Difference between OS GB and Garmin maps

Former Member over 8 years ago

If I plan a road cycling route on BC, using the OS GB map, but then follow that route using my Edge 810, which has city navigator only on it, inevitably, some of the waypoints are off the road, so it keeps telling me to do a U turn. No problem, I have got used to that, and if I want real accuracy, I now how to (tediously) get round it.

But the reason I'm posting this is simply to ask why there is a difference between the oldest, supposedly most accurate mapping system in the world (OSGB), and perhaps the newest (Garmin / citynavigator). Does this situation apply in other countries? Again, just a query, not a complaint.

Thanks.

0 SUSSAMB over 8 years ago

The OS map isn't routeable but underlying it is Garmins CN map. How old that map is I don't know but I suspect it hasn't been updated for a while, which may explain the issue.

0 BTLAAKE over 8 years ago

Your question isn’t an easy one to answer. One issue could be the underlying navigational road system beneath(?) the OS map. Or it may be a difference between the road system on your device versus the OS GB software. Or it may be related to OS map layer.

Mapping software designed specifically for GPS usage is likely to operate only within the GPS system. In other words all data needs to be in the WGS84 datum and the units used by GPS infrastructure. For any particular county, the most accurate data will be in whatever datum that country suits their needs best.

This means at some point, a transformation from one datum to another is required. Unfortunately these transformations are not exact. The amount of error will depend on the method used.

Another issue might be the map image layer (OS map). 2D maps are projections of a 3D surface and will be distorted. As an example, let’s assume that this layer in the Garmin software is based on actual images of 1:50000 scale maps. I would guess that these images would have to be distorted to obtain something that would be come close to correct due to the change from the local datum to the WGS84 datum. The visual affect may not be desirable, while the error in leaving the images unchanged may be acceptable. In effect, you would have different projections for the visible map and the road system beneath it. Each may be equally accurate but they will not align.

If you want to go down the rabbit hole, this might be a reasonable place to learn about datums, coordinate systems and reference systems (possibly projections as well, but maybe not) relative to the GB survey national survey.
https://www.ordnancesurvey.co.uk/docs/support/guide-coordinate-systems-great-britain.pdf

By the way, this are just some of my thoughts and should not be considered authoritative. My understanding of the necessary subjects is far to insufficient; so much of the above could be irrelevant or flat out wrong.

0 Former Member over 8 years ago

Thanks.

I hadn't thought of them using different datums / projections so that probably explains it. I realised that when planning routes I was using the OSGB for planning and CN for actual navigation, and know that I could, if I was really that bothered, I could plan initially in OSGB and then move any offset points to match the CN map as a second operation. This answer is good enough to satisfy my curiosity.

I'll check out that link and see if I can make sense of it - not that it will change anything of course, but always good to know. This might explain one or two discrepancies I have noticed when out walking or mountain biking, when the Garmin grid reference is noticeably different from the OS map (I don't worry about high accuracy most of the time, as it is normally very obvious, and rarely that critical as far as I am concerned when pointed to the vicinity on the map).

Thanks again for the replies