Printing - Scale Printing Explained

It’s not obvious, but printing a photo from your computer is almost the logical opposite process technically of printing a map.

This difference in approach is all due to the scale of the map.

First of all a few definitions:

Map Scale – Ordnance Survey Explorer maps are 1:25,000 scale. This simply means that if you measure something on the map, and multiply that answer by 25,000, you will have its length in the real world. So, if a lake on the map is 2cm long, then in real life it is 2cm*25,000 = 50,000 cm = 500m long.

Wikipedia Map Scale Article

Dots Per Inch (DPI) – Printers produce images through a series of dots of ink. The higher the DPI of the printer, the more dots can be used to create the image. Using more dots can improve the image quality through smoothing of edges. In simple terms, if you remember trying to draw a circle in your school maths book by shading squares you probably remember it not being very circular. If you can now imagine the squares on the page being smaller which would represent a higher DPI, and you could improve your circle.

Wikipedia Dots Per Inch Article

Printing a photo – There is no scale involved. So we just take the photo, the user decides how large they want the image – say 6”*4”, and we send it to the printer. The amount of information to be printed is already decided.

Printing a map – Because scale is important, one of the first things we have to do is communicate with the printer to find out how much paper we can print on. Although your printer may have A4 paper loaded in it, it most likely can’t print to the absolute edge of the paper.

A4 paper is 297mm * 210mm. To make the maths easy, let’s say the printer can only print on 290mm by 200mm. If we are printing Ordnance Survey 1:25,000 mapping, we now know how much mapping we are going to print:

  • 290mm = 29cm. 29cm * 25,000 = 725,000cm = 7,250m = 7.25km
  • 200mm = 20cm. 20cm * 25,000 = 500,000 cm = 5,000m = 5km.

So, to fill the paper available with mapping at the correct scale we need an area of mapping 7.25km by 5km.

How to control printing maps through Outdoor Map Navigator

The OMN printing interface is very powerful, but involves control of two zooms:

Map View Zoom – This is simply how you normally zoom maps on screen when not printing. This simply alters the amount of mapping being displayed on your screen. It does not alter the amount or scale of what is being printed.

Print Zoom – On the Print Map window is a drop down combo box called Zoom. This alters the scale at which a map print will be made. It does not alter the map on screen.

These two zooms are therefore independent of each other.

A worked example based on the calculation above:

If on my computer, I open up OMN, select OS 1:25,000 mapping on screen, and press the print button, then the below screenshot is not very informative. It does’t show me what mapping will in fact be printed. If we count the map grid squares, we can see that on my monitor I can view approximately 5km * 3.5km which is less than the 7.25km*5km we are able to print

If I then alter my Map View Zoom and zoom out, I now have more mapping on screen, and I can now see the print area.

The black outline in the above screenshot is showing the area of mapping that I can print of Ordnance Survey 1:25,000 mapping at scale to my printer. I can now drag the map using the mouse to line up exactly the area of mapping I wish to print.

Please note if you are following this example on your PC the Map View Zoom requirement may well differ from this example due to different monitor resolutions.