lat/long v OS grid ref

[spiderwebb]

thanks everyone

I was just curious as to how good a downloaded route was in eg whiteouts, exposed ground etc etc as a 100m squared ref is of limited use...

as an ML grad (soon to embark on my Winter ML!) I always use paper maps but feel I should be more tech savvy....

I've not used the downloaded routes from this site, I make up my own using the GPS Planner, just marking key Waypoints and also writing them on my map. The Waypoints used from the GPS Planner (and probably the routes too) will be accurate as they too use the 10 figure ref and given they are taken from the OS Map, and pinpointed using the mouse/pc they will be very accurate, even though you won't be able to see to that level on the map yourself. Once you've done this a few times, you'll realise how accurate and be able to trust the GPS.


When it comes to Winter, white outs etc. other skills come into play, such as knowing the area you're walking in, pacing, timings, and escape routes. Study of the map and route before the walk are equally as important. Although I've relied on my GPS in these situations, given its easier than trying to hold a map etc and navigate, concentrate in such weather. Important not to lose focus, as I have been roaming around a summit in near white out trying to pin down the summit with the GPS, which was changing direction, then realising that I was on it, it was just buried, but one shouldn't be roaming about on a summit that is a few feet from a huge drop

[mynthdd2]

Thanks S/web


I do have the technical skills to deal with white outs etc (eg leading groups in the Cairngorms in January) but I think I probably didn't frame my questions very well - it all comes down to how accurate is GPS...? I also wanted to know the relationship between lat/long and OS grid ref...

[davekeiller]

How accurate is GPS? It depends where you are and how good the signal is. It's often accurate to <10m, but in dense woodland it might struggle to get a signal at all. The co-ordinates in a .gpx or similar file are likely to consistently go to maximum resolution (10 figure grid ref, more than high enough resolution for navigating). Whether those points are where you actually want to be is another question entirely!

What's the relationship between lat/long and OS Grid ref ? They're just two different ways of expressing your location. Lat/long expresses your position based on a "polar" co-ordinate system. Imagine drawing a line from the centre of the earth to a point on the equator due south of Greenwich. Then draw a line from the centre of the earth to your location. The angle north/south between the two lines is your latitude and the angle east/west is your longitude. OS Grid Ref on the other hand, takes the whole of the UK (plus a margin) and projects it onto a flat grid (making a correction for the curvature of the earth) The co ordinates of each square on that grid being your OS Grid Ref.
GPS units likely work internally with lat/long and convert your position into OS Grid Ref (in the UK) so that you can work with a map that expresses your location using the OS grid Ref co ordinate system, because that's the standard in the UK. Go to another country and maps likely won't use OS Grid Ref so you can set your GPS to display whatever they use.

[spiderwebb]

Thanks S/web


I do have the technical skills to deal with white outs etc (eg leading groups in the Cairngorms in January) but I think I probably didn't frame my questions very well - it all comes down to how accurate is GPS...? I also wanted to know the relationship between lat/long and OS grid ref...

I can only add that in my experience, using my GPS with the GPS Planner on this site, which if you are logged in, gives the 1:25 map scale, then the GPS has proved to be very accurate, most certainly down to a few feet. The key being to pinpoint with the sharp end of the mouse pointer to the spot you wish to mark (Waypoint). Although 1:25 as per a map, on screen it is shown much larger.


Technology being technology, and us only being human, it is obviously advisable to take other factors into account when navigating under extreme conditions, but when I have had doubts in the past, the GPS has been true for Waypoints that I have carefully plotted in the comfort of home whilst studying the route beforehand.


Its also worth mentioning that GPS Units themselves are clearly varied, my old Etrex 10, which I believe went sailing off the roof of my car after a Glen Dessary Epic (Sgurr Mor a Hill to Far, walk report), in its later stages of life started to become slow in identifying a fix i.e. where I was simply trying to pin point my location to then refer to the map. The issue with this even with a fully working GPS is that your average human cannot discern down to the level of detail given by the GPS grid Ref from a 10 figure reference, as opposed to plotting it beforehand via the GPS Planner. Although the GPS Planner is still showing a map at 1:25k, it is shown on screen at a much larger scale.... think I've explained that right, same scale but magnified. But it allows you to see the detail, for accurate Waypoint marking, something that is not so easy in the dark, white out etc. when you are out in it.

As for the Etrex 10, now deceased, the previous one, to pick up on an earlier comment was terrible at locating itself in woods. I have recently upgraded to a GPS64S and found it to be highly accurate including in woods. I still carry map/compass, although the GPS comes with full 1:50 mapping, same issue as above, it is difficult to discern the level of detail for such micro navigation, compared to my used method above.

Marking the same waypoints on the map (in pencil) helps to find yourself or a particular Waypoint quickly, not to mention progress overall on your day and as long as the waypoints are plotted as accurately as described above, the marks on the map do not have to be spot on (you couldn't mark to that level anyway (as per the 10 digit issue, too small).

Hope that helps