Thus you need to go by the dimensions, diameter, radius, and do the math. I do draw by hand, the curves look great, but unless the dimensions are noted it's just another cool looking drawing without required info.

So…AI is generating articles on how to work with construction drawings back in the 1980’s? Do yourself a favor. Obtain a digital copy of the drawings (almost no one draws by hand any more) in a PDF format and bring them into any number of drawing programs (even Adobe Acrobat) and do your measurements there using the digital tools and features which are much more accurate and much faster. Bluebeam Revu is one such application made specifically for Architects, Engineers, and Constructors.

It's interesting to know how these instruments work; and both would be very useful if there was a power cut. It took me a while to find my rotring compass and haven't found my old small set square protractor ruler all in one but will be pleased when I do.