How to Draw a Scale Bar

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When you're reading a map, it's helpful to know the relative size of features on the map compared to their sizes in real life. That's where scale bars come in handy. You can draw a scale bar when making a map to let readers know distances between objects on the map.

Drawing a Scale Bar

All scale bars compare a unit of distance, such as feet or miles, to the distances of the locations on the map. A 1:200 scale ruler on the map tells you that, for one unit you measure on the map, that distance is 200 times that unit in the real world. There are two different methods of drawing one, the first that begins with locations and calculates distance between them, and the second that begins with a fixed distance and draws a scale that fits it.

To do the first method, you start by figuring out the real distance between two easily distinguishable locations or points on the map. Make sure to choose locations that are far enough from one another on the map (typically an inch or greater) such that you can draw a more easy-to-use scale bar on the map.

After you've measured the distances between the locations in the real world, use a ruler or other equipment to measure the distance between those locations on the map. Compare the distances as a fraction and draw the scale bar accordingly. For example, if the distance between two points in the real world is 1,000 m compared to 2 inches on the map, the scale would be 1 inch long with a measurement of 500 m.

To perform the second method, begin by choosing a fixed distance that you would like to measure in the real world, such as 100 miles. Then, use measuring tape or some other method of measuring a long distance (such as calculating how far a car travels on a straight road) to determine the start and stop point that covers this distance in the real world when traveling in a straight line. Compare the start and stop points on your map, and draw the scale bar accordingly.

Types of Scale Drawing

In addition to scale bars, there are other methods of representing the relative scale of objects on a map. The first is to simply write the scale in text form as a ratio or fraction, such as writing 1:2,000, which indicates that one unit of measurement of distance on the map equals 2,000 of that unit in the real world.

Another method is using a specific stated scale that doesn't scale precisely by individual units. This could be 1 cm: 25 m which is another way of writing 1:2,500. These can be fit for certain rules and measuring tapes that rely on specific units, unlike a general 1:200 scale ruler.

Finally, some maps contain inset or locator maps within them. This lets the reader zoom in on a part of a map with a given scale that can let the reader see more details in smaller areas of geography. This could be useful for scaling in on Vatican City from a broad map of Europe. These types of scale drawing show how readers can understand distance between features on the map.

Scale Bars in Science

Scientists taking pictures of phenomena on the cellular or similar minuscule levels rely on scaling their images appropriately to represent size. This could be helpful to, for example, communicate the relative size of cells in a population or neurons in a network of the nervous system. The ways to do this depends on the specific software used in imaging.

Other methods of defining a scale can be more straightforward with simple photography. You may consider placing a specimen or cell culture next to a ruler before taking a photograph to make it easy and simple for readers to determine length and size.

Scale Bars in Photoshop

Some of the later versions of Photoshop can make it convenient and quick to add scale bars to microscope images. You need to, first, figure out the pixel size of the camera used to generate the images alongside whether you used any binning in producing the image. You should also determine the lens magnification and the magnification for both C mount or objective magnification lenses.

From there, you can calculate the actual pixel size of the microscopy images using the following formula: Actual Pixel size = (CCD Pixel x Binning) / Lens Mag x C mount x Objective Mag.

Scale Bars in ImageJ

In ImageJ, there are two methods of adding a scale bar. The first method is taking an image of the scale bar (such as a ruler or micrometer), selecting the straight line selection tool and drawing a line over the scale to define a known distance. Select the "Analyze" menu, and choose "Set Scale" and set the appropriate distance in the boxes given. Click "Global" so that it applies to all images.

The second method is directly changing the scale by the "Set Scale" menu options without directly measuring. If you know the scale of your imaging method, then you can use this method.

After that, figure out which images you want to add a scale bar to and, from the "Analyze/Tools" menu, choose "Scale Bar." This should place a scale bar on your image. You can change the size, color and location of the scale bar, too.

Designing a Scale Bar

Think about the best way to visualize a scale bar. Generally, in scientific and engineering research, professionals want to communicate information as effectively as possible. This means valuing simplicity and straightforwardness, functionality and conciseness when designing the features of maps and images such as scale bars or types of scales.

Make the process as easy as possible for your audience to determine the relative size of objects in the images and on the maps you create. Choose simple lengths such as 100 μm for microscopy images or 100 m for maps.

Use colors that contrast well with the background that are easy on the eyes. Using black and white scale bars for bright microscopy colors such as green and pink may be ideal, but also consider the color settings of the available printers or projectors through which you will either print out an image or display a presentation.

Generating Images

On the subject of printing and presenting, make sure you're aware how the image on your computer may be scaled up for the purposes of a poster or presentation. Make sure that, when generating images, they have the appropriate resolution to scale to these sizes without losing image quality. Use vector graphics, which scale much better when their size is changed, instead of raster graphics.

For positioning, stick to corners such as the lower left or lower right corners of the image. Don't place them too far from the main features of the image that it makes it difficult for readers to actually use on the image or map. Make sure you pay attention to the proportion of the scale and how easy it is for your audience to identify key features of the image you want to show using that scale proportion.

References

About the Author

S. Hussain Ather is a Master's student in Science Communications the University of California, Santa Cruz. After studying physics and philosophy as an undergraduate at Indiana University-Bloomington, he worked as a scientist at the National Institutes of Health for two years. He primarily performs research in and write about neuroscience and philosophy, however, his interests span ethics, policy, and other areas relevant to science.