One of my projects to maintain interest during lockdown walks has been to follow the route of the old Dundee-Newtyle railway. My main reference for that trip was a Six-Inch Ordnance Survey map dating from 1903, which I consulted on the National Library of Scotland’s excellent “georeferenced maps” webpage. If you follow this link, you should be able to see the set-up I used. There’s a little blue slider at the bottom of the control panel at top left, which will allow you to fade between the 1903 map and a modern street map from OpenStreetMap.

The good people at the National Library of Scotland have gone to the trouble of georeferencing a large collection of out-of-copyright historical maps of Scotland (and some of the wider UK), and this is a fabulous resource for anyone who wants to explore their local history and geography. And it got me hankering for the ability to load such detailed maps into a portable GPS-enabled device.

Now, my go-to service for georeferenced electronic Ordnance Survey maps is usually Anquet. Mainly, I use them on my PC or laptop, but I also keep a few local topographic maps on my mobile phone, and use them for the occasional bit of GPS navigation. Anquet also used to sell a variety of historical Ordnance Survey maps, but they were fairly pricey, and I anyway discover that the service now seems to have been discontinued.

So I began to wonder if I could parasitize the work of the National Library of Scotland, and get a copy of their georeferenced map on to my phone. And it turns out I could. Here’s what I did.

I dusted off and updated my old copy of the venerable OziExplorer software on my PC. OziExplorer has been around for decades, dating back to a time when it was expensive or impossible to get good quality maps into a hand-held navigation device. The unique feature it offers is the ability to import map images (in those days, from scanned paper maps) and “calibrate” them with latitude and longitude information. I bought my own copy of the program years ago. It’s nowadays fairly expensive, and probably not something you’d purchase for a one-off project. However, I’m pretty sure the trial version will let you do everything I’m describing here, if you’re prepared to put up with restarting it every hour.

My next step was to take a screenshot of the Ordnance Survey map from the NLS website. I use Greenshot for these tasks, but there are many options.

I fed this image to OziExplorer, using the “Load And Calibrate Map Image” option from the File menu.

OziExplorer is extremely versatile in how it calibrates map images. If the map gridlines run parallel to the edges of the image (as they do in the NLS maps), it only requires three calibration points, preferably close to three corners of the image. For skewed maps, or maps with curved gridlines, more points are needed.

But first I need to tell OziExplorer what map projection was used, in the Setup tab of the calibration window at top right.

From the drop-down menus, I choose “Ord Srvy Grt Britn” for my Map Datum, and “[BNG] British National Grid” for Map Projection. The next three tabs in this window are the set-up for the three calibration points.

So now it’s back to the NLS map, with a notepad and pencil, to write down coordinates for three points. I just place my cursor over a suitable point, and then read off the coordinates at the bottom right of the screen. When I started doing this, I spent some time casting around for suitable natural features or buildings on the map, before I had the blinding revelation that the text on the map would work just as well for this purpose. So here I am with the cursor on the dot of the first “i” of Menzieshill.

And here are the associated coordinates for that point:

What I want to feed to OziExplorer are the letters and numbers in bold in the top line—these are the Ordnance Survey grid square letters, and the easting and northing values. It’s important not to use the latitude and longitude provided by the NLS, since this will create a position error on the order of a hundred metres if transferred to OziExplorer. The NLS is providing the global standard WGS84 coordinates, which is what your GPS receiver tells you. But once you’ve stipulated to OziExplorer that you’re using the British National Grid, it then assumes (I think) that any latitude and longitude you enter pertain to coordinates on the specific ellipsoid on which the BNG is based, which is not the same shape and orientation as the WGS84 ellipsoid.

The underlying reason for the mismatch in latitude and longitude doesn’t really matter for practical purposes, though—just be sure to use the grid letters and numbers offered by the National Library of Scotland as your input to OziExplorer.

Going back to OziExplorer armed with my three calibration points, I enter the first set of coordinates by opening the “Point 1” tab in the calibration window at top right. This changes the cursor to a set of cross-hairs that I use to select the same points I copied off the NLS map:

Positioning the cross-hairs accurately is aided by the little magnified square that appears on the screen at top left—you can see it to the left of my screenshot above.

Once I have the position right, I click to set my calibration point:

And then I enter the grid reference for Point 1:

Then it’s just a matter of repeating the process for Point 2 and Point 3, and hitting Save. OziExplorer saves a little file with the same name as the map image file, but with the suffix *.map, and the map image is now calibrated.

In a minute I’ll go on to explain how I moved a calibrated map to my phone, but there’s one other thing that’s worth dealing with at this point. Even with a UHD monitor, you may want to capture more than one screenful to get complete coverage of an area of interest. This is where OziExplorer‘s free “Map Merge” utility comes in. It will combine any overlapping array of calibrated OziExplorer maps into a single large image.

So for my little project relating to Dundee’s abandoned railway lines, I captured a series of screenshots of the 1903 Ordnance Survey map from NLS, and calibrated them in OziExplorer as described above. This involves jotting down quite a lot of calibration coordinates, but not as many as you might expect—because the screenshot edges must overlap to produce a single large map, and because the calibration points need to be at the corners of each image, then calibration points can and should be reused, to ensure that the images are perfectly aligned in the final map.

Then I open Map Merge, and point it at the folder on my hard drive containing all the map images and their associated *.map files. When these are imported, Map Merge tiles them together to display the coverage of the final map:

When I’m happy with the coverage, I tell Map Merge to create a map from the selected maps:

I also need to tell it what projection and scale to use:

And then I just sit back and wait for Map Merge to zip all the individual files together into one calibrated map, which is saved to the hard drive as two files—an image file with extension *.ozfx3, and a *.map. calibration file for that image. I can load these back into OziExplorer to make sure everything is aligned as it should be.

To get this final map on to my phone, I needed to download and install the OziExplorer Android app. There’s nothing for Apple users, unfortunately, but there is a version for PocketPC handheld devices, which is a bit of a legacy market these days. You can find details on the OziExplorer website. Again, the full version of the Android app is distinctly pricey, but the trial version will do what I describe here, if you don’t mind a prominent watermark on your map display, and having to restart the app every fifteen minutes.

With the app installed on my phone, I plugged it into my PC via a USB cable, and used Windows Explorer to navigate my way to the phone’s OziExplorer\Maps folder. Then I copied across the *.ozfx3 and *map files created by Map Merge.

And that was that. When I opened the OziExplorer app on my phone, I was able to call up my Victorian OS map and follow the line of my disappeared railway using the phone’s GPS. So here I am on the Perth Road, just about to set off cross-country:

That’s neat, isn’t it?