OSM vs OS
Contents: OSM vs OS
Why not just use OS maps?
Good reasons to use OSM maps
Key facts
Why not just use OS maps?
I suspect that many of us in the UK who are unfamiliar with OSM (OpenStreetMap) may be thinking “OS mapping is the best, why would I use anything else”?
OpenStreetMap, the Wikipedia of mapping, is here to stay and will just get better as time passes. OSM maps are now everywhere and offer lots of things that OS maps don’t. For example: All over the high ground in Scotland OS maps are often path-blanks on the map. OSM mapping marks many of the paths that are high in the hills.
With electronics, you can easily add weightless OSM topo maps to your toolkit for free. Why wouldn’t you take one along?
Good reasons to use OSM maps
- Many good paths aren’t marked on OS mapping but they are on OSM maps.
- Lots of other useful details are on OSM maps but not on OS maps.
- Hybrid OS/OSM maps may suit some adventures better than the 1:25k OS mapping.
- OSM mapping can be customised to suit the activity.
- OSM topo maps for phones, handhelds and watches are available for free.
- OSM map files are small. Store whole countries on your device.
- Great for travel. OSM maps are readily available for almost anywhere on earth.
- Edit OSM mapping yourself. Add a missing path, your house or anything else.
- A phone app with OSM mapping makes a great free navigation tool.
- An OSM map can help identify non-existent PRoW ‘paths’.
- Get up-to-date OSM mapping at any time for free.
- Print your own OSM maps maps for free.
- With OS & OSM mapping, you’re better equipped than with just an OS map
Key facts
It’s important to understand the pros and cons of both map types to use them safely. So, I’ve compared them here. It will help explain what you’re missing if you don’t carry OSM mapping and also highlights some pitfalls to watch out for with both map types.
There will be many more differences between OS and OSM mapping than I’ve listed here. These are just the things that have caught my attention the most. If you want to learn more then click the links in the ‘feature’ column.
Feature | OS (government-funded) | OSM (crowd-sourced) |
Cost | Expensive | Free or very cheap |
File size | Large – 1:50k about 3GB UK at 1:25k about 12GB | Small (vector maps are data-efficient) UK about 2GB |
Overall quality | Excellent | Usually very good. OS/OSM hybrid maps can be great. |
Paths | Well marked. Updated infrequently. Beware non-existent paths shown as public rights of way. | Many good paths are marked on OSM maps that are not on the best 1:25k OS maps. A few will be ‘patchy’ on the ground. |
PRoWs | Public rights of way are reliably marked including the misleading non-existent paths. | A hybrid OS/OSM map may mark them too. OSM maps can help identify non-existent ‘PRoW’ paths. |
Contours | As accurate as the scale allows. | NASA contours don’t show fine details. Hybrid OS/OSM maps may use OS ‘Terrain 50’ contours. |
Crags | Well represented but many individual crags are not named. | Marked poorly and often not at all, but individual crags may be named. |
Ground | Grass, marsh, scree etc all marked | Key features sometimes marked. Often blank. |
Access land | Reliably marked | A hybrid OS/OSM map might show access land. |
Boundaries | Farmers dry stone walls and fences are often well marked. Not on the 1:50k maps. | Can be very good but not all are marked. |
Towns | Mostly building outlines. Symbols mark a few key buildings. | Excellent. Find what you want in an unfamilar town. |
Fine detail | Good for the scale | Useful details are marked that aren’t on OS maps. |
Zoom friendly? | No, but better apps auto-switch maps when zooming. | Yes. Text and map features remain readable and the map sharp. |
Routable? | No, but external routing services can provide snap-to-path plotting using OS maps. | Garmin maps can be. Some app & maps offer snap-to-path plotting offline. |
Edit the map? | No. | Yes. Changes quickly propagate around the world. |
Customise | No, although simple ‘overlays’ may be offered to highlight cycle tracks or similar. | Yes. You may be able to choose what the map shows and how those elements display. |
Points of Interest | Some apps may superimpose these over the map. | POIs may be part of the map and link to useful information. |
Cost
Approximate costs in Jan 2023…
OS
Smartphones
£13 – BackCountry Navigator Pro can download all the OS mapping!
£30pa – Subscriptions to phone apps OS Maps, Outdooractive, Outdoor Map Navigator.
£150 – Whole UK to keep at 1:25k & 1:50k. The Memory Map app and desktop software.
£3 – To keep popular ‘OS’ map sheets. A download includes all scales. eg Locus Map 4.
GNSS units
£315 – For Garmin handhelds. UK to keep at 1:25k & 1:50k.
OSM
Smartphones
Free: OpenAndroMaps are good and the UK maps incorporate OS crag OpenData.
Free: The Mapy.cz app and accompanying Mapy OSM maps.
£15 – The JohnThornMap of Great Britain is a good OS/OSM hybrid you can keep.
Some OSM-based apps charge more or offer a subscription service.
GNSS units
Free: Good topo maps for Garmin handhelds and watches are available from third-party map makers.
£15: You shouldn’t need to pay more for a Garmin-compatible topo map of Britain to keep, including OS/OSM hybrid maps.
For other makes of dedicated devices OSM maps will likely be more expensive.
File size
Purpose-built for electronics, OSM vector maps only use a fraction of the data that would be occupied by the equivalent converted paper map. If you want the 1:25k and 1:50k OS mapping for the whole of Britain on your phone, you’ll need about 15GB of spare storage. If you don’t do that, then every time you head out somewhere new, you must remember to download the mapping before you get off-grid.
OSM-based vector maps use about 2GB for the whole UK. You can easily keep the entire country permanently stored on your phone. If you have plenty of space on your phone, or have a micro-SD card slot, you could carry detailed topo mapping for the whole of western Europe around with you.
Overall quality
OS maps are reliably accurate with consistent coverage everywhere. There may be areas on OSM maps that are better mapped and show more details than OS maps but the crowd-sourced nature of OSM mapping means that coverage is not so consistent.
Looking to far-flung places, a region which is not well-served with internet connections is less likely to be well-mapped by OSM. Europe is well-mapped.
Paths
Keep at least one OSM map with you for extra paths not marked on the OS maps.
In the highlands of Scotland particularly, there are countless mountain thoroughfares on the high ground that are not marked on OS maps. Even in honeypot areas like Glencoe, the Mamores and Torridon, large areas are path-blanks on the OS 1:25k Explorer mapping. Maps that contain OSM paths show many of these popular routes.
The following image shows two maps of a Glencoe hotspot. If you were planning a trip over those high tops, which map would you prefer to use?
A good map showing OSM paths will often be a much better choice for plotting classic routes in the Scottish Highlands as the OS simply haven’t mapped the paths. A few examples… Beinn Eighe traverse in Torridon, Liatach ridge via the Am Fasarinen pinnacles in Torridon, An Teallach traverse in Wester Ross, Slioch loop from Kinlochewe, the CMD arete on Ben Nevis, Binnein Mor traverse in the Mamores etc. I could go on, it’s a long list.
The same is true across the country but maybe to a lesser extent. WalkLakes.co.uk offer a split screen with a ‘WayMap’ on the left showing OSM paths and OS mapping on the right. Register for free to make some comparisons for yourself…
Caution: An OSM path could have been added by anyone. I’ve come across a few that were at best ‘patchy’ on the ground (although they were safe lines through the terrain). I’ve yet to come across an OSM path that goes through dangerous ground where there isn’t a pre-existing route, but bear in mind their open-source nature. Check the contours and crags that any paths go through and make sure you know what you’re taking on. If an OSM path isn’t what you were expecting, don’t abandon good sense just because it’s marked on the map. Maybe you should consider a different line.
PRoWs
‘Public rights of way’ are marked on OS maps and some OS/OSM hybrid maps. Lots of PRoWs aren’t paths on the ground. Some go through dangerous terrain. Just in case you missed that…many PRoWs are just legal markings on the map where there isn’t even the hint of a path in real life. Avoid. Look for the black dashes that mark real paths. This ‘feature’ of the OS maps has encouraged people into bad spots. Learn how to avoid the non-existent ‘paths’ in Public Rights of Way.
Pure OSM maps don’t reliably mark PRoWs.
Contours
- Best: On the 1:25,000 scale OS maps.
- Next best: On the 1:50,000 scale OS maps
- OS ‘Terrain 50’ contours are close behind the 1:50k OS maps and can be found on OS/OSM hybrid maps.
- NASA/SRTM contours are the least detailed and typically found on much OSM mapping. eg Popular apps like AllTrails and Komoot will likely be using these. Web-based maps like OpenTopoMap and Thunderforest maps too.
Crags
Crag markings are a real weakness of 100% OSM mapping for adventures in mountainous regions.
Cliff edges and crags are usually represented by ‘bumpy’ lines or simple symbols depending on the map, possibly with accompanying shading to represent a craggy area. They fall well short of the crag detail on a 1:25k Explorer OS map and often only hint at how serious the ground is. The lack of any fine detail in the lower resolution NASA-derived contours doesn’t help. Even worse, there are still countless places where seriously steep rocky ground has no OSM crag markings at all.
To venture into mountainous terrain with a pure OSM map and no other knowledge of your route exposes you to unnecessary risk. The map alone is not enough to judge how tricky a path may be. You could be following a path towards dangerous ground and a pure OSM map might not give you any clues. Proper planning and a good map with well-modelled terrain is required to better understand a route.
Popular apps like AllTrails, Komoot and Mapy.cz rely on pure OSM mapping with OSM crag markings and NASA contours. They can keep you on marked paths accurately and can be useful for plotting routes but you must carry better mapping for mountain navigation. This article shares two AllTrails app stories which demonstrate the problem.
eg This Mapy.cz map of Liatach in Torridon does mark the route up and down but there’s hardly a crag or cliff marking on it. With few clues from the coarse NASA contours you could be forgiven for thinking that Liatach was a steep-sided grassy lump.
That couldn’t be further from the truth…
The path along the ridge is not shown but the OS map represents the mountain much better.
In the UK the OS has made ‘Terrain 50’ contours and crag markings OpenData. Free for anyone to use. Third party map-makers have combined this with OSM data to produce some good hybrid maps. You might prefer one of these maps to the OS maps for some routes.
For example, the hybrid OS/OSM WayMap of Liatach below offers pretty good OS terrain modelling and shows the OSM paths. The different ground shading at the 800m contour helps point up the higher ground too. You can get excellent prints of these maps using normal browser printing at WalkLakes.co.uk
You can compare these two maps side by side at the WalkLakes website.
Ground markings
The nature of the ground underfoot is often well marked on the 1:25,000 OS maps. Although OSM data does contain some of this detail it’s not nearly as comprehensive as the OS mapping. However it’s worth noting that the 1:50,000 OS mapping doesn’t contain this information either.
Access land
The 1:25k OS mapping shows this, the 1:50k OS mapping doesn’t. A 100% OSM map won’t reliably show any access land. However a hybrid OS/OSM map might. eg The John Thorn map of Britain puts a yellow wash over inland access land that’s hard to miss.
Boundaries
Knowing where dry stone walls and fences are can be handy for navigation ‘handrailing’. Only the 1:25k OS maps reliably show these. They are not shown on the OS 1:50k maps. OSM mapping is somewhere between the two. Some areas are remarkably well marked and may match the 1:25k OS mapping for detail and elsewhere those details may be absent.
Towns
The detailed 1:25k OS mapping doesn’t do much more than provide building outlines in towns.
OSM maps can be brilliant and will contain all the kind of information you’d like to have at your fingertips when entering an unfamiliar town or city.
Fine detail
It’s not just in towns that great detail may exist on an OSM map. Unconstrained by scale, zooming in on OSM mapping can reveal details that can’t easily be marked on an OS map.
Individual crag names, individual cairns marking a safe route off a mountain summit, river fords, small waterfalls, hiking signposts, stiles and gates, drinking water sources etc may all be marked on an OSM map.
No detail is too small to be marked and the image remains crystal clear however far in you zoom.
Zoom friendly?
An OS map doesn’t reveal any more detail with a zoom-in. Zoom-out and the map details become too small to read.
Good app and OS map combinations will switch between different scale maps automatically with the zoom. An OSM map will typically work well at any zoom level.
One OSM map does everything, from an overview of the whole country to showing exactly where the public benches are in a village.
Routable?
Routable third-party mapping for Garmins is available. OsmAnd mapping is routable and offers snap-to-path routing offline. Locus Map 4 stores OSM routing data and has a built-in router for offering snap-to-path plotting offline. This might be the only app that offers offline snap-to-path plotting with OS maps.
Nearly all apps offering OS maps need to be online to offer snap-to-path routing.
Edit the map?
OpenStreetMap can be edited by anyone at any time. Those edits will quickly propagate around the world as map-makers update their mapping.
OS maps can’t be edited and may get updated infrequently.
Customise
The appearance of some OSM mapping may be customised significantly to suit the activity.
OS maps can’t be customised.
Points of interest (POIs)
Stored OSM data can provide useful information for POIs offline. Tapping a POI for a restaurant may reveal useful invisible information. You may learn what type of food it sells, opening times and a telephone number. All stored in an OSM map that doesn’t require an internet connection.