Most of us probably understand that 1 measurement unit in our N scale model world is the equivalent to 160 measurement units in the real world. I use the term measurement unit because it could be feet, metres, inches, centimetres, millimetres etc. I prefer to use metric units so as an example 160 real world centimetres would be 1 centimetre in N scale.

Scale ruler by filmingilman – Own work, CC BY-NC-ND 2.0, View Image

All pretty straight-forward but what happens when we’re making a model of an object like the Derwent Way swing bridge and we don’t have access to the prototype to take measurements?

Well, aside from accepting you’re unlikely to be 100% accurate we’ll need to convert the measurements on the drawing, satellite images or if we’re really lucky, the plan of the real World object we want to build to N scale.

Read on to find out how to do that…

Once you’ve got your image, you need to establish the scale of the drawing or image and calculate how the measurement units of this image relate to both the real and N scale worlds.

If you model modern prototypes, in most cases you’ll probably be using maps or satellite images from a source like Google Maps, Bing Maps or OpenStreetMap and these will include a measurement scale somewhere on the map or image. The scale is usually shown in both metric and imperial but it’s best to pick one scale and stick with it throughout your build. I’ll use metres.

I tend to work directly on the computer without ever printing the images I’m using. I open the image in a vector graphics package such as Inkscape, zoom-in and as accurately as I can draw a line directly over the metre scale. Inkscape shows you how long your line is in whatever measurement unit you choose. In this example, 50 metres in the map equates to 17.24mm within my Inkscape drawing.

We’ll need to break that measurement down a bit to make measuring and calculating the size of other parts of the image easier. So, 50 divided by 17.24mm is 2.9, i.e. 1mm in the image is 2.9m in the real World. Let’s call this our image scale.

Now all we need to do is measure part of the image in mm and we can easily figure out how big it is in the real World by multiplying by the image scale of 2.9. For example, the largest bridge support island is 36.36mm long, multiplying this measurement by the image scale of 2.9 results in 105.444 or 105.44m.

It’s probably worth rounding that to 105.4m to make subsequent calculations a bit easier (you could even round up to 105m!). How much rounding you do will probably depend on how accurate your measurements are, how big the object your modelling is and how clear/detailed the image you are working from is. In this case I’m happy with 105m.

It’s quite easy to convert this measurement to N scale if we know the following:

1000mm (or 1 metre) divided by 160 = 6.25mm. So a metre in the real World is 6.25mm in N scale.

So how long is that bridge support island in N scale?

105m multiplied by 6.25 = 656.25mm. 65.6cm or just over 2 foot for those on imperial.

Wow, that’s bigger than I expected! It’s actually wider than the modules I’m intending to use. I’ll need to think about how to overcome that problem and still create an accurate representation of the bridge but that’s another story altogether.

Once you’ve done the initial image scale calculation and written it down somewhere, subsequent measurements and calculations should be quicker. You simply take your next measurement from the image, multiply by your image scale and convert to N scale by dividing by 160.

Using maps/satellite images and these calculations you’ll find you can put together a fairly accurate scale drawing to use as the basis for a model of a real World prototype.